Land Manager's Monitoring Guide: soil pH indicator
Land Manager's Monitoring Guide Soil pH indicator
Prepared by: Environment and Resource Sciences Department of Environment and Resource Management © State of Queensland (Department of Environment and Resource Management) 2010
This document has been prepared with all due diligence and care, based on the best available information at the time of publication. The department holds no responsibility for any errors or omissions within this document. Any decisions made by other parties based on this document are solely the responsibility of those parties. Information contained in this document is from a number of sources and, as such, does not necessarily represent government or departmental policy. If you need to access this document in a language other than English, please call the Translating and Interpreting Service (TIS National) on 131 450 and ask them to telephone Library Services on +61 7 3224 8412.
This publication is available in alternative formats (including large print and audiotape) on request. Contact (07) 322 48412 or email August 2010
Contents What is it? ..................................................................................................................................................................................1 Other factors and related indicators........................................................................................................................................1 Why monitor this indicator?.......................................................................................................................................................1 Planning to monitor this indicator ..............................................................................................................................................2 How will your data be used? ..................................................................................................................................................3 What will you monitor?..........................................................................................................................................................3 Where will you monitor?........................................................................................................................................................3 When and how often will you monitor? .................................................................................................................................4 What legal or regulatory requirements will affect your monitoring? .....................................................................................5 How do you measure it?.............................................................................................................................................................5 How do you measure it? - Level 1 monitoring ...........................................................................................................................5 Skills needed ..........................................................................................................................................................................5 Equipment and costs...............................................................................................................................................................6 Time taken..............................................................................................................................................................................6 Setting up ...............................................................................................................................................................................6 Monitoring procedure.............................................................................................................................................................6 Data quality considerations ....................................................................................................................................................7 How do you measure it? – Level 2 monitoring ..........................................................................................................................7 Skills needed ..........................................................................................................................................................................8 Equipment and costs...............................................................................................................................................................8 Time taken..............................................................................................................................................................................9 Setting up ...............................................................................................................................................................................9 Monitoring procedure...........................................................................................................................................................10 Data quality considerations ..................................................................................................................................................11 How to record your results .......................................................................................................................................................11 Metadata ...............................................................................................................................................................................12 What does your data mean?......................................................................................................................................................13 What are some management options? ......................................................................................................................................17 Managing soil pH for production .........................................................................................................................................17 Adding lime or dolomite to your soil ...................................................................................................................................18 Some tips about lime ............................................................................................................................................................19 Adding organic matter to your soil.......................................................................................................................................19 Other information sources ........................................................................................................................................................19 Facts sheets...........................................................................................................................................................................19 Websites ...............................................................................................................................................................................19 Glossary....................................................................................................................................................................................19 References ....................................................................................................................................................................................22 Authors.........................................................................................................................................................................................23
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Land Manager's Monitoring Guide - Soil pH indicator
What is it? Soil pH indicates the concentration of acidity or alkalinity within a soil profile (Rayment & Higginson 1992). The pH scale is the means by which the degree of acidity is measured. Scientifically it is a measure of the hydrogen ion (H+) activity. It is represented in a logarithmic scale ranging from less than zero (strongly acidic) to 14 (strongly alkaline) (Powell & Ahern 1999). Soils can be naturally acidic, neutral or alkaline and this can be measured by testing the pH value of the soil. The pH value can differ within and across a paddock or the property and vertically though the soil’s profile and can change over time at the same location and depth. The pH of the soil in a paddock can change as a result of the use of the paddock, management practices and natural processes. The pH level within the soil influences soil conditions and plant growth. Change in soil pH refers to regular monitoring of any variation over time in soil pH. It is very important to note in coastal areas that the presence of acid sulfate soils (ASS) can make for very high acidity readings. For more information on this, go to the acid sulfate soils website <www.derm.qld.gov.au/land/ass/>. This indicator refers to general soil acidity derived from sources other than from acid sulfate soils.
Other factors and related indicators Monitoring change in soil pH will give you an understanding of the acidity and alkalinity in the soil and how this may change not only down the soil profile but also across your property. This indicator focuses upon change in soil pH only. It will not provide you with information on change of soil structure, infiltration, chemistry or biological activity. Monitoring of all or some of these parameters will provide a more comprehensive picture of overall soil health and potential productivity. Related indicators in the Land Manager’s Monitoring Guide include: ‘Saline land’, ‘Soil erosion’, ‘Ground cover’, ‘Hillslope erosion’, ‘Wind erosion’, ‘pH of water’, ‘Soil chemistry’, ‘Soil structure’ and ‘Soil infiltration’. The pH of soil can vary in most soils with levels of moisture in the soils and prolonged periods of very wet or dry conditions. Working out the pH response of your soils to rainfall, seasonal or longer-term climatic variation (El Niño) is critical to understanding and interpreting what is happening. Monitoring rainfall and irrigation on your property will assist in this. There are a number of aspects of your irrigation practices that you may wish to monitor. Further information is available from the Queensland Government’s Rural Water Use Efficiency Initiative <www.derm.qld.gov.au/rwue>.
Why monitor this indicator? Having the correct pH is important for healthy plant growth and maintaining the natural soil biota (see ‘Glossary’). The pH of all soils change over time; some agricultural practices accelerate this change. (see Soil acidification fact sheet <www.derm.qld.gov.au/factsheets/pdf/land/l45.pdf>) The reason behind monitoring pH over time is to understand the baseline pH level, the rate of change and what this means for management. “Queensland has more than 500 000 ha of agricultural and pastoral land that has acidified or is at risk of acidification. The higher rainfall coastal areas used for intensive agriculture are most susceptible. Soils most at risk are lightertextured sands and loams with low organic matter levels, and the naturally acidic red clay loam soils commonly found in areas such as the South Burnett and Atherton Tableland. Soils least at risk are the neutral to alkaline clay soils, for example, the black clay soils of the Darling Downs and Central Queensland and brigalow soils” (DERM 2009). “Growing plants lower the alkalinity in the soil. When harvested products are removed, the soil becomes more acidic. The impact is greatest where a large quantity of material is removed, as in the production of silage, hay and sugarcane” (DERM 2009). Research has shown that some agricultural practices (such as fertilising with ammonium-based nitrogen or deep ripping) significantly alter soil pH over time by influencing soil conditions, affecting plant growth, reducing nutrient availability and ultimately resulting in land degradation (Baker & Eldershaw 1993). Factors that accelerate acidification include: •
The application of ammonium-based nitrogen fertilisers to naturally acid soils at rates in excess of plant requirements
•
Leaching of nitrate nitrogen, originally applied as ammonium-based fertilisers, out of the root zone
•
Continued removal of alkaline plant and animal produce and waste products from the paddock (DERM 2009).
Conducting a baseline assessment and monitoring for change in soil pH will provide important information to assist your management action decision-making. As with all monitoring activities, you will need to clearly identify why you 1
Land Manager's Monitoring Guide - Soil pH indicator
want to monitor soil pH and how monitoring this indicator may support your property or business planning objectives. Figure 1 gives you an idea of how acid or alkaline some well-known products are and shows the typical ranges for optimal plant growth. Acidity has an indirect effect on plant growth by decreasing the availability of essential plant nutrients such as phosphorus and molybdenum, and by increasing the availability of certain elements to toxic levels, particularly aluminium and manganese. Most Australian soils are within a pH range of pH 3–10 (Stace et al 1968) and most Australian agricultural soils are in the range of pH 4.5–9, with plant growth in most situations being optimum at pH 5–7. Soil testing for pH will identify the most likely limitations due to soil acidity (and where pH is low, subsequent soil chemical analysis for aluminium toxicity may be necessary) (GRDC & DNRE 2003).
Figure 1: The pH scale, Australian soils, agriculture, plant growth and some common products (Powell & Ahern 1999) Understanding of the pH of your soils is essential to determine how suitable your soils are for long-term productive agriculture. Gaining an understanding of the variation in soil pH throughout your soil and across your property can help in making decisions about how you use the land. Monitoring the soil pH both at the surface and at depth (subsurface) will give an indication of any trends in soil pH over time. The pH of soil naturally changes very slowly over geological time and is influenced by factors including parent material (the rock from which it was formed), weathering and the gradual accumulation or loss of plant and animal matter in the soil. Agricultural practices may accelerate or alter these natural processes. The pH of soils may also fluctuate during the year depending on moisture levels. Monitoring soil pH will indicate whether your land use and management practices are changing this key parameter of soil health.
Planning to monitor this indicator The ‘Developing your monitoring plan’ part of the ‘Monitoring overview’ section of the Land Manager’s Monitoring Guide discusses seven key questions that help to define the why, what, who and how of monitoring: 1. What are your monitoring objectives? 2. How will your data be used? 3. What will you monitor? 4. Where will you monitor? 5. When and how often will you monitor? 6. Who will be involved and how? 7. How will your data be managed? These seven questions should be considered for every indicator that you plan to monitor. In addition to the concepts 2
Land Manager's Monitoring Guide - Soil pH indicator
discussed in the ‘Monitoring overview’ you should also considered the following issues that are specific to this indicator when planning your monitoring.
How will your data be used? Soil acidification is a major soil degradation issue in many parts of Australia and is one of the indicators listed in the National framework for natural resource management standards and targets (AG, 2004). Regional natural resource management bodies are now required to report on soil condition including soil acidity and they along with other natural resource management groups may be interested in your data. If you are interested in participating in the monitoring programs of your regional natural resource management body <www.regionalnrm.qld.gov.au> you should contact these organisations for further information. Assessments by national and state governments and regional natural resource management bodies have identified limited information and data on soil condition (including soil acidity) and the impact of management actions. Your monitoring of soil pH to level 1 will give you some information about what is happening to the soils on your property which will be beneficial for your management. However, if you choose to monitor soil pH to level 2 you can contribute to developing a deeper understanding of what is occurring in your region and help in managing this national natural resource priority.
What will you monitor? Existing monitoring in your area Before you start monitoring any indicator it is strongly recommended that you explore who else is monitoring in your area, what they are monitoring and how they are monitoring it. Doing this will not only make sharing your data easier if you chose to do so but will also help you become more familiar with: •
Any area specific issues that may influence your monitoring
•
What strategies and/or methods have proven successful within your area
Other land managers, your catchment group, your regional natural resource management body <www.regionalnrm.qld.gov.au> or the Queensland Department of Environment and Resource Management may be monitoring for soil acidity in your area. They may be able to provide information on other local monitoring and if there are any specific recommendations for your area.
Where will you monitor? Gathering baseline and ongoing monitoring information from different areas on your property will enhance your overall understanding of the way pH changes over time across each area and how they may be affected by the use and management of the land. This will provide you with an understanding of the pH of your soils in highly productive areas as well as in any areas that might be showing signs of declining yields or unexplained failures. You may have a property management plan that includes a layer identifying your soil types that you have previously surveyed. If you are unfamiliar with your soils or the history of the use of the property, you may want to do a survey of the soils on your property or ask your neighbours what they know about the previous uses of the areas that you are most interested in. You may be able to obtain soil maps from your local Queensland Department of Environment and Resource Management business centre . pH values can vary from one sample spot to another spot within one paddock. To ensure that you obtain a representative sample and more reliable result, you will need to collect from a number of spots within your chosen area/s for each pH test you take. Figure 2 provides some guidelines on sampling strategies and the number of samples you will need to collect. As indicated in the figure you will need to blend a number samples from a uniform depth in each spot, then mix the sub-samples in a clean plastic bucket to form one uniform or composite sample from which you can then take a sample to get your pH measurement.
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Land Manager's Monitoring Guide - Soil pH indicator
Figure 2: Soil sampling strategy for general use (amended from NR&M, 2005) If you have a number of locations of interest, you should identify very similar (near uniform) locations that you may be able to monitor as one area. These are paddocks or areas of your property that have similar soils, occupy a similar position in the landscape (e.g. river flats) and have a similar history of use and management practices. Avoid areas close to fences, roads and buildings as these could influence your results. You should also avoid highly variable locations. However, where these areas are of particular interest you should treat each one of them as a separate monitoring location. The number of soil sample locations will depend on the soil type and land practice on your property. In general, it is good to have soil pH measurements for each individual cropping area. It is also good to have an idea of the soil pH of each soil type. You may be able to obtain a soil map from your nearest Queensland Department of Environment and Resource Management business centre .You can use this and any other information you have about your soil type to determine where best to locate your sampling points. Soil sampling means soil is disturbed and removed. Be careful to space your sampling holes to minimise damage. Sampling depths You will get the clearest picture if you sample from the surface and the subsurface of the soil and across the various soils on your property. The surface layer is from the soil surface to a depth of about 10 cm for pastures and crops and to around 15 cm for trees, vegetables and ornamentals. There is no convention for subsurface sampling depth, but the aim is to be near the lower part of the root system at approximately 30–40 cm. This would identify any subsurface acidity problems in a soil layer where most crops would have an active root system (pers comm. Moody 2004). Whole of soil profile sampling Additional samples can be collected from other depths to observe and monitor a trend down a soil profile. Note that in traditional soil surveys, tests are conducted at intervals of 30 cm down the soil profile (including one sample at the surface) or at least one test per soil horizon or layer whichever is lesser. This procedure gives an excellent idea of what is happening down the soil profile in terms of surface and subsurface soil pH and can indicate trends. If you use this technique, include a sample at 10 cm as well.
When and how often will you monitor? As soil pH in general tends to change slowly over time, soil pH can be monitored: •
Annually
•
Before and after major changes in land use or farming system on soils vulnerable to significant pH change 4
Land Manager's Monitoring Guide - Soil pH indicator
The frequency of monitoring will be influenced by property activities and climate. Some land practices will speed up the rate of change in soil pH and so regular monitoring should aim to identify these changes. Do not sample paddocks for at least one month after fertiliser, lime or gypsum has been applied.
What legal or regulatory requirements will affect your monitoring? Generally, there are no legal requirements if you wish to monitor for soil pH on your property.
How do you measure it? To monitor soil pH you will need to select the monitoring technique you want to use, identify the areas that you wish to monitor (see ‘Planning to monitor this indicator’) and then carry out routine measurements of the soil pH over time. After gathering your first (or baseline) information you can then monitor your soil regularly to see how the soil pH is changing over time. Two levels of monitoring soil pH are provided in this guide. Level 2 monitoring is recommended if you plan to share your information with others. Level 2 monitoring includes the two options of self-testing or laboratory testing. At whichever level you choose to monitor, it is a good idea to mark the next monitoring date on your planning calendar. You may be able to coordinate other on-site monitoring activities and save time. There are four common methods for monitoring soil pH: pH strips, field pH kits (Raupach & Tucker method), electronic pH meter and laboratory testing. Because of technical limitations, pH strips are not recommended and are not discussed further here. The cheapest recommended method (Level 1 monitoring) for monitoring soil pH in the middle range of the pH scale is using the field pH kit (Raupach & Tucker 1959 method). This technique uses a barium sulfate (BaSO4) powder and a ‘universal indicator’ solution that changes colour as pH level changes. Where pH extremes are being monitored (very low or very high pH measurements, such as acid sulfate soils), the field pH kit method can be in error by up to 2 pH units. Level 2 monitoring using an electronic pH meter or laboratory analysis and precise location of the sample site provides more accurate results and high quality standards.
How do you measure it? - Level 1 monitoring Key aspects of level 1 monitoring •
Choose locations for soil sampling
•
Collect soil samples
•
Test soil samples using field pH kit
•
Monitor regularly
Skills needed Locate soil sampling sites •
Ability to identify areas of poor productivity and/or signs of acidity such as patchy growth, nutrient deficiency, aluminium or manganese toxicity
•
Understanding of the general distribution of different soil types on your property
Collect soil core •
Ability to use appropriate collection tools
•
Knowledge of the correct sampling strategy
•
Ability to accurately collect and label samples
Measure soil pH •
Ability to use soil pH field kit
•
Ability to accurately determine soil field pH measurement using field kit colour cards (field kits are not recommended for people with colour blindness)
•
Ability to record information accurately 5
Land Manager's Monitoring Guide - Soil pH indicator •
Illustrate data and analyse trends
•
Ability to prepare basic graphs
•
Understanding of the relationship between results and land practices
Equipment and costs Monitoring soil pH is relatively inexpensive using a field pH kit. The main costs associated with monitoring soil pH for Level 1 are shown below. Shovel or soil auger Soil augers can be purchased from scientific supply outlets or specialist auger manufacturers (engineering firms). Augers cost about $300. Soil pH test kit Soil pH test kits are available from pool supply stores, hardware stores and chemical supply stores and cost about $30. Note: Soil pH test kits are only suitable for mid-range pH soils; they should not be used to measure the pH of highly acidic or alkaline soils. Minor items •
Tape measure – 50 m fibreglass with 1 cm increments
•
Wooden skewers or stirrers to break down lumps in the soil
•
Bucket and brush – to rinse and clean the pH tray ready for next use
•
Recording sheets
•
Stationery – writing pad, pens, pencils, eraser, ruler, printing ink, etc.
Time taken The time taken to monitor soil pH to Level 1 will depend on: •
How long it takes to travel to and between your sampling sites
•
The number of samples collected and tested
•
The hardness of the soil
•
The sampling procedure you use – it will take more time if you are blending or mixing a number of samples.
Setting up 1. Store barium sulfate powder and indicator solution in a cool dry place. For quality assurance and reliable results the powder will need to be replaced if it has become moist. 2. You need to select the location/s, depth and frequency of monitoring that are most likely to provide you with reliable information from the various soil types on your property (see ‘Planning to monitor this indicator’). 3. Record your sample locations on your property management plan or in some other way that enables you to reliably sample from the same location each time (remember to space holes to minimise soil damage). 4. Make notes on why you have chosen each location, such as land use and/or signs of acidity or potential toxicity due to soil acidity. 5. Where practical you may want to mark the sampling site with painted star pickets or some other marker.
Monitoring procedure Initial recording 1. Collect the soil sample and where necessary break up any large clumps to ensure the sample can be mixed well before sub-sampling for testing (see ‘Planning to monitoring this indicator’). It is recommended that you use a soil auger to collect the sample. 2. For each sample in turn, place a small amount of the sampled soil (¼–½ teaspoon) on the tray provided with the pH kit.
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Land Manager's Monitoring Guide - Soil pH indicator
3. Add a few drops of indicator solution and mix thoroughly with a clean toothpick, skewer or stirrer. 4. Puff a small amount (enough to put a thin layer over the soil) of barium sulfate over the mixture and wait for the soil to absorb the moisture and for the colour to become stable. This should only take about 10 seconds. If it is left too long, the mixture will ‘go off’ (especially in high temperatures) and give an inaccurate reading. 5. Compare the colour of the powder to the chart provided in the kit. 6. Record the following on your datasheet: •
Date
•
Site name or number
•
Site location
•
Sample method (shovel, auger, mechanical equipment)
•
Sample number (Unique identifier for sample)
•
Sampling depths for each sample taken from each soil core
•
Measurement in pH units
•
Test method (pH test kit)
•
Recording person
•
Comments, such as any new information on signs of land use and/or signs of acidity or potential toxicity due to soil acidity and/or water status of soil (wet, dry, waterlogged, field capacity), weather conditions or days since rain or irrigation and any problems.
7. Repeat steps 2–6 above on the remaining samples (at different locations or depths). Note: You may also want to carry out other monitoring activities from your soil samples, such as soil chemistry, soil structure, plant available water content, soil life, soil infiltration and/or soil salinity. These other measures will involve additional steps or sampling throughout the soil profile and it is a good idea to logically order your monitoring measurement; some will be recorded prior to pH sampling. Ongoing monitoring Repeat steps of the initial recording at the frequency you have decided to monitor.
Data quality considerations Correctly using a soil auger rather than a shovel to collect soil samples will in general give a more accurate representation of the soil profile because of reduced risk of cross-contamination between upper and lower soil profile. Soil augers can be used with extension handles to retrieve soil from depths in excess of 5 metres. Augers may not be ideal where there is a hard clay pan or lots of rocks in the soil profile. Mechanical drilling equipment may be more suitable in this case. Augers need to be cleaned after each hole is dug to minimise cross-contamination. Store and replace barium sulfate powder and indicator solution as soon as it becomes moist or 3 months old (whichever comes first) (see manufacturer’s instruction for specific details). In extreme weather conditions such as high humidity the powder can become moist and ineffective or can absorb carbon dioxide and become acid.
How do you measure it? – Level 2 monitoring Key aspects of level 2 monitoring •
Choose locations for soil sampling and record locations with GPS
•
Collect soil samples
•
Test soil samples using (a) electronic pH meter or (b) laboratory analysis
•
Monitor regularly
Level 2a & b monitoring allows you to monitor across the whole pH range if necessary with accurate results and is suitable if you want to share your data with others. Level 2a involves field-testing using your own or borrowed equipment that provides you with immediate results and requires regular calibration of the equipment. Level 2b involves getting your soil samples analysed at a chemical laboratory and can be combined with analysis for a number of soil chemical indicators.
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Land Manager's Monitoring Guide - Soil pH indicator
Skills needed Locate soil sampling sites •
Ability to identify areas of poor productivity and/or signs of acidity such as patchy growth, nutrient deficiency, aluminium or manganese toxicity
•
Understanding of the general distribution of different soil types on your property
Use of GPS and GIS •
Ability to use a Geographic Positioning System (GPS) unit for recording locations
•
Ability to use a personal computer and Geographic Information System (GIS) software
•
Ability to manage GIS data
Collect soil core •
Ability to use appropriate collection tools
•
Knowledge of the correct sampling strategy
•
Ability to accurately collect and label samples
Measure soil pH •
Ability to (a) use an electronic pH meter or (b) prepare samples for laboratory analysis
•
Ability to record information accurately
•
Illustrate data and analyse trends
•
Ability to prepare basic graphs
•
Understanding of the relationship between results and land practices
Equipment and costs Monitoring soil pH using an electronic pH meter (Level 2a monitoring) is slightly more expensive than using the field pH kit (Level 1 monitoring) but the results you obtain will be more accurate. The additional expense is the purchase an electronic pH meter and the occasional replacement of electrodes. Using laboratory analysis (Level 2b monitoring) means your results will be more accurate and you do not need your own testing equipment but you will incur a cost for each sample you get analysed. The main costs associated with monitoring soil pH to Level 2 are shown below. GPS receiver GPS receivers are now commonplace, are frequently updated and are available from camping, boating, scientific and electronics stores and cost about $250+. GIS software GIS software is constantly being upgraded and there are now many vendors. Use of online search engines set to Australia will readily identify some of these providers. Basic functions include display of digital images and overlaying of data layers. Costs vary depending on software. Shovel or soil auger Soil augers can be purchased from scientific supply outlets or specialist auger manufacturers (engineering firms). Augers cost about $300. Electronic pH meter with electrode (spear point) (for Level 2a monitoring) Electronic pH meters are available from specialist scientific equipment stores and can cost between $150 and $600+. When purchasing you need to consider the level of accuracy required and the durability of the meter. You will also need to decide if you want to use multipurpose electronic meters that can test a number of parameters. Buffer solutions and electrode gel (for Level 2a monitoring) You will need pH 4.0 buffer solution, pH 6.88 buffer solution and electrode gel (potassium chloride) to maintain a reference solution in the electrode. These are available from specialist scientific equipment stores and most department stores (solutions are commonly used in steam irons). Buffer solutions do not tolerate extreme temperatures, so should be stored in a cool place. Buffer solutions and electrode gel should be checked and/or replaced every 12 months and cost 8
Land Manager's Monitoring Guide - Soil pH indicator
about $30 each for 500 ml quantities. Basic computer spreadsheet software Basic computer spreadsheet software is available from computer and office supply stores. The cost depends on the software, although it is often included with a computer purchase. Plastic test tubes and rack or other small containers (for Level 2a monitoring) Plastic test tubes and racks are available from specialist scientific equipment stores and cost about $25 for 24 tubes and $20 for a rack. It is important that test tubes have incremental volume markings and that the rack snugly fits the test tubes and keeps the tubes upright. Some racks are collapsible making storage easy. Minor items •
Tape measure – 50 m fibreglass with 1 cm increments
•
Wooden skewers or stirrers to break down lumps in the soil
•
Bucket and brush – to rinse and clean the pH tray ready for next use
•
Recording sheets
•
Stationery – writing pad, pens, pencils, eraser, ruler, printing ink etc.
Level 2a monitoring: •
Bucket and brush – to rinse and clean the pH tray ready for next use
•
Deionised water (pH 5.5) and squirt bottle for testing and rinsing test tubes and electrode
•
Tissues
Level 2b monitoring: •
Plastic ziplock bags – to place soil samples in.
•
Waterproof labels – marked with identification code, date, and sample depth
•
Copy of data sheet – to send with samples to the laboratory
Time taken The time taken to monitor soil pH to level 2 will depend on: •
How long it takes to travel to and between your sampling sites
•
The number of samples collected and tested
•
The hardness of the soil
•
The sampling procedure you use – it will take more time if you are blending or mixing a number of samples
•
Calibration of equipment for level 2a monitoring (approximately 5 minutes)
•
Sending samples off to a laboratory and awaiting results for level 2b monitoring (may be several weeks).
Setting up 1. You need to select the location/s, depth and frequency of monitoring that are most likely to provide you with reliable information from the various soil types on your property (see ‘Planning to monitor this indicator’). 2. Cleaning your sampling equipment in between sampling events will prevent cross-contamination of soil samples and encourage reliability of results. 3. Record your sample locations on your property management plan or in some other way that enables you to reliably sample from the same location each time (remember to space holes to minimise soil damage). 4. Make notes on why you have chosen each location, such as land use and/or signs of acidity or potential toxicity due to soil acidity. 5. Where practical you may want to mark the sampling site with painted star pickets or some other marker. 6. Take a GPS reading to obtain the location coordinates. This will enable you to reliably sample from the same location each time and this will be required if you wish to share your information with others.
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Land Manager's Monitoring Guide - Soil pH indicator
Monitoring procedure Initial recording 1. Collect the soil sample and where necessary break up any large clumps to ensure the sample can be mixed well before sub-sampling for testing (see ‘Planning to monitor this indicator’). It is recommended that you use a soil auger to collect the sample. 2. Cleaning your sampling equipment in between sampling events will prevent cross-contamination of soil samples and encourage reliability of results. 3. Measure the pH of your sample by using: An electronic pH meter (Level 2a monitoring): •
Ensure the meter has been calibrated (see ‘Data quality considerations’ below).
•
Ensure the buffer solutions are fresh and if replace necessary.
•
Record the pH of the buffer solutions and any other quality assurance actions.
•
Prepare test tubes and rack or similar small clean containers to hold each sample.
•
Place approximately half a teaspoon of the soil sample into the corresponding test tube.
•
Place enough deionised water in the test tube to make a paste similar to a ‘tiling grout mix’. Stir with a skewer or similar to ensure all soil ‘lumps’ are broken down. Never stir the paste with the electrode as it will damage the delicate glass membrane of the electrode.
•
Place the pH electrode into the test tube ensuring that the point is totally submerged in the soil/water paste.
•
Wait for the reading to stabilise and record the pH measurement. Some meters have a data logging facility that enables measurements to be downloaded directly from the unit to a computer, thus reducing the possibility of error during data transfer. Record the result.
•
Clean your pH electrode after each measurement.
•
Clean out your test tubes prior to the next round of sampling.
•
Repeat this procedure for each sample.
Laboratory analysis (Level 2b monitoring): •
Place samples in 500 g lots in sealable airtight bags (ziplock sandwich bags are good).
•
Mark your samples clearly so that location and depth of the sample is readily identified. Sending your soil samples to a NATA accredited laboratory is recommended for quality assurance. You should phone the laboratory first so they know your samples are coming.
•
Once your results are returned from the laboratory you will need to transfer the results to your data sheet. Some laboratories offer an interpretation service for an additional fee that will help you interpret the results.
4. Record the following on your datasheet: •
Date
•
Site name or number
•
Site location
•
Sample method (shovel, auger, mechanical equipment)
•
Sample number (Unique identifier for sample)
•
Sampling depths for each sample taken from each soil core
•
Measurement in pH units
•
Test method (electronic pH meter or laboratory analysis)
•
Recording person
•
Buffer solutions used and any other quality assurance actions (Level 2a monitoring only)
•
Comments, such as any new information on signs of land use and/or signs of acidity or potential toxicity due to soil acidity and/or water status of soil (wet, dry, waterlogged, field capacity), weather conditions or days since rain or irrigation and any problems. 10
Land Manager's Monitoring Guide - Soil pH indicator
5. Repeat steps 2–4 above on the remaining samples (at different locations or depths). Note: You may also want to carry out other monitoring activities from your soil samples such as soil chemistry, soil structure, plant available water content, soil life, soil infiltration and/or soil salinity. These other measures will involve additional steps or sampling throughout the soil profile and it is a good idea to logically order your monitoring measurement as some should be recorded prior to pH sampling. On-going monitoring Repeat steps of the initial recording at the frequency you have decided to monitor.
Data quality considerations Correctly using a soil auger rather than a shovel to collect soil samples will in general give a more accurate representation of the soil profile because of reduced risk of cross-contamination between upper and lower soil profile. Soil augers can be used with extension handles to retrieve soil from depths in excess of 5 metres. Augers may not be ideal where there is a hard clay pan or lots of rocks in the soil profile. Mechanical drilling equipment may be more suitable in this case. Augers need to be cleaned after each hole is dug to minimise cross-contamination. Field test kits are only suitable for middle range pH soils, while electronic pH meters or laboratory analysis is suitable for the whole pH range of soils. If you are sending samples to a laboratory it is recommended that you use a NATA accredited laboratory. These laboratories regularly undergo quality assurance procedures through an independent body. Care of pH electrodes For level 2a monitoring it is recommended that you use a combination (IJ) electrode (the reference and glass electrode are combined together). You should store the electrode tip in deionised water, potassium chloride solution or a pH buffer solution and ensure the protective cap is on the probe when it is not being used. Never let the electrode dry out. Check the electrode regularly for scratches or damage and if it is damaged replace it. Calibration of pH meter It is recommended that you calibrate the pH meter and electrode, and replace electrolyte in the electrode each day prior to use (see manufacturer’s instruction for specific details). If you are taking measurements all day recalibrate your pH meter after a few hours as the change in outside temperature may affect the calibration. •
Remove the protective cap and probe sleeve, refresh the saturated potassium chloride solution or gel in the electrode junction and replace the sleeve.
•
Rinse the probe with deionised water and dry with a clean tissue before putting the protective cap back on.
•
Calibrate the meter according to the manufacturer’s instructions with standard pH buffer solution (e.g. pH 6.88 and pH 4).
•
Ensure the buffer solutions are fresh and clean by placing a small amount in a small tube for use. Never insert the probe into the bulk bottle and don’t return used buffer solution to the bulk bottle. You should replace the buffers every 12 months.
How to record your results When you are working in the field the simplest way to record your data is to have a field recording sheet (Table 1) with you. A field recording sheet will help ensure that your data is recorded in a way that is easy to read later and will also act as a checklist to ensure that you don’t miss recording any important information. The data recorded on the field recording sheet should include: •
Date
•
Site name or number
•
Site location
•
Sample method (shovel, auger, mechanical equipment)
•
Sample number (Unique identifier for sample)
•
Sampling depths for each sample taken from each soil core 11
Land Manager's Monitoring Guide - Soil pH indicator •
Measurement in pH units
•
Test method (pH test kit, electronic pH meter, laboratory analysis)
•
Recording person
•
Buffer solutions used and any other quality assurance actions (Level 2a monitoring only)
•
Comments, such as any new information on signs of land use and/or signs of acidity or potential toxicity due to soil acidity and/or water status of soil (wet, dry, waterlogged, field capacity), weather conditions or days since rain or irrigation and any problems.
Table 1: Field recording sheet for level 1 monitoring with minimum example data
Date
Site number
Site location
Sample method
Sample number
Sample depth (cm)
pH value
Test method
Recording person
Comments
23/05/2005
1
Bottom of back paddock
Auger
1
8.6) can lead to: •
Surface sealing and crusting problems due to excessive sodium
•
Reduced availability of iron, manganese, zinc, phosphorus and copper
•
Reduced microbial activity and reduction in fungal population.
Some more specific problems associated with soil acidity and alkalinity A soil of pH 5.5–7.5 results in a good balance in availability of most nutrients. Very high and very low soil pH can influence the soil’s ability to retain nutrients, which can lead to leaching of some nutrients. Assessing the impact of significant change in pH levels requires a soil chemical analysis to obtain a detailed analysis of the availability of each nutrient. Some crops, for example sugar cane, are quite tolerant of low soil pH and high amounts of aluminium and manganese. However, over time, nutrient deficiencies are likely to become an issue if unmanaged. Soil pH can be influenced by the use of some fertilisers or simply by growing and harvesting crops. Often the soil can be quite acidic at the surface with an increase to alkaline at depth. This trend can be the result of repeated fertiliser application or organic matter build up in the case of mild acidity at the surface. Alkaline soil pH can be found above a hardpan or impermeable layer at depth in the soil profile if the soil is sodic. Chemical testing by a laboratory can confirm this. This is one example of a vertical trend that may require a change in management or land use at the sample location. Table 5 lists some additional causes and implications of various pH ranges in surface and subsurface soils.
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Land Manager's Monitoring Guide - Soil pH indicator
Table 5: Causes and implications of soil pH value (amended from Moody 2004) Diagnostic range
Causes and implications
Extremely acidic soil (pH 9.1)
Soils are extremely alkaline and dominated by sodium, calcium and magnesium carbonates. In these alkaline soils, micronutrients (e.g. copper, zinc, iron, manganese), potassium and/or phosphorus deficiencies can occur. Boron toxicity can exist. The soil will almost certainly have a very poor nutritional and structural status.
What are some management options? Managing soil pH for production Regular soil pH monitoring is an important tool in managing soil pH as it gives an indication of what nutrients may be limited or required for healthy crops (Sugar CRC 2001). It is important from a cropping perspective to maintain a soil pH of at least pH 5.5. If the pH of the surface soil is less than 5.5, long-term acidification of subsoils will result as the acid moves down the soil profile (Sugar CRC 2001). There are several management practices that can be used to reduce the rate of acidification. These are: •
Closely matching nitrogen fertiliser inputs to crop demand
•
Using nitrate forms of nitrogen fertiliser
•
Efficient irrigation management to minimise nitrate leaching
•
Early sowing after fallow to minimise nitrate leaching
•
Minimal removal of crop residues off-site
•
Adoption of minimum-till or zero-till practices
•
Growing deep-rooted perennial species to maximise nitrate uptake from the subsoil
•
Reducing legume dominance in a mixed pasture system
•
Managing stock to disperse campsites. 17
Land Manager's Monitoring Guide - Soil pH indicator
Table 6 provides some general property management options relating to soil pH with respect to productivity. The acidity monitoring tools package compiled by the Grains Research and Development Corporation (GRDC & DNRE 2003) provides detailed information on managing acidity that may applicable on your property. Table 6: General property management options relating to soil pH with respect to plant productivity (amended from Moody 2004) Diagnostic range
Plant management options
Extremely acidic soil (pH 9.1)
Only alkaline-tolerant plants will survive, and micronutrients may be required. If soil electrical conductivity (EC) is greater than 1.9 dS/m then the soil may be saline and groundwater will need to be lowered. If EC is less than 0.95 dS/m then the soil is sodic and will require acidifying; legumes and gypsum may be effective at reducing exchangeable sodium.
Adding lime or dolomite to your soil The acidity monitoring tools package (GRDC & DNRE 2003) provides a concise toolkit on when and how to add lime to your soil. In general, the pH of low pH soil can be raised by applying neutralising materials, such as agricultural lime (calcium carbonate), dolomite (magnesium carbonate and calcium carbonate) or other materials with a liming effect on the soil. The efficiency of these materials for neutralising acidity depends on their neutralising value (relative to pure calcium carbonate, which has a value of 100) and their particle size, for example, the finer the material, the more quickly it reacts with the soil. Agricultural lime does not move much from where it is applied within the soil. It relies on direct contact with soil particles and moisture for effect (Moody 2004). You will need to make sure the agricultural lime or neutralising product is well mixed with the soil to the depth required. Soils vary in the amount of added alkalinity required to raise the soil pH by 1 unit (defined as ‘pH buffer capacity’), and a calibrated lime requirement soil test such as the Mehlich buffer method (Moody 2004) should be used to calculate the amount of lime required to raise the soil pH to the desired target value. A soil pH of 5.5 is considered to be an appropriate target pH that is economic (Moody 2004). Most neutralising products will have directions for application, including an indication of how much of the product is required to raise the pH to the nominated target value. The amount of lime or dolomite (or any other neutralising product) to use will vary depending on soil type. Some soils (e.g. highly organic or clay-rich soils) will be more resistant to pH changes than others, so that they may require higher application rates. Some soils (including clays) have an internal buffering or neutralising capacity; however, these may take some time to change pH due to their texture. By contrast, sands do not have a great buffering capacity and also do not store water or nutrients well. In these soils, the change in pH in response to lime application is often the greatest. On average an increase of 0.2–0.3 pH units can be expected from one tonne of lime per hectare (Sugar CRC, 2001). A typical commercial application rate of around 2 tonnes of lime per hectare to sandy loam soils is likely to increase the soil pH by about 0.5 of a pH unit. This is a general comment and caution will need to be used as this estimation is strongly dependent on soil texture, its buffering capacity, soil moisture content, the solubility of the neutralising product, its fineness and other soil constituents such as organic matter. 18
Land Manager's Monitoring Guide - Soil pH indicator
Some tips about lime One of the benefits of liming that is often not recognised is that the soil’s capacity to retain nutrients is increased. The increase in pH as a consequence of liming increases the soil’s cation exchange capacity. There are some things you should remember when using lime. These are: •
Lime quality is measured by neutralising value (NV) and fineness.
•
Lime needs to be finely ground to react quickly with the soil (most liming materials are not very soluble and the rate of reaction with soil is affected by particle size).
•
The higher the soil pH, the lower the solubility of lime; lime that is applied to alkaline soils will react very slowly.
•
Lime will need to be mixed well into the soil as it does not move very far from where it is placed.
•
Do your liming on a wind-free day if possible to reduce the possibility of it blowing away.
•
Gypsum is a good source of calcium and sulfur but will not change the soil pH if it is acid.
Adding organic matter to your soil Organic materials, such as plant residues, can also increase pH because they produce alkaline products as they decompose or if they are burned. The amount of alkalinity produced depends on the content of calcium, magnesium, sodium and potassium relative to sulfate, nitrate and phosphate (Moody 2004). As some plant materials decompose, organic compounds are produced that have the ability to bind to toxic soil aluminium rendering it harmless to the growth of plant roots. This can be a real benefit of adding plant residues to excessively acidic soils.
Other information sources Facts sheets The Queensland Department of Environment and Resource Management has several fact sheets relating to soils and soil pH . The topics include: •
Understanding soil (L1)
•
Understanding soil pH (L47)
•
Soil acidification (L45).
Websites Understanding soil pH leaflet, NSW Department of Industry and Development - Primary Industries Soil acidity, NSW Department of Industry and Development - Primary Industries
Glossary Acid soil An acid soil is one that gives an acid reaction throughout most or all of the soil profile (precisely, below a pH of 7.0; practically, below pH of 6.5). Acid sulfate soils Acid sulfate soils are those containing iron sulphides which, when drained or disturbed, produce sulphuric acid. Acidity Acidity is the chemical activity of hydrogen ions in soil expressed in terms of pH. Alkaline soil An alkaline soil is one that gives an alkaline reaction throughout most or all of the soil profile (precisely, above a pH of 7.0; practically, above a pH of 8.0). Soils with a pH above 9.5 are generally unsuitable for agriculture.
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Land Manager's Monitoring Guide - Soil pH indicator
B horizon (of soil) The B horizon consists of one or more mineral soil layers characterised by one or more of the following: a concentration of silicate clay, iron, aluminium, organic material or several of these; a structure and/or consistency unlike that of the above horizon or below horizon; stronger colours than those of the above and below horizons. Calibrate In terms of electronic meters, calibration refers to standardising the meter to a set standard (buffers). It can also refer to the adjusting of the meter’s set standard. Cation exchange capacity (CEC) Cation exchange capacity refers to the total amount of exchangeable cations that a soil can hold. Cations are positive ions such as calcium, magnesium, potassium, sodium, hydrogen, aluminium, and manganese, these being the most important ones found in soils. Electrical conductivity (EC) Electrical conductivity is a measure of the amount of electricity transferred across an area, and increases with salt concentration. Units are siemens per unit length (dS/m). Exchangeable cations In most soils calcium Ca2+, magnesium Mg2+, sodium Na+, and potassium K+ comprise the bulk of the cations which are present in exchangeable form. Ammonium ions, NH4+, as well as those of micronutrient elements, for example, copper Cu2+, cobalt Co2+ and zinc Zn2+, can also occur as exchangeable cations. Acidic soils may also contain aluminium Al3+, and perhaps iron Fe2+, Fe3+, as exchangeable cations, and hydrogen H+ may also be present in this form. Fertility (soil) Fertility is the capacity of the soil to provide an adequate supply of nutrients in proper balance for the growth of specified plants, when other growth factors such as light, moisture and temperature are favourable. Fertility is generally divided into chemical, physical and biological fertility. Fineness Fineness refers to the percentage of particles less than 0.25 mm in diameter. A good lime should have at least 60% fine particles. Leaching Leaching is the removal in solution of the more soluble minerals and salts by water seeping through a soil, rock, ore body or waste material. Neutralising value (NV) Neutralising value is a measure of the ability of a liming product to neutralise acidity compared to pure calcium carbonate or limestone. pH pH is a measure of the acidity or alkalinity of a soil. A pH of 7.0 denotes neutrality, higher values indicate alkalinity, and lower values indicate acidity. Soil biota Soil biota includes micro-organisms (bacteria, fungi, etc.), micro-fauna (protozoa, nematodes, etc.), mesofauna (acari, springtails, etc.) and macrofauna (insects, earthworms, etc.). It also includes the roots that grow in the soil and interact with other species above and below ground (FAO, 2005). Soil horizon A soil horizon is a layer of soil, with morphological properties different from layers below and/or above it. Soil profile A soil profile is a vertical section of a soil from the soil surface through all its horizons to parent material, other consolidated substrate material or selected depth in unconsolidated material.
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Land Manager's Monitoring Guide - Soil pH indicator
Subsurface soil (subsoil) Subsurface soil is material comprising the B horizons of soils with distinct profiles. In soil with a weak profile development, the subsoil can be defined as the soil below the topsoil. Toxicity Toxicity relates to a soil’s content of elements or minerals that at specific levels can adversely affect plant growth. It is of particular concern in acid soils.
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Land Manager's Monitoring Guide - Soil pH indicator
References AG (Australian Government) 2004, Natural resource management monitoring and evaluation and standards and targets, Australian Government, http://www.nrm.gov.au/publications/frameworks/standards-targets-framework.html (accessed 10 July 2007). Aitken, RL, Moody, PW & McKinley, PG 1990, Lime requirement of acidic Queensland soils: Relationships between soil properties and pH buffer capacity, Australian Journal of Soil Research 28, 695–701. Baker, DE & Eldershaw, VJ 1993, Interpreting soil analyses – for agricultural land use in Queensland, Department of Primary Industries, Brisbane, Project Report Series Q093014, ISSN 0727 6281. DERM 2009, Soil acidification, Land series: L45, Queensland Department of Environment and Resource Management, http://www.derm.qld.gov.au/factsheets/pdf/land/l45.pdf (accessed 1 August 2010). FAO 2005, Soil biodiversity and agricultural context: conservation and management of soil biodiversity and its role in sustainable agriculture, Soil Biodiversity Portal, http://www.fao.org/ag/agl/agll/soilbiod/fao.stm (accessed 10 July 2007). GRDC & DNRE 2003, Riverina Environmental Management System Package – Acidity Monitoring Tools, Grains Research and Development Corporation and Department of Natural Resources and Environment, Victoria, Melbourne. Hey, KM (ed.) 2002, Field testing, sampling, and safety for acid sulfate soils, Queensland Department of Natural Resources and Mines, Indooroopilly, Queensland. Houghton, PD & Charman, PEV 1986, Glossary of terms used in soil conservation, Soil Conservation Service of NSW, ISBN 0 7305 1525 7. Maas, EV & Hoffman, GJ 1977, Crop salt tolerance – current assessment, Journal of the Irrigation and Drainage Division, Proceedings of the American Society of Civil Engineers 103, 115–130. Moody, PW & Aitken, RL 1997, Soil acidification under some tropical agricultural systems: Rates of acidification and contributing factors, Australian Journal of Soil Research 35, 163–173. Moody, PW 2004, Excerpts from soil constraints and management package, Queensland Department of Natural Resources, Mines and Energy and ACIAR, Brisbane. NR&M 2005, Soil sampling instructions, Natural Resource Sciences, Queensland Department of Natural Resources and Mines, http://www.nrw.qld.gov.au (accessed 8 June 2005). Peverill, KI, Sparrow, LA & Reuter, DJ 1999, Soil analysis: an interpretation manual, CSIRO Publishing, Melbourne. Powell, B & Ahern, CR 1999, Nature, origin, and distribution of acid sulfate soils: issues for Queensland, in Acid sulfate soils and their management in coastal Queensland: forum and technical papers, KM Hey, CR Ahern, VJ Eldershaw, JM Anorov & KM Watling (eds), Brisbane 21–23 April, 1999, Department of Natural Resources, Indooroopilly, Queensland, ISBN 0 7242 7476 6, DNRQ990058. Rayment, GE & Higginson, FR 1992, Australian laboratory handbook of soil and water chemical methods, Inkata Press, Sydney, ISBN 0 909605 68 8. Raupach, M & Tucker, BM 1959, The field determination of soil reaction, Journal of the Australian Institute of Agricultural Science 25, 129–133. Stace, HCT, Hubble, GD, Brewer, R, Northcote, KH, Sleeman, JR, Mulcahy, MJ & Hallsworth, EG (1968). A handbook of Australian soils, Rellim Technical Publications, Glenside, South Australia. Sugar CRC 2001, What is pH? CRC Sugar Information Sheets No 43 1995–2003, CRC for Sustainable Sugar Production, James Cook University, Townsville.
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Land Manager's Monitoring Guide - Soil pH indicator
Authors Kylie Hey and Andy Grodecki, Queensland Department of Environment and Resource Management
23
Prepared by: Environment and Resource Sciences Department of Environment and Resource Management © State of Queensland (Department of Environment and Resource Management) 2010
This document has been prepared with all due diligence and care, based on the best available information at the time of publication. The department holds no responsibility for any errors or omissions within this document. Any decisions made by other parties based on this document are solely the responsibility of those parties. Information contained in this document is from a number of sources and, as such, does not necessarily represent government or departmental policy. If you need to access this document in a language other than English, please call the Translating and Interpreting Service (TIS National) on 131 450 and ask them to telephone Library Services on +61 7 3224 8412.
This publication is available in alternative formats (including large print and audiotape) on request. Contact (07) 322 48412 or email August 2010
Contents What is it? ..................................................................................................................................................................................1 Other factors and related indicators........................................................................................................................................1 Why monitor this indicator?.......................................................................................................................................................1 Planning to monitor this indicator ..............................................................................................................................................2 How will your data be used? ..................................................................................................................................................3 What will you monitor?..........................................................................................................................................................3 Where will you monitor?........................................................................................................................................................3 When and how often will you monitor? .................................................................................................................................4 What legal or regulatory requirements will affect your monitoring? .....................................................................................5 How do you measure it?.............................................................................................................................................................5 How do you measure it? - Level 1 monitoring ...........................................................................................................................5 Skills needed ..........................................................................................................................................................................5 Equipment and costs...............................................................................................................................................................6 Time taken..............................................................................................................................................................................6 Setting up ...............................................................................................................................................................................6 Monitoring procedure.............................................................................................................................................................6 Data quality considerations ....................................................................................................................................................7 How do you measure it? – Level 2 monitoring ..........................................................................................................................7 Skills needed ..........................................................................................................................................................................8 Equipment and costs...............................................................................................................................................................8 Time taken..............................................................................................................................................................................9 Setting up ...............................................................................................................................................................................9 Monitoring procedure...........................................................................................................................................................10 Data quality considerations ..................................................................................................................................................11 How to record your results .......................................................................................................................................................11 Metadata ...............................................................................................................................................................................12 What does your data mean?......................................................................................................................................................13 What are some management options? ......................................................................................................................................17 Managing soil pH for production .........................................................................................................................................17 Adding lime or dolomite to your soil ...................................................................................................................................18 Some tips about lime ............................................................................................................................................................19 Adding organic matter to your soil.......................................................................................................................................19 Other information sources ........................................................................................................................................................19 Facts sheets...........................................................................................................................................................................19 Websites ...............................................................................................................................................................................19 Glossary....................................................................................................................................................................................19 References ....................................................................................................................................................................................22 Authors.........................................................................................................................................................................................23
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Land Manager's Monitoring Guide - Soil pH indicator
What is it? Soil pH indicates the concentration of acidity or alkalinity within a soil profile (Rayment & Higginson 1992). The pH scale is the means by which the degree of acidity is measured. Scientifically it is a measure of the hydrogen ion (H+) activity. It is represented in a logarithmic scale ranging from less than zero (strongly acidic) to 14 (strongly alkaline) (Powell & Ahern 1999). Soils can be naturally acidic, neutral or alkaline and this can be measured by testing the pH value of the soil. The pH value can differ within and across a paddock or the property and vertically though the soil’s profile and can change over time at the same location and depth. The pH of the soil in a paddock can change as a result of the use of the paddock, management practices and natural processes. The pH level within the soil influences soil conditions and plant growth. Change in soil pH refers to regular monitoring of any variation over time in soil pH. It is very important to note in coastal areas that the presence of acid sulfate soils (ASS) can make for very high acidity readings. For more information on this, go to the acid sulfate soils website <www.derm.qld.gov.au/land/ass/>. This indicator refers to general soil acidity derived from sources other than from acid sulfate soils.
Other factors and related indicators Monitoring change in soil pH will give you an understanding of the acidity and alkalinity in the soil and how this may change not only down the soil profile but also across your property. This indicator focuses upon change in soil pH only. It will not provide you with information on change of soil structure, infiltration, chemistry or biological activity. Monitoring of all or some of these parameters will provide a more comprehensive picture of overall soil health and potential productivity. Related indicators in the Land Manager’s Monitoring Guide include: ‘Saline land’, ‘Soil erosion’, ‘Ground cover’, ‘Hillslope erosion’, ‘Wind erosion’, ‘pH of water’, ‘Soil chemistry’, ‘Soil structure’ and ‘Soil infiltration’. The pH of soil can vary in most soils with levels of moisture in the soils and prolonged periods of very wet or dry conditions. Working out the pH response of your soils to rainfall, seasonal or longer-term climatic variation (El Niño) is critical to understanding and interpreting what is happening. Monitoring rainfall and irrigation on your property will assist in this. There are a number of aspects of your irrigation practices that you may wish to monitor. Further information is available from the Queensland Government’s Rural Water Use Efficiency Initiative <www.derm.qld.gov.au/rwue>.
Why monitor this indicator? Having the correct pH is important for healthy plant growth and maintaining the natural soil biota (see ‘Glossary’). The pH of all soils change over time; some agricultural practices accelerate this change. (see Soil acidification fact sheet <www.derm.qld.gov.au/factsheets/pdf/land/l45.pdf>) The reason behind monitoring pH over time is to understand the baseline pH level, the rate of change and what this means for management. “Queensland has more than 500 000 ha of agricultural and pastoral land that has acidified or is at risk of acidification. The higher rainfall coastal areas used for intensive agriculture are most susceptible. Soils most at risk are lightertextured sands and loams with low organic matter levels, and the naturally acidic red clay loam soils commonly found in areas such as the South Burnett and Atherton Tableland. Soils least at risk are the neutral to alkaline clay soils, for example, the black clay soils of the Darling Downs and Central Queensland and brigalow soils” (DERM 2009). “Growing plants lower the alkalinity in the soil. When harvested products are removed, the soil becomes more acidic. The impact is greatest where a large quantity of material is removed, as in the production of silage, hay and sugarcane” (DERM 2009). Research has shown that some agricultural practices (such as fertilising with ammonium-based nitrogen or deep ripping) significantly alter soil pH over time by influencing soil conditions, affecting plant growth, reducing nutrient availability and ultimately resulting in land degradation (Baker & Eldershaw 1993). Factors that accelerate acidification include: •
The application of ammonium-based nitrogen fertilisers to naturally acid soils at rates in excess of plant requirements
•
Leaching of nitrate nitrogen, originally applied as ammonium-based fertilisers, out of the root zone
•
Continued removal of alkaline plant and animal produce and waste products from the paddock (DERM 2009).
Conducting a baseline assessment and monitoring for change in soil pH will provide important information to assist your management action decision-making. As with all monitoring activities, you will need to clearly identify why you 1
Land Manager's Monitoring Guide - Soil pH indicator
want to monitor soil pH and how monitoring this indicator may support your property or business planning objectives. Figure 1 gives you an idea of how acid or alkaline some well-known products are and shows the typical ranges for optimal plant growth. Acidity has an indirect effect on plant growth by decreasing the availability of essential plant nutrients such as phosphorus and molybdenum, and by increasing the availability of certain elements to toxic levels, particularly aluminium and manganese. Most Australian soils are within a pH range of pH 3–10 (Stace et al 1968) and most Australian agricultural soils are in the range of pH 4.5–9, with plant growth in most situations being optimum at pH 5–7. Soil testing for pH will identify the most likely limitations due to soil acidity (and where pH is low, subsequent soil chemical analysis for aluminium toxicity may be necessary) (GRDC & DNRE 2003).
Figure 1: The pH scale, Australian soils, agriculture, plant growth and some common products (Powell & Ahern 1999) Understanding of the pH of your soils is essential to determine how suitable your soils are for long-term productive agriculture. Gaining an understanding of the variation in soil pH throughout your soil and across your property can help in making decisions about how you use the land. Monitoring the soil pH both at the surface and at depth (subsurface) will give an indication of any trends in soil pH over time. The pH of soil naturally changes very slowly over geological time and is influenced by factors including parent material (the rock from which it was formed), weathering and the gradual accumulation or loss of plant and animal matter in the soil. Agricultural practices may accelerate or alter these natural processes. The pH of soils may also fluctuate during the year depending on moisture levels. Monitoring soil pH will indicate whether your land use and management practices are changing this key parameter of soil health.
Planning to monitor this indicator The ‘Developing your monitoring plan’ part of the ‘Monitoring overview’ section of the Land Manager’s Monitoring Guide discusses seven key questions that help to define the why, what, who and how of monitoring: 1. What are your monitoring objectives? 2. How will your data be used? 3. What will you monitor? 4. Where will you monitor? 5. When and how often will you monitor? 6. Who will be involved and how? 7. How will your data be managed? These seven questions should be considered for every indicator that you plan to monitor. In addition to the concepts 2
Land Manager's Monitoring Guide - Soil pH indicator
discussed in the ‘Monitoring overview’ you should also considered the following issues that are specific to this indicator when planning your monitoring.
How will your data be used? Soil acidification is a major soil degradation issue in many parts of Australia and is one of the indicators listed in the National framework for natural resource management standards and targets (AG, 2004). Regional natural resource management bodies are now required to report on soil condition including soil acidity and they along with other natural resource management groups may be interested in your data. If you are interested in participating in the monitoring programs of your regional natural resource management body <www.regionalnrm.qld.gov.au> you should contact these organisations for further information. Assessments by national and state governments and regional natural resource management bodies have identified limited information and data on soil condition (including soil acidity) and the impact of management actions. Your monitoring of soil pH to level 1 will give you some information about what is happening to the soils on your property which will be beneficial for your management. However, if you choose to monitor soil pH to level 2 you can contribute to developing a deeper understanding of what is occurring in your region and help in managing this national natural resource priority.
What will you monitor? Existing monitoring in your area Before you start monitoring any indicator it is strongly recommended that you explore who else is monitoring in your area, what they are monitoring and how they are monitoring it. Doing this will not only make sharing your data easier if you chose to do so but will also help you become more familiar with: •
Any area specific issues that may influence your monitoring
•
What strategies and/or methods have proven successful within your area
Other land managers, your catchment group, your regional natural resource management body <www.regionalnrm.qld.gov.au> or the Queensland Department of Environment and Resource Management may be monitoring for soil acidity in your area. They may be able to provide information on other local monitoring and if there are any specific recommendations for your area.
Where will you monitor? Gathering baseline and ongoing monitoring information from different areas on your property will enhance your overall understanding of the way pH changes over time across each area and how they may be affected by the use and management of the land. This will provide you with an understanding of the pH of your soils in highly productive areas as well as in any areas that might be showing signs of declining yields or unexplained failures. You may have a property management plan that includes a layer identifying your soil types that you have previously surveyed. If you are unfamiliar with your soils or the history of the use of the property, you may want to do a survey of the soils on your property or ask your neighbours what they know about the previous uses of the areas that you are most interested in. You may be able to obtain soil maps from your local Queensland Department of Environment and Resource Management business centre . pH values can vary from one sample spot to another spot within one paddock. To ensure that you obtain a representative sample and more reliable result, you will need to collect from a number of spots within your chosen area/s for each pH test you take. Figure 2 provides some guidelines on sampling strategies and the number of samples you will need to collect. As indicated in the figure you will need to blend a number samples from a uniform depth in each spot, then mix the sub-samples in a clean plastic bucket to form one uniform or composite sample from which you can then take a sample to get your pH measurement.
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Land Manager's Monitoring Guide - Soil pH indicator
Figure 2: Soil sampling strategy for general use (amended from NR&M, 2005) If you have a number of locations of interest, you should identify very similar (near uniform) locations that you may be able to monitor as one area. These are paddocks or areas of your property that have similar soils, occupy a similar position in the landscape (e.g. river flats) and have a similar history of use and management practices. Avoid areas close to fences, roads and buildings as these could influence your results. You should also avoid highly variable locations. However, where these areas are of particular interest you should treat each one of them as a separate monitoring location. The number of soil sample locations will depend on the soil type and land practice on your property. In general, it is good to have soil pH measurements for each individual cropping area. It is also good to have an idea of the soil pH of each soil type. You may be able to obtain a soil map from your nearest Queensland Department of Environment and Resource Management business centre .You can use this and any other information you have about your soil type to determine where best to locate your sampling points. Soil sampling means soil is disturbed and removed. Be careful to space your sampling holes to minimise damage. Sampling depths You will get the clearest picture if you sample from the surface and the subsurface of the soil and across the various soils on your property. The surface layer is from the soil surface to a depth of about 10 cm for pastures and crops and to around 15 cm for trees, vegetables and ornamentals. There is no convention for subsurface sampling depth, but the aim is to be near the lower part of the root system at approximately 30–40 cm. This would identify any subsurface acidity problems in a soil layer where most crops would have an active root system (pers comm. Moody 2004). Whole of soil profile sampling Additional samples can be collected from other depths to observe and monitor a trend down a soil profile. Note that in traditional soil surveys, tests are conducted at intervals of 30 cm down the soil profile (including one sample at the surface) or at least one test per soil horizon or layer whichever is lesser. This procedure gives an excellent idea of what is happening down the soil profile in terms of surface and subsurface soil pH and can indicate trends. If you use this technique, include a sample at 10 cm as well.
When and how often will you monitor? As soil pH in general tends to change slowly over time, soil pH can be monitored: •
Annually
•
Before and after major changes in land use or farming system on soils vulnerable to significant pH change 4
Land Manager's Monitoring Guide - Soil pH indicator
The frequency of monitoring will be influenced by property activities and climate. Some land practices will speed up the rate of change in soil pH and so regular monitoring should aim to identify these changes. Do not sample paddocks for at least one month after fertiliser, lime or gypsum has been applied.
What legal or regulatory requirements will affect your monitoring? Generally, there are no legal requirements if you wish to monitor for soil pH on your property.
How do you measure it? To monitor soil pH you will need to select the monitoring technique you want to use, identify the areas that you wish to monitor (see ‘Planning to monitor this indicator’) and then carry out routine measurements of the soil pH over time. After gathering your first (or baseline) information you can then monitor your soil regularly to see how the soil pH is changing over time. Two levels of monitoring soil pH are provided in this guide. Level 2 monitoring is recommended if you plan to share your information with others. Level 2 monitoring includes the two options of self-testing or laboratory testing. At whichever level you choose to monitor, it is a good idea to mark the next monitoring date on your planning calendar. You may be able to coordinate other on-site monitoring activities and save time. There are four common methods for monitoring soil pH: pH strips, field pH kits (Raupach & Tucker method), electronic pH meter and laboratory testing. Because of technical limitations, pH strips are not recommended and are not discussed further here. The cheapest recommended method (Level 1 monitoring) for monitoring soil pH in the middle range of the pH scale is using the field pH kit (Raupach & Tucker 1959 method). This technique uses a barium sulfate (BaSO4) powder and a ‘universal indicator’ solution that changes colour as pH level changes. Where pH extremes are being monitored (very low or very high pH measurements, such as acid sulfate soils), the field pH kit method can be in error by up to 2 pH units. Level 2 monitoring using an electronic pH meter or laboratory analysis and precise location of the sample site provides more accurate results and high quality standards.
How do you measure it? - Level 1 monitoring Key aspects of level 1 monitoring •
Choose locations for soil sampling
•
Collect soil samples
•
Test soil samples using field pH kit
•
Monitor regularly
Skills needed Locate soil sampling sites •
Ability to identify areas of poor productivity and/or signs of acidity such as patchy growth, nutrient deficiency, aluminium or manganese toxicity
•
Understanding of the general distribution of different soil types on your property
Collect soil core •
Ability to use appropriate collection tools
•
Knowledge of the correct sampling strategy
•
Ability to accurately collect and label samples
Measure soil pH •
Ability to use soil pH field kit
•
Ability to accurately determine soil field pH measurement using field kit colour cards (field kits are not recommended for people with colour blindness)
•
Ability to record information accurately 5
Land Manager's Monitoring Guide - Soil pH indicator •
Illustrate data and analyse trends
•
Ability to prepare basic graphs
•
Understanding of the relationship between results and land practices
Equipment and costs Monitoring soil pH is relatively inexpensive using a field pH kit. The main costs associated with monitoring soil pH for Level 1 are shown below. Shovel or soil auger Soil augers can be purchased from scientific supply outlets or specialist auger manufacturers (engineering firms). Augers cost about $300. Soil pH test kit Soil pH test kits are available from pool supply stores, hardware stores and chemical supply stores and cost about $30. Note: Soil pH test kits are only suitable for mid-range pH soils; they should not be used to measure the pH of highly acidic or alkaline soils. Minor items •
Tape measure – 50 m fibreglass with 1 cm increments
•
Wooden skewers or stirrers to break down lumps in the soil
•
Bucket and brush – to rinse and clean the pH tray ready for next use
•
Recording sheets
•
Stationery – writing pad, pens, pencils, eraser, ruler, printing ink, etc.
Time taken The time taken to monitor soil pH to Level 1 will depend on: •
How long it takes to travel to and between your sampling sites
•
The number of samples collected and tested
•
The hardness of the soil
•
The sampling procedure you use – it will take more time if you are blending or mixing a number of samples.
Setting up 1. Store barium sulfate powder and indicator solution in a cool dry place. For quality assurance and reliable results the powder will need to be replaced if it has become moist. 2. You need to select the location/s, depth and frequency of monitoring that are most likely to provide you with reliable information from the various soil types on your property (see ‘Planning to monitor this indicator’). 3. Record your sample locations on your property management plan or in some other way that enables you to reliably sample from the same location each time (remember to space holes to minimise soil damage). 4. Make notes on why you have chosen each location, such as land use and/or signs of acidity or potential toxicity due to soil acidity. 5. Where practical you may want to mark the sampling site with painted star pickets or some other marker.
Monitoring procedure Initial recording 1. Collect the soil sample and where necessary break up any large clumps to ensure the sample can be mixed well before sub-sampling for testing (see ‘Planning to monitoring this indicator’). It is recommended that you use a soil auger to collect the sample. 2. For each sample in turn, place a small amount of the sampled soil (¼–½ teaspoon) on the tray provided with the pH kit.
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Land Manager's Monitoring Guide - Soil pH indicator
3. Add a few drops of indicator solution and mix thoroughly with a clean toothpick, skewer or stirrer. 4. Puff a small amount (enough to put a thin layer over the soil) of barium sulfate over the mixture and wait for the soil to absorb the moisture and for the colour to become stable. This should only take about 10 seconds. If it is left too long, the mixture will ‘go off’ (especially in high temperatures) and give an inaccurate reading. 5. Compare the colour of the powder to the chart provided in the kit. 6. Record the following on your datasheet: •
Date
•
Site name or number
•
Site location
•
Sample method (shovel, auger, mechanical equipment)
•
Sample number (Unique identifier for sample)
•
Sampling depths for each sample taken from each soil core
•
Measurement in pH units
•
Test method (pH test kit)
•
Recording person
•
Comments, such as any new information on signs of land use and/or signs of acidity or potential toxicity due to soil acidity and/or water status of soil (wet, dry, waterlogged, field capacity), weather conditions or days since rain or irrigation and any problems.
7. Repeat steps 2–6 above on the remaining samples (at different locations or depths). Note: You may also want to carry out other monitoring activities from your soil samples, such as soil chemistry, soil structure, plant available water content, soil life, soil infiltration and/or soil salinity. These other measures will involve additional steps or sampling throughout the soil profile and it is a good idea to logically order your monitoring measurement; some will be recorded prior to pH sampling. Ongoing monitoring Repeat steps of the initial recording at the frequency you have decided to monitor.
Data quality considerations Correctly using a soil auger rather than a shovel to collect soil samples will in general give a more accurate representation of the soil profile because of reduced risk of cross-contamination between upper and lower soil profile. Soil augers can be used with extension handles to retrieve soil from depths in excess of 5 metres. Augers may not be ideal where there is a hard clay pan or lots of rocks in the soil profile. Mechanical drilling equipment may be more suitable in this case. Augers need to be cleaned after each hole is dug to minimise cross-contamination. Store and replace barium sulfate powder and indicator solution as soon as it becomes moist or 3 months old (whichever comes first) (see manufacturer’s instruction for specific details). In extreme weather conditions such as high humidity the powder can become moist and ineffective or can absorb carbon dioxide and become acid.
How do you measure it? – Level 2 monitoring Key aspects of level 2 monitoring •
Choose locations for soil sampling and record locations with GPS
•
Collect soil samples
•
Test soil samples using (a) electronic pH meter or (b) laboratory analysis
•
Monitor regularly
Level 2a & b monitoring allows you to monitor across the whole pH range if necessary with accurate results and is suitable if you want to share your data with others. Level 2a involves field-testing using your own or borrowed equipment that provides you with immediate results and requires regular calibration of the equipment. Level 2b involves getting your soil samples analysed at a chemical laboratory and can be combined with analysis for a number of soil chemical indicators.
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Land Manager's Monitoring Guide - Soil pH indicator
Skills needed Locate soil sampling sites •
Ability to identify areas of poor productivity and/or signs of acidity such as patchy growth, nutrient deficiency, aluminium or manganese toxicity
•
Understanding of the general distribution of different soil types on your property
Use of GPS and GIS •
Ability to use a Geographic Positioning System (GPS) unit for recording locations
•
Ability to use a personal computer and Geographic Information System (GIS) software
•
Ability to manage GIS data
Collect soil core •
Ability to use appropriate collection tools
•
Knowledge of the correct sampling strategy
•
Ability to accurately collect and label samples
Measure soil pH •
Ability to (a) use an electronic pH meter or (b) prepare samples for laboratory analysis
•
Ability to record information accurately
•
Illustrate data and analyse trends
•
Ability to prepare basic graphs
•
Understanding of the relationship between results and land practices
Equipment and costs Monitoring soil pH using an electronic pH meter (Level 2a monitoring) is slightly more expensive than using the field pH kit (Level 1 monitoring) but the results you obtain will be more accurate. The additional expense is the purchase an electronic pH meter and the occasional replacement of electrodes. Using laboratory analysis (Level 2b monitoring) means your results will be more accurate and you do not need your own testing equipment but you will incur a cost for each sample you get analysed. The main costs associated with monitoring soil pH to Level 2 are shown below. GPS receiver GPS receivers are now commonplace, are frequently updated and are available from camping, boating, scientific and electronics stores and cost about $250+. GIS software GIS software is constantly being upgraded and there are now many vendors. Use of online search engines set to Australia will readily identify some of these providers. Basic functions include display of digital images and overlaying of data layers. Costs vary depending on software. Shovel or soil auger Soil augers can be purchased from scientific supply outlets or specialist auger manufacturers (engineering firms). Augers cost about $300. Electronic pH meter with electrode (spear point) (for Level 2a monitoring) Electronic pH meters are available from specialist scientific equipment stores and can cost between $150 and $600+. When purchasing you need to consider the level of accuracy required and the durability of the meter. You will also need to decide if you want to use multipurpose electronic meters that can test a number of parameters. Buffer solutions and electrode gel (for Level 2a monitoring) You will need pH 4.0 buffer solution, pH 6.88 buffer solution and electrode gel (potassium chloride) to maintain a reference solution in the electrode. These are available from specialist scientific equipment stores and most department stores (solutions are commonly used in steam irons). Buffer solutions do not tolerate extreme temperatures, so should be stored in a cool place. Buffer solutions and electrode gel should be checked and/or replaced every 12 months and cost 8
Land Manager's Monitoring Guide - Soil pH indicator
about $30 each for 500 ml quantities. Basic computer spreadsheet software Basic computer spreadsheet software is available from computer and office supply stores. The cost depends on the software, although it is often included with a computer purchase. Plastic test tubes and rack or other small containers (for Level 2a monitoring) Plastic test tubes and racks are available from specialist scientific equipment stores and cost about $25 for 24 tubes and $20 for a rack. It is important that test tubes have incremental volume markings and that the rack snugly fits the test tubes and keeps the tubes upright. Some racks are collapsible making storage easy. Minor items •
Tape measure – 50 m fibreglass with 1 cm increments
•
Wooden skewers or stirrers to break down lumps in the soil
•
Bucket and brush – to rinse and clean the pH tray ready for next use
•
Recording sheets
•
Stationery – writing pad, pens, pencils, eraser, ruler, printing ink etc.
Level 2a monitoring: •
Bucket and brush – to rinse and clean the pH tray ready for next use
•
Deionised water (pH 5.5) and squirt bottle for testing and rinsing test tubes and electrode
•
Tissues
Level 2b monitoring: •
Plastic ziplock bags – to place soil samples in.
•
Waterproof labels – marked with identification code, date, and sample depth
•
Copy of data sheet – to send with samples to the laboratory
Time taken The time taken to monitor soil pH to level 2 will depend on: •
How long it takes to travel to and between your sampling sites
•
The number of samples collected and tested
•
The hardness of the soil
•
The sampling procedure you use – it will take more time if you are blending or mixing a number of samples
•
Calibration of equipment for level 2a monitoring (approximately 5 minutes)
•
Sending samples off to a laboratory and awaiting results for level 2b monitoring (may be several weeks).
Setting up 1. You need to select the location/s, depth and frequency of monitoring that are most likely to provide you with reliable information from the various soil types on your property (see ‘Planning to monitor this indicator’). 2. Cleaning your sampling equipment in between sampling events will prevent cross-contamination of soil samples and encourage reliability of results. 3. Record your sample locations on your property management plan or in some other way that enables you to reliably sample from the same location each time (remember to space holes to minimise soil damage). 4. Make notes on why you have chosen each location, such as land use and/or signs of acidity or potential toxicity due to soil acidity. 5. Where practical you may want to mark the sampling site with painted star pickets or some other marker. 6. Take a GPS reading to obtain the location coordinates. This will enable you to reliably sample from the same location each time and this will be required if you wish to share your information with others.
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Land Manager's Monitoring Guide - Soil pH indicator
Monitoring procedure Initial recording 1. Collect the soil sample and where necessary break up any large clumps to ensure the sample can be mixed well before sub-sampling for testing (see ‘Planning to monitor this indicator’). It is recommended that you use a soil auger to collect the sample. 2. Cleaning your sampling equipment in between sampling events will prevent cross-contamination of soil samples and encourage reliability of results. 3. Measure the pH of your sample by using: An electronic pH meter (Level 2a monitoring): •
Ensure the meter has been calibrated (see ‘Data quality considerations’ below).
•
Ensure the buffer solutions are fresh and if replace necessary.
•
Record the pH of the buffer solutions and any other quality assurance actions.
•
Prepare test tubes and rack or similar small clean containers to hold each sample.
•
Place approximately half a teaspoon of the soil sample into the corresponding test tube.
•
Place enough deionised water in the test tube to make a paste similar to a ‘tiling grout mix’. Stir with a skewer or similar to ensure all soil ‘lumps’ are broken down. Never stir the paste with the electrode as it will damage the delicate glass membrane of the electrode.
•
Place the pH electrode into the test tube ensuring that the point is totally submerged in the soil/water paste.
•
Wait for the reading to stabilise and record the pH measurement. Some meters have a data logging facility that enables measurements to be downloaded directly from the unit to a computer, thus reducing the possibility of error during data transfer. Record the result.
•
Clean your pH electrode after each measurement.
•
Clean out your test tubes prior to the next round of sampling.
•
Repeat this procedure for each sample.
Laboratory analysis (Level 2b monitoring): •
Place samples in 500 g lots in sealable airtight bags (ziplock sandwich bags are good).
•
Mark your samples clearly so that location and depth of the sample is readily identified. Sending your soil samples to a NATA accredited laboratory is recommended for quality assurance. You should phone the laboratory first so they know your samples are coming.
•
Once your results are returned from the laboratory you will need to transfer the results to your data sheet. Some laboratories offer an interpretation service for an additional fee that will help you interpret the results.
4. Record the following on your datasheet: •
Date
•
Site name or number
•
Site location
•
Sample method (shovel, auger, mechanical equipment)
•
Sample number (Unique identifier for sample)
•
Sampling depths for each sample taken from each soil core
•
Measurement in pH units
•
Test method (electronic pH meter or laboratory analysis)
•
Recording person
•
Buffer solutions used and any other quality assurance actions (Level 2a monitoring only)
•
Comments, such as any new information on signs of land use and/or signs of acidity or potential toxicity due to soil acidity and/or water status of soil (wet, dry, waterlogged, field capacity), weather conditions or days since rain or irrigation and any problems. 10
Land Manager's Monitoring Guide - Soil pH indicator
5. Repeat steps 2–4 above on the remaining samples (at different locations or depths). Note: You may also want to carry out other monitoring activities from your soil samples such as soil chemistry, soil structure, plant available water content, soil life, soil infiltration and/or soil salinity. These other measures will involve additional steps or sampling throughout the soil profile and it is a good idea to logically order your monitoring measurement as some should be recorded prior to pH sampling. On-going monitoring Repeat steps of the initial recording at the frequency you have decided to monitor.
Data quality considerations Correctly using a soil auger rather than a shovel to collect soil samples will in general give a more accurate representation of the soil profile because of reduced risk of cross-contamination between upper and lower soil profile. Soil augers can be used with extension handles to retrieve soil from depths in excess of 5 metres. Augers may not be ideal where there is a hard clay pan or lots of rocks in the soil profile. Mechanical drilling equipment may be more suitable in this case. Augers need to be cleaned after each hole is dug to minimise cross-contamination. Field test kits are only suitable for middle range pH soils, while electronic pH meters or laboratory analysis is suitable for the whole pH range of soils. If you are sending samples to a laboratory it is recommended that you use a NATA accredited laboratory. These laboratories regularly undergo quality assurance procedures through an independent body. Care of pH electrodes For level 2a monitoring it is recommended that you use a combination (IJ) electrode (the reference and glass electrode are combined together). You should store the electrode tip in deionised water, potassium chloride solution or a pH buffer solution and ensure the protective cap is on the probe when it is not being used. Never let the electrode dry out. Check the electrode regularly for scratches or damage and if it is damaged replace it. Calibration of pH meter It is recommended that you calibrate the pH meter and electrode, and replace electrolyte in the electrode each day prior to use (see manufacturer’s instruction for specific details). If you are taking measurements all day recalibrate your pH meter after a few hours as the change in outside temperature may affect the calibration. •
Remove the protective cap and probe sleeve, refresh the saturated potassium chloride solution or gel in the electrode junction and replace the sleeve.
•
Rinse the probe with deionised water and dry with a clean tissue before putting the protective cap back on.
•
Calibrate the meter according to the manufacturer’s instructions with standard pH buffer solution (e.g. pH 6.88 and pH 4).
•
Ensure the buffer solutions are fresh and clean by placing a small amount in a small tube for use. Never insert the probe into the bulk bottle and don’t return used buffer solution to the bulk bottle. You should replace the buffers every 12 months.
How to record your results When you are working in the field the simplest way to record your data is to have a field recording sheet (Table 1) with you. A field recording sheet will help ensure that your data is recorded in a way that is easy to read later and will also act as a checklist to ensure that you don’t miss recording any important information. The data recorded on the field recording sheet should include: •
Date
•
Site name or number
•
Site location
•
Sample method (shovel, auger, mechanical equipment)
•
Sample number (Unique identifier for sample)
•
Sampling depths for each sample taken from each soil core 11
Land Manager's Monitoring Guide - Soil pH indicator •
Measurement in pH units
•
Test method (pH test kit, electronic pH meter, laboratory analysis)
•
Recording person
•
Buffer solutions used and any other quality assurance actions (Level 2a monitoring only)
•
Comments, such as any new information on signs of land use and/or signs of acidity or potential toxicity due to soil acidity and/or water status of soil (wet, dry, waterlogged, field capacity), weather conditions or days since rain or irrigation and any problems.
Table 1: Field recording sheet for level 1 monitoring with minimum example data
Date
Site number
Site location
Sample method
Sample number
Sample depth (cm)
pH value
Test method
Recording person
Comments
23/05/2005
1
Bottom of back paddock
Auger
1
8.6) can lead to: •
Surface sealing and crusting problems due to excessive sodium
•
Reduced availability of iron, manganese, zinc, phosphorus and copper
•
Reduced microbial activity and reduction in fungal population.
Some more specific problems associated with soil acidity and alkalinity A soil of pH 5.5–7.5 results in a good balance in availability of most nutrients. Very high and very low soil pH can influence the soil’s ability to retain nutrients, which can lead to leaching of some nutrients. Assessing the impact of significant change in pH levels requires a soil chemical analysis to obtain a detailed analysis of the availability of each nutrient. Some crops, for example sugar cane, are quite tolerant of low soil pH and high amounts of aluminium and manganese. However, over time, nutrient deficiencies are likely to become an issue if unmanaged. Soil pH can be influenced by the use of some fertilisers or simply by growing and harvesting crops. Often the soil can be quite acidic at the surface with an increase to alkaline at depth. This trend can be the result of repeated fertiliser application or organic matter build up in the case of mild acidity at the surface. Alkaline soil pH can be found above a hardpan or impermeable layer at depth in the soil profile if the soil is sodic. Chemical testing by a laboratory can confirm this. This is one example of a vertical trend that may require a change in management or land use at the sample location. Table 5 lists some additional causes and implications of various pH ranges in surface and subsurface soils.
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Land Manager's Monitoring Guide - Soil pH indicator
Table 5: Causes and implications of soil pH value (amended from Moody 2004) Diagnostic range
Causes and implications
Extremely acidic soil (pH 9.1)
Soils are extremely alkaline and dominated by sodium, calcium and magnesium carbonates. In these alkaline soils, micronutrients (e.g. copper, zinc, iron, manganese), potassium and/or phosphorus deficiencies can occur. Boron toxicity can exist. The soil will almost certainly have a very poor nutritional and structural status.
What are some management options? Managing soil pH for production Regular soil pH monitoring is an important tool in managing soil pH as it gives an indication of what nutrients may be limited or required for healthy crops (Sugar CRC 2001). It is important from a cropping perspective to maintain a soil pH of at least pH 5.5. If the pH of the surface soil is less than 5.5, long-term acidification of subsoils will result as the acid moves down the soil profile (Sugar CRC 2001). There are several management practices that can be used to reduce the rate of acidification. These are: •
Closely matching nitrogen fertiliser inputs to crop demand
•
Using nitrate forms of nitrogen fertiliser
•
Efficient irrigation management to minimise nitrate leaching
•
Early sowing after fallow to minimise nitrate leaching
•
Minimal removal of crop residues off-site
•
Adoption of minimum-till or zero-till practices
•
Growing deep-rooted perennial species to maximise nitrate uptake from the subsoil
•
Reducing legume dominance in a mixed pasture system
•
Managing stock to disperse campsites. 17
Land Manager's Monitoring Guide - Soil pH indicator
Table 6 provides some general property management options relating to soil pH with respect to productivity. The acidity monitoring tools package compiled by the Grains Research and Development Corporation (GRDC & DNRE 2003) provides detailed information on managing acidity that may applicable on your property. Table 6: General property management options relating to soil pH with respect to plant productivity (amended from Moody 2004) Diagnostic range
Plant management options
Extremely acidic soil (pH 9.1)
Only alkaline-tolerant plants will survive, and micronutrients may be required. If soil electrical conductivity (EC) is greater than 1.9 dS/m then the soil may be saline and groundwater will need to be lowered. If EC is less than 0.95 dS/m then the soil is sodic and will require acidifying; legumes and gypsum may be effective at reducing exchangeable sodium.
Adding lime or dolomite to your soil The acidity monitoring tools package (GRDC & DNRE 2003) provides a concise toolkit on when and how to add lime to your soil. In general, the pH of low pH soil can be raised by applying neutralising materials, such as agricultural lime (calcium carbonate), dolomite (magnesium carbonate and calcium carbonate) or other materials with a liming effect on the soil. The efficiency of these materials for neutralising acidity depends on their neutralising value (relative to pure calcium carbonate, which has a value of 100) and their particle size, for example, the finer the material, the more quickly it reacts with the soil. Agricultural lime does not move much from where it is applied within the soil. It relies on direct contact with soil particles and moisture for effect (Moody 2004). You will need to make sure the agricultural lime or neutralising product is well mixed with the soil to the depth required. Soils vary in the amount of added alkalinity required to raise the soil pH by 1 unit (defined as ‘pH buffer capacity’), and a calibrated lime requirement soil test such as the Mehlich buffer method (Moody 2004) should be used to calculate the amount of lime required to raise the soil pH to the desired target value. A soil pH of 5.5 is considered to be an appropriate target pH that is economic (Moody 2004). Most neutralising products will have directions for application, including an indication of how much of the product is required to raise the pH to the nominated target value. The amount of lime or dolomite (or any other neutralising product) to use will vary depending on soil type. Some soils (e.g. highly organic or clay-rich soils) will be more resistant to pH changes than others, so that they may require higher application rates. Some soils (including clays) have an internal buffering or neutralising capacity; however, these may take some time to change pH due to their texture. By contrast, sands do not have a great buffering capacity and also do not store water or nutrients well. In these soils, the change in pH in response to lime application is often the greatest. On average an increase of 0.2–0.3 pH units can be expected from one tonne of lime per hectare (Sugar CRC, 2001). A typical commercial application rate of around 2 tonnes of lime per hectare to sandy loam soils is likely to increase the soil pH by about 0.5 of a pH unit. This is a general comment and caution will need to be used as this estimation is strongly dependent on soil texture, its buffering capacity, soil moisture content, the solubility of the neutralising product, its fineness and other soil constituents such as organic matter. 18
Land Manager's Monitoring Guide - Soil pH indicator
Some tips about lime One of the benefits of liming that is often not recognised is that the soil’s capacity to retain nutrients is increased. The increase in pH as a consequence of liming increases the soil’s cation exchange capacity. There are some things you should remember when using lime. These are: •
Lime quality is measured by neutralising value (NV) and fineness.
•
Lime needs to be finely ground to react quickly with the soil (most liming materials are not very soluble and the rate of reaction with soil is affected by particle size).
•
The higher the soil pH, the lower the solubility of lime; lime that is applied to alkaline soils will react very slowly.
•
Lime will need to be mixed well into the soil as it does not move very far from where it is placed.
•
Do your liming on a wind-free day if possible to reduce the possibility of it blowing away.
•
Gypsum is a good source of calcium and sulfur but will not change the soil pH if it is acid.
Adding organic matter to your soil Organic materials, such as plant residues, can also increase pH because they produce alkaline products as they decompose or if they are burned. The amount of alkalinity produced depends on the content of calcium, magnesium, sodium and potassium relative to sulfate, nitrate and phosphate (Moody 2004). As some plant materials decompose, organic compounds are produced that have the ability to bind to toxic soil aluminium rendering it harmless to the growth of plant roots. This can be a real benefit of adding plant residues to excessively acidic soils.
Other information sources Facts sheets The Queensland Department of Environment and Resource Management has several fact sheets relating to soils and soil pH . The topics include: •
Understanding soil (L1)
•
Understanding soil pH (L47)
•
Soil acidification (L45).
Websites Understanding soil pH leaflet, NSW Department of Industry and Development - Primary Industries Soil acidity, NSW Department of Industry and Development - Primary Industries
Glossary Acid soil An acid soil is one that gives an acid reaction throughout most or all of the soil profile (precisely, below a pH of 7.0; practically, below pH of 6.5). Acid sulfate soils Acid sulfate soils are those containing iron sulphides which, when drained or disturbed, produce sulphuric acid. Acidity Acidity is the chemical activity of hydrogen ions in soil expressed in terms of pH. Alkaline soil An alkaline soil is one that gives an alkaline reaction throughout most or all of the soil profile (precisely, above a pH of 7.0; practically, above a pH of 8.0). Soils with a pH above 9.5 are generally unsuitable for agriculture.
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Land Manager's Monitoring Guide - Soil pH indicator
B horizon (of soil) The B horizon consists of one or more mineral soil layers characterised by one or more of the following: a concentration of silicate clay, iron, aluminium, organic material or several of these; a structure and/or consistency unlike that of the above horizon or below horizon; stronger colours than those of the above and below horizons. Calibrate In terms of electronic meters, calibration refers to standardising the meter to a set standard (buffers). It can also refer to the adjusting of the meter’s set standard. Cation exchange capacity (CEC) Cation exchange capacity refers to the total amount of exchangeable cations that a soil can hold. Cations are positive ions such as calcium, magnesium, potassium, sodium, hydrogen, aluminium, and manganese, these being the most important ones found in soils. Electrical conductivity (EC) Electrical conductivity is a measure of the amount of electricity transferred across an area, and increases with salt concentration. Units are siemens per unit length (dS/m). Exchangeable cations In most soils calcium Ca2+, magnesium Mg2+, sodium Na+, and potassium K+ comprise the bulk of the cations which are present in exchangeable form. Ammonium ions, NH4+, as well as those of micronutrient elements, for example, copper Cu2+, cobalt Co2+ and zinc Zn2+, can also occur as exchangeable cations. Acidic soils may also contain aluminium Al3+, and perhaps iron Fe2+, Fe3+, as exchangeable cations, and hydrogen H+ may also be present in this form. Fertility (soil) Fertility is the capacity of the soil to provide an adequate supply of nutrients in proper balance for the growth of specified plants, when other growth factors such as light, moisture and temperature are favourable. Fertility is generally divided into chemical, physical and biological fertility. Fineness Fineness refers to the percentage of particles less than 0.25 mm in diameter. A good lime should have at least 60% fine particles. Leaching Leaching is the removal in solution of the more soluble minerals and salts by water seeping through a soil, rock, ore body or waste material. Neutralising value (NV) Neutralising value is a measure of the ability of a liming product to neutralise acidity compared to pure calcium carbonate or limestone. pH pH is a measure of the acidity or alkalinity of a soil. A pH of 7.0 denotes neutrality, higher values indicate alkalinity, and lower values indicate acidity. Soil biota Soil biota includes micro-organisms (bacteria, fungi, etc.), micro-fauna (protozoa, nematodes, etc.), mesofauna (acari, springtails, etc.) and macrofauna (insects, earthworms, etc.). It also includes the roots that grow in the soil and interact with other species above and below ground (FAO, 2005). Soil horizon A soil horizon is a layer of soil, with morphological properties different from layers below and/or above it. Soil profile A soil profile is a vertical section of a soil from the soil surface through all its horizons to parent material, other consolidated substrate material or selected depth in unconsolidated material.
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Land Manager's Monitoring Guide - Soil pH indicator
Subsurface soil (subsoil) Subsurface soil is material comprising the B horizons of soils with distinct profiles. In soil with a weak profile development, the subsoil can be defined as the soil below the topsoil. Toxicity Toxicity relates to a soil’s content of elements or minerals that at specific levels can adversely affect plant growth. It is of particular concern in acid soils.
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Land Manager's Monitoring Guide - Soil pH indicator
References AG (Australian Government) 2004, Natural resource management monitoring and evaluation and standards and targets, Australian Government, http://www.nrm.gov.au/publications/frameworks/standards-targets-framework.html (accessed 10 July 2007). Aitken, RL, Moody, PW & McKinley, PG 1990, Lime requirement of acidic Queensland soils: Relationships between soil properties and pH buffer capacity, Australian Journal of Soil Research 28, 695–701. Baker, DE & Eldershaw, VJ 1993, Interpreting soil analyses – for agricultural land use in Queensland, Department of Primary Industries, Brisbane, Project Report Series Q093014, ISSN 0727 6281. DERM 2009, Soil acidification, Land series: L45, Queensland Department of Environment and Resource Management, http://www.derm.qld.gov.au/factsheets/pdf/land/l45.pdf (accessed 1 August 2010). FAO 2005, Soil biodiversity and agricultural context: conservation and management of soil biodiversity and its role in sustainable agriculture, Soil Biodiversity Portal, http://www.fao.org/ag/agl/agll/soilbiod/fao.stm (accessed 10 July 2007). GRDC & DNRE 2003, Riverina Environmental Management System Package – Acidity Monitoring Tools, Grains Research and Development Corporation and Department of Natural Resources and Environment, Victoria, Melbourne. Hey, KM (ed.) 2002, Field testing, sampling, and safety for acid sulfate soils, Queensland Department of Natural Resources and Mines, Indooroopilly, Queensland. Houghton, PD & Charman, PEV 1986, Glossary of terms used in soil conservation, Soil Conservation Service of NSW, ISBN 0 7305 1525 7. Maas, EV & Hoffman, GJ 1977, Crop salt tolerance – current assessment, Journal of the Irrigation and Drainage Division, Proceedings of the American Society of Civil Engineers 103, 115–130. Moody, PW & Aitken, RL 1997, Soil acidification under some tropical agricultural systems: Rates of acidification and contributing factors, Australian Journal of Soil Research 35, 163–173. Moody, PW 2004, Excerpts from soil constraints and management package, Queensland Department of Natural Resources, Mines and Energy and ACIAR, Brisbane. NR&M 2005, Soil sampling instructions, Natural Resource Sciences, Queensland Department of Natural Resources and Mines, http://www.nrw.qld.gov.au (accessed 8 June 2005). Peverill, KI, Sparrow, LA & Reuter, DJ 1999, Soil analysis: an interpretation manual, CSIRO Publishing, Melbourne. Powell, B & Ahern, CR 1999, Nature, origin, and distribution of acid sulfate soils: issues for Queensland, in Acid sulfate soils and their management in coastal Queensland: forum and technical papers, KM Hey, CR Ahern, VJ Eldershaw, JM Anorov & KM Watling (eds), Brisbane 21–23 April, 1999, Department of Natural Resources, Indooroopilly, Queensland, ISBN 0 7242 7476 6, DNRQ990058. Rayment, GE & Higginson, FR 1992, Australian laboratory handbook of soil and water chemical methods, Inkata Press, Sydney, ISBN 0 909605 68 8. Raupach, M & Tucker, BM 1959, The field determination of soil reaction, Journal of the Australian Institute of Agricultural Science 25, 129–133. Stace, HCT, Hubble, GD, Brewer, R, Northcote, KH, Sleeman, JR, Mulcahy, MJ & Hallsworth, EG (1968). A handbook of Australian soils, Rellim Technical Publications, Glenside, South Australia. Sugar CRC 2001, What is pH? CRC Sugar Information Sheets No 43 1995–2003, CRC for Sustainable Sugar Production, James Cook University, Townsville.
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Land Manager's Monitoring Guide - Soil pH indicator
Authors Kylie Hey and Andy Grodecki, Queensland Department of Environment and Resource Management
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