The saying commonly goes, ‘You can’t manage well what you don’t measure’. You can manage pastures without measuring, using observation and experience; however, your decisions are unlikely to be made with a high level of precision and things can and do go wrong when conditions change and go outside of what you have experience with.

Changes of nutrient levels and balances, pH and salinity can occur over time when using a standard fertiliser program for many years, especially if the focus is just on nitrogen (N) or N plus phosphorus (P) and potassium (K). The risk is that both macronutrient and trace element defieciencies will be created or worsened. While animals can be directly supplied with trace elements, they will still be missing in the soil and affect pasture growth. If pastures or fodder are held back by deficiencies of one or two elements, NPK fertilisers will not be well utilised, resulting in a waste of money and effort. A soil test to get nutrition right would be cheaper!

Agricultural inputs are costly. A soil test is a relatively cheap and easy way to gain a wealth of knowledge about soil conditions, and provide indications on pasture and fodder crop performance. This in turn reflects fodder quality and nutrition, which affects animal health and productivity. By assessing the status of the soil, valuable dollars can be more accurately spent where they will make the most difference, maximising the feed base performance and increasing overall productivity.

Soil fertility testing is the process of taking and analysing a representative soil sample to determine its nutrient content and other properties that affect plant growth. The goal of soil fertility testing is to use the information, together with other data, observations, and experience, to make informed decisions about fertiliser and soil amendment inputs and other soil management grazing practices.

Typically, a soil fertility test will measure the levels of major nutrients such as nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulphur (S), in addition to trace metals such as iron (Fe), manganese (Mn), copper (Cu) and zinc (Zn), as well as boron (B) and chloride (Cl). The test will also include a measurement of soil pH, organic matter/carbon content, electric conductivity, as well as physical and chemical indicators that can affect nutrient availability to a crop or pasture and thus plant growth. Examples of indicators are cation exchange capacity including sodium (Na), electrochemical stability index or P loss risks.

Soil fertility testing is important because plants require specific nutrients in certain amounts and ratios to grow and develop properly. By testing the soil, land managers can identify any nutrient deficiencies or other factors that may be limiting plant growth and productivity. They can then adjust their nutrient budgets and soil amendment programs to provide the necessary nutrients and improve soil health over time.

Regular soil testing is more valuable than standalone or diagnostic soil testing because it can show trends over time in a given paddock.

A farm sampling plan² should allow for each paddock to be sampled every three to four years or approximately a quarter of paddocks in production each year.

Representative sampling is essential for getting meaningful results and value for money. Sampling has to be well planned and executed. Consideration needs to be given to the volume of soil required for the suite of tests, the conditions in which the samples need to arrive at the lab, and the technique or tool with which the samples are taken. Samples for a sampling area which may be a paddock, paddock area or even a group of paddocks with known, similar conditions, may be taken at random, by transect, grid, or zig-zag. An NDVI map (normalised difference vegetation index map) can provide information to assist delineating sampling areas with comparable biomass production to form a sampling area within a paddock.

Usually, sampling depths for pastures are from 0-10 cm, although some samples are even from shallower depth, because the topsoil is shallow. The main limitation of shallow sampling is that it does not provide accurate information about soil conditions in the entire root zone, or throughout the profile. Shallow sampling provides an indication of topsoil conditions and nutrient levels. Research to determine desirable nutrient levels in pastures has focussed on the 0-10 cm sampling depth. Nutrient levels to this depth have been related to pasture biomass production to arrive at the desirable nutrient level ranges for pastures³.

It is generally good practice to dig below the typical sampling depth of 10 cm to gain a better understanding of soil condition and root penetration. This may also be useful for informing sampling depth i.e. if it should deviate from the standard 10 cm for a good reason. If there are physical soil constraints beyond 10 cm depth, these are unlikely to be ameliorated through additional nutrients and deeper testing may not be required. If the topsoil condition supports deeper rooting than 10 cm and there is interest in determining the nutrient level over the main rooting depth, a deeper sample may be taken. Desirable level ranges calibrated to 10 cm depth can still be used as an interpretation guide. The farmer or agronomist may want to build a picture of nutrient trends for the farm or paddock where deeper samples have been taken and compare them to dry matter production trends.

Separate subsoil sampling can also to be done when pasture root systems grow deeper than 10 cm, i.e. if the topsoil is deeper. This would mean taking samples to 10 cm and also samples from 10-20 cm or 10-30 cm separately.

This gives information about nutrient availability across the rootzone and the desirable ranges for 0-10 cm apply. When sampling the top 10 cm of topsoil of longer term pastures only, it needs to be considered that some nutrients, especially phosphorus, may have accumulated in the top centimetres of the soil. A stratification of pH may also have occurred so that the topsoil pH may be in a desirable range, but the subsoil that is accessed by roots is not.

Depending on pasture composition and purpose for testing, different depths of testing may be appropriate. Commonly tested segments are 0-10cm,10-30cm, 30-60cm, 60-90cm. For issues with acidity, segmented pH tests of 0-10cm, 10-20cm, and 20-30cm are adequate to assess whether or not the subsoil is acid, and the degree to with which it is acid.

Rooting depth for common Tasmanian pasture species in ideal soil conditions is as follows⁴:

Pasture renovations are generally a good time to conduct soil sampling, as it provides a good opportunity to incorporate fertilisers or other soil amendments (e.g., lime, gypsum, or organic material).

Comprehensive and up to date information on how to sample can be found in the Fertcare Soil Sampling Guide available from the Fertilizer Australia website:

https://fertilizer.org.au/Fertcare/Nutrients-And-Fertilizer-Information/Soil-Sampling.

Nutrient Management for Farming in Tasmania by Bill Cotching (2023) contains further information on soil sampling methods and limitations.

There are a number of different soil testing services available within the state and nationally, as well as for a range of different production systems. Most soil testing laboratories offer a mix of chemical analyses, fertility indicators, and soil properties e.g., texture. Some labs offer specialty testing for contaminants such as heavy metals and pesticides. There are labs that perform physical soils test; these are not commonly used for agronomy purposes. They are at times used for research to better understand soil compaction or water relations.

There is no one-size-fits-all approach for the appropriate soil fertility testing methodology. Determining the appropriate test for your situation should be discussed with an advisor. In determining which soil tests are necessary, the following should be considered:

• The reason for soil testing, i.e., what the information is going to be used for.
• Soil type.
• Soil texture (proportions of sand, clay, and silt).
• Any known limitations such as physical or chemical subsoil constraints likely leading to restrictions in root growth.
• Pasture species and rooting depth.
• Previous soil testing history, especially when looking at trends.
• Any variability in the paddock e.g., change in soil type, texture or soil conditions due to previous management (paddock has been created by combining two previous paddocks).

The following points are important for all soil analyses:

1. The lab should be a member of the Australasian Soil and Plant Analysis Council Inc (ASPAC)⁵, participate in the ASPAC proficiency program and be proficient in the test methods requested.
2. The lab should ideally operate under a quality assurance system such as NATA⁶ or ISO/IEC17025-2017⁷.
3. Sampling must be done correctly for the required testing service; the laboratory can provide information on how to sample and how to handle and send the sample. The abovementioned Guide will provide relevant information.
4. How to best get the sample(s) to the chosen lab and avoid spoilage. If a sample arrives just before the weekend, it will be stored. Sending samples on a Friday is not recommended; it will be stuck in the mail for a while, e.g., when testing for N, samples must remain cold until analysed.
5. Sampling should occur early enough to get results in time to determine
6. inputs / applications; some labs have a longer turnaround time than others; factor in time needed to interpret the test results and to organise the fertiliser.

ASPAC provides further advice on selecting the right testing service on its public web site.

A complete reference on testing methods for Australia is Soil Chemical Methods – Australasia; by George E. Rayment and David J. Lyons, CSIRO Publishing, 2011.

Soil tests give us information about one point in time, and are critical in informing the best application of fertilisers and amendments. However, while chemical soil properties are important indicators of soil health, physical and biological properties are equally crucial. Soil is a complex and dynamic ecosystem, and its physical and biological properties play essential roles in maintaining soil fertility, supporting plant growth, and sustaining overall ecosystem health.

Physical properties such as soil structure, texture, and porosity influence soil water retention, nutrient availability, and aeration. For example, compacted soils with poor structure and low porosity can limit root growth and lead to reduced water infiltration and increased runoff, while well-structured soils with good porosity can promote healthy root growth and water infiltration, reducing the need for irrigation.

Biological properties such as soil biodiversity, organic matter content, and microbial activity contribute to soil fertility, nutrient cycling, and disease suppression. Soil microorganisms, such as bacteria and fungi, are essential for breaking down organic matter, releasing nutrients into the soil, and suppressing soil-borne pathogens. Organic matter also helps to improve soil structure and water-holding capacity, enhancing plant growth and reducing erosion.

Therefore, assessing soil health solely based on chemical properties will not provide a complete picture of soil fertility, productivity, and ecosystem health. A comprehensive evaluation of soil health should consider the physical, chemical, and biological properties of the soil, taking into account the interrelationships between these properties and how they contribute to soil function and overall ecosystem health.

The more information that can be considered when interpreting soil test results, the better. A good start is a visual soil assessment. Many methods of visual soil assessment (VSA) have been published and can be used as a guide. Refer to: Visual soil examination techniques as part of a soil appraisal framework for farm evaluation in Australia, by David C. McKenzie. Soil and Tillage Research Volume 127, March 2013, pages 26–33, and Soil Health for Farming in Tasmania, by Bill Cotching (2009), page 16. In short, a VSA is used to assess a range of physical, chemical, and biological properties, as detailed below.

Based on the observations of soil properties, a VSA can provide insight into the overall health and quality of the soil, as well as identify areas where soil management practices may need to be adjusted to improve soil function and productivity. A good way to contextualise a chemical soil test and VSA is to think about these three P’s:

Soil tests and visual soil assessments do not take into account topography, climate, plant growth or grazing management, nor are they able to observe pasture conditions such as insect pests, diseases, and pasture composition. Simply relying on soil tests to add fertilisers or amendments is often treating the ‘symptom’ instead of providing the ‘cure’. To have a complete understanding of soil condition and therefore pasture performance, it’s important to look at the farming system in conjunction with the soil test results. A working knowledge of the landscape and the context of the entire farm is needed to make sensible decisions around nutrient management.

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² Fertilizer Australia soil sampling guide: https://fertilizer.org.au/Fertcare/Nutrients-And-Fertilizer-Information/Soil-Sampling.

³ Cameron J P Gourley, C.J.P. et al. 2007. Making Better Fertiliser Decisions for Grazed Pastures in Australia. Published by the Victorian Government Department of Primary Industries © The State of Victoria, Department of Primary Industries, June 2007. https://www.asris.csiro.au/downloads/BFD/Making%20Better%20Fertiliser%20Decisions%20for%20Grazed%20Pastures%20in%20Australia.pdf

Bill Cotching 2023. Nutrient Management for Farming in Tasmania.

⁴ Heritage seeds: Perennial Pasture Guide 2017

⁵ http://www.aspac-australasia.com/laboratories.

⁶ http://www.nata.com.au or

⁷ AS ISO/IEC 17025-2017 ‘General requirements for the competence of testing and calibration laboratories’

⁸ https://www.agriculturalsolutions.com.au/shop/books/organic-biodynamic/visual-soil-assessment-vsa-field-guide/