New guidance to make sense of soil health indicators

How to monitor soil health more frequently within the rotation is the subject of our new evidence-based web pages. Senior Environment Manager Amanda Bennett describes the approaches and how the results can support management decisions.

Routine measurement of soil health indicators on a rotational basis should be standard practice. So, what is the appetite for new indicators to be added to the toolbox?

Interest in soil biology is surging and the role of soil organisms in delivering multiple functions in agricultural systems is becoming widely recognised.

Biological indicators of soil health are often well researched academically, but are less understood on farm.

A recent project investigated the strength of evidence for a range of indicators that could be used on a more frequent basis to monitor changes in soil health on the farm.

Assessing topsoil structure, pH, soil nutrients, organic matter levels and earthworm presence integrates physical, chemical and biological aspects of soil health.

Importantly, these core indicators can be interpreted in the context of soil management to inform and guide practical decision making. 

The AHDB soil health scorecard provides benchmark values (and associated management notes) for these indicators across different soil types and rainfall regions. 

The recommendation is to do the assessments on a rotational basis, once every three to five years.

As soils can take a long time to change, the research project explored indicators that were more responsive to change on a shorter time frame. Many of these are related to biological functioning.  

Criteria for reviewing the indicators included simplicity, sensitivity, cost effectiveness and the extent of data to support findings. 

There is still a lot to learn about assessing soil biology, whether that relates to diversity, biomass, activity or functions.

Test results can be confusing. Confidence in interpretation of results is often low and there may be little evidence to robustly support practical management decisions.

Rather than going with a ‘gut feeling’ or basing decisions on what ‘feels right’, it is important to gain a better understanding of why things work when and where they do. 

Data and evidence are needed to increase confidence in using tests and interpreting results for the context in which they are used.

Insights gained from a series of workshops during the research, which involved both expert opinion and industry (end user) feedback, revealed that there was an appetite for tests that farmers can do themselves, rather than sending samples to a laboratory.

However, many biological tests require specialist laboratory equipment or knowledge for interpretation. 

Alternative perspectives were also revealed during the workshops. For example, aggregate stability is tested by wet sieving in laboratories (used in research), but slake testing is a more recognised method on farm.

While these methods may indicate the same outcome, the approaches are different.

Active carbon (also referred to as permanganate oxidisable carbon) is the biologically available carbon in soil organic matter.

It is sensitive to changes in soil management and may be an earlier indicator to monitor changes in soil carbon compared to soil organic matter.

It is offered by commercial laboratories in the UK as part of soil carbon testing packages. However, it can be an expensive test and there is still some uncertainty about the interpretation of results.

Potentially mineralisable nitrogen (PMN) is the proportion of organic nitrogen in soil that can be converted into plant available nitrogen through mineralisation by soil microbes.

As such, measuring PMN provides information on potential (future) nitrogen supply from organic matter, and it is also an indicator of microbial activity in the soil. It is available as a commercial test in some UK laboratories.

Soil biology is fascinating. Yet the myriad of life below ground is vastly unexplored and little understood.

There is still work to be done to interpret biological soil health assessments.

However, the practical management to improve and maintain soil health (and biological functioning) largely remains the same: reduce cultivation intensity where possible, optimise organic material inputs and ensure cropping diversity (in both space and time).