- Managing soil fertility by undertaking soil chemical tests is one of the tools in assessing the health of your soils
- Soil sampling is possibly the most neglected step in soil testing, and the greatest source
of error in the whole process � It will save you money if you sample correctly
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- Ensure that you are using accredited soil testing labs
- Check which type of soil tests your advisor is using � commercial or alternative
approaches
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Understanding the question
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| Why is it important to me as a farmer?
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- Soil testing can be a relatively simple, cost effective management strategy to help
optimise profitability and correctly identify important soil problems; test results are
particularly useful when making decisions on inputs
- In an agricultural context, soil testing is commonly used:
- To help identify the main reasons for poor plant performance (diagnostic tests)
- To confirm nutrient status and warn of deficiencies
- To predict likely response to fertilizer rate and type for individual situations
- To monitor changes in soil health in response to changes in land use and management
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- However, there are many risks in soil testing, and unless care is taken with sampling, transport and laboratory procedures test, results can be misleading
- Getting soil test results that are meaningful and accurate is critical to be able to
effectively use them as management tool
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| How to ensure your soil tests are as accurate as possible
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Introduction
- Although there is some degree of uncertainty associated with a soil test, in-field
variability and inappropriate soil sampling techniques are likely to introduce larger
variations and errors than the laboratory analyses
- Regular soil testing and analysis will aid in building up a history which shows trends
through time, but care must be taken in ensuring that analyses done from year to year
are undertaken using the same methods for analysis, to reduce errors associated with
different analytical techniques or sample handling
- To appreciate just how crucial it is to ensure that a representative sample is submitted
for analysis, consider the fact that a hectare of soil to a depth of 10 cm weighs roughly
1500 tonnes, while the sample submitted for testing typically amounts to about 0.5 kg (or
about 0.00003% of the surface soil on 1 ha � just 1 part in 3 million). If such a tiny
fraction is to be representative of the target area, then your sampling needs to be spot
on. Otherwise, the test results will be of little or no value
- Since soil characteristics are variable both in time (temporal) and space (location in the
paddock or on the farm), it is essential to get an accurate measurement of soil attributes
by adjusting the sampling strategy and number of samples to adequately represent the
variation in soil and land use
- Use all the information available to develop your soil sampling plan with the overall aim
to know what you are sampling for as well as where, when and how to undertake it
Figure 1 � Using precision agriculture mapping output to target soil sampling. - Source: Precision Cropping Technologies [View larger image]
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- Sampling method:
- Remove excess plant material before sampling
- Use a sampling tube to extract the soil core. Soil sampling tubes or soil cores are
generally available from your local Department of Environement and Primary Industries office
- Collect a sub-sample of about 0.5 to 1 kg of soil
- Be careful not to get any topsoil in the sample
- Mix sample well:
- Bulking or mixing soil from several locations to create an average (or
composite) sample is a common procedure. However, averages can hide
problems if there is a big range in the soil you have sampled
Figure 2 - A typical soil sampler with bucket for sampling at 0-10cm depth. - Source: Source: Arborline Pty. Ltd
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- Sampling location
- There are two common sampling patterns; zig zag which is used for larger
paddocks and the line transect (see below), for smaller paddocks
Figure 3 � Zig zag (left) and transect (right). - Source: DEPI Victoria
- Avoid sampling along fence lines and in headlands, stock camps, gateways, wet
areas, troughs or fertiliser dumps, on tracks, under trees, near buildings and in
areas where timber has been stacked and burnt. Such areas will have different
fertility to the majority of the paddock and will influence the result
- Avoid areas of poor growth or excessively good growth, for example, urine and
dung patches
- Nutrient concentrations can be highly variable within paddocks. See example
below, where one core taken at each point, results in mg P/kg soil. This illustrates
the need to sample numerous points in strategic locations
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Figure 4 - Olsen P testing on permanent pasture at Macalister Research Farm. - Source: A Brown, DEPI Victoria [View larger image]
- Sampling depth and number
- Take samples of the topsoil (0 to 10 cm)
- Sample the subsurface soil (10 to 60 cm) if testing for nitrogen
Figure 5 - Effect of sampling depth on Olsen P (mg/kg). - Source: A Brown, DEPI Victoria [View larger image]
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Table 1 - How many 0-10cm soil cores are enough?. � Adapted from Peverill et. al. 1999
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Number of Cores* |
Soil Parameters |
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5 - 10 |
Total N and organic C |
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10 - 20 |
Above plus, pH (not limed), EC (low salinity), exchangeable cations (no amendments applied) |
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20 - 30 |
Above plus, extractable P, K, S (virgin sites or no fertiliser applied for many years |
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30 - 40 |
Suitable for all tests except where fertiliser or a soil amendment recently applied, fertiliser banded or high stocking rate |
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40 - 60 |
Suitable for all test (most situations) |
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>60 |
Generally significant no gain in precision |
*Surface samples Sub-surface: min 8-10 cores
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- Sampling time
- As soil nutrient levels can vary throughout the year, it is best to sample at the
same time each year
- Late winter to early spring are usually the recommended sampling times
- During summer and autumn, it is difficult to obtain a good soil sample. Dry, hard
soils are likely to make collection of a 10-cm-deep core impossible. Even if the
dry ground can be penetrated to 10cm, some of the core bottom may drop out of
the core sampler before being deposited in the sample bag. Also, it is difficult to
see the urine patches (brown or yellowing coloured grass) in dry pasture or on
bare ground
- Paddocks should not be sampled for at least 6 to 8 weeks after the last fertiliser
application
- Sample handling
- Label bag immediately, on outside, in permanent pen. Include your name,
address and paddock name or number
- Avoid leaving samples in a hot car or the back of a ute
- Avoid handling soil, as perspiration from hands can affect the sample
- Send to the laboratory as soon as possible (e.g. on the day they are collected), or
alternately should be dried at low temperature (e.g. 40�C in an oven or spread on
plastic sheets in the sun) as quickly as possible to minimise chemical changes
that may occur during storage or in transit
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- Different approaches to soil nutrient management
- You need to be aware of which approach to soil nutrient management is used by your advisor
- Two main approaches to soil nutrient testing:
- Traditional soil tests (conventional view) � known as the Sufficiency Level of Available Nutrients (SLAN)
- SLAN provides the nutrients to lead to plant response
- Has been used in Victoria for past 80+ years & based on extensive research (e.g. DEPI, Incitec Pivot, etc.)
- Has prescribed sampling depths, usually 10cm increments and interpretation of results recognise differences in soil types
- Is the current best management practice from DEPI
- Alternative soil tests (Ecological view) � known as the Base Cation
Saturation Ratio (BCSP)
- Some consider lays foundation for �system� health i.e. including
impacts on soil and plant health
- Based on maintaining an �ideal� balance of cations
- Increased focus on interactions between fertilisers e.g. Ca:Mg,
Ca:B, P:Zn., and importance of trace elements
- From the US � much of it is yet to be fully tested by DPI
- Not yet the DPI best management practice
- Both approaches generally can use the same laboratories, but it is the interpretation of the tests that differ
- Followers of the BCSR approach often use non accredited labs using unknown analytical methods such that results that are often not comparable to more traditional soil testing results
- The results from one or the other are a BIG issue in the region. For example, you might get an interpretation of a P test result and you do not know what type (SLAN or BCSR)
- Accredited soil testing laboratories
- How can you be sure that the lab is giving you the right results?
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Other related questions in the Brown Book
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Brown Book content has been based on published information listed in the Resources and References sections below
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- Johnston T (2011),Soil Types and Structures Module. Victorian Department of Primary
Industries.
- Soil Fertility Monitoring Tools Choosing a soil testing laboratory. Department of Primary Industries, Victoria.
- Loch Dr D.S. Soil Nutrient Testing: How to Get Meaningful Results. Formerly
Department of Primary Industries and Fisheries, Redlands Research Station, Cleveland.
- Soil Sampling and Analyses. Soil Health Knowledge Bank.
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- Do's and don'ts of soil sampling. (Part C2-5) SOILpak - southern dryland farmers. -�
Department of Primary Industries, NSW.
- Suggested soil sampling guidelines. Assessing the Nutrient Status of Dairy Pastures �
(Section 7.2) � Department of Primary Industries, Victoria.
- How to interpret your soil test. Department of Primary Industries, NSW.
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