By SRUC’s Professor Bryan Griffiths, Samantha Dolan and Dr Paul Hargreaves,

for the Farm Advisory Service

The headline to this story begs three questions: What is soil health? Can you test it? What is benchmarking?

A healthy soil is ‘fit for purpose’ and should maintain a sustainable production system that may not be exclusively fit for growing crops. Soils can also provide environmental goods and services that benefit both the farmer and society in general.

Reduced environmental pollution from run-off, leaching and greenhouse gas emissions, prevention of soil erosion and better water management to smooth out the extremes of flood and drought are all benefits of a healthy soil. These goods and services are also beneficial to the bottom line, encouraging yield resilience, enabling efficient production with fewer inputs and increasing the number of workable days on the soil.

Soil health means getting the biology, chemistry and physics of the soil in balance and optimised so that the soil is working for you as much as possible. Health tests are now readily available and can be likened to an MOT for your car, a check-up at the doctors or an end of term report.

Soil is the fabric that your farming business relies on, so it makes sense to monitor its health. Soil health tests augment a routine chemical nutrient test (usually pH, P, K, Mg, Ca and Na) with additional chemical information such as sulphur, or organic matter content (which is a well-known long-term indication of soil health) and also add physical and biological parameters. The former includes a visual evaluation of soil structure (VESS) and compaction, together with measures of biological activity.

These measures are then compared with threshold values or a normal operating range to give an individual score for each of the tests. Taking soil phosphorus (P) as an example, there are well established values for the soil ‘P status’ (measured in mg/L for Scottish soils), which are analysed using the Modified Morgan’s extraction and tests will indicate if levels are so low as to be a risk to production, at an optimum or excessive with a risk to the environment.

Mineralisable nitrogen (also called PMN) is a composite measure of biological activity and organic matter quality. The typical PMN ranges of <20, 20-50 and >50 mg/kg are classified as low, medium and high in the SAC soil health test.

Soil health is the balance between the biological, physical and chemical properties of the soil, and a soil health test gives a quantifiable measure of that balance at the time of sampling. This snapshot of soil health, leads neatly to the notion of benchmarking.

Your first soil health test result is the benchmark against which all subsequent tests can be compared. Establishing what your baseline soil health and nutrient status looks like will allow you to make management decisions which could reduce your future inputs costs and improve the efficiency of the farm business. You can benchmark in two directions, vertically (over time) and horizontally (across locations).

Benchmarking over time will show how that field is changing, whether in response to a new management option or just over the course of typical farming practice. Soils respond relatively slowly to change, so monitoring soil health regularly makes sense. It also opens up the possibility of testing rented land before and after a rental period.

Benchmarking across locations will show how your soil compares with similar soils in the same district and answer the question ‘is my soil as productive as it could be?’ While benchmarking the same field over time is something a single farmer can easily do, benchmarking across locations is only possible with access to a larger database of other farms analysis, so that fields from different farms across a region can be compared without giving away any confidential information.

The ability to interrogate information in this way is a major advantage of using a commercial soil health test and allows the results of a test to be seen in the context with the range of results from that area.