New guidance to make sense of soil health indicators

by organicresearchcentre

The outcomes of ORC’s project Early indicators to monitor changes in soil health: evidence to update guidance for UK farmers, have been published on the AHDB website. ORC Principal Researcher – Soil Dr Julia Cooper explains:

The project’s aim was to identify ‘early indicators of soil health.’ These were indicators that a farmer might be able to use soon after they had introduced some change in management to find out if it was having a positive impact on soil health. AHDB’s current suite of soil health indicators are meant to be measured every 3 to 5 years to get a feeling for the long-term trajectory of changes in soil health. Early indicators of soil health would ideally detect a change in soil health as early as six months following a change in soil management. The challenge with finding these indicators was to identify a test that was sensitive to management, but not so sensitive that it would be affected by recent weather or land operations.

Ultimately, we identified three indicators with potential as ‘early indicators of soil health.’

1. The slake test is a farmer-led measure of aggregate stability that appealed to our stakeholders. This test is simple to conduct using a free app and is a quick and easy measure of the soil’s physical health and resistance to soil erosion. More UK-relevant data is needed to improve interpretation of slake scores i.e., what is a ‘good’ slake score versus a ‘poor’ score.
2. Potentially mineralisable nitrogen (PMN) is already included in the soil health scorecard and is sensitive to recent additions of organic nitrogen, so it shows potential as an early indicator of one aspect of soil health: the ability of the soil microbial community to decompose organic matter and release nutrients. However, we were unable to update the traffic light scores for this indicator due to a lack of UK-relevant data on how PMN links to soil health and function in different soil types and cropping systems
3. Potassium permanganate oxidisable carbon (POXC), is a measure of one component of the soil’s organic matter pool. There is some evidence that links POXC to the quality and accessibility of soil organic matter to soil microorganisms; however, other studies have shown a high correlation with good old-fashioned organic matter by loss on ignition, suggesting it doesn’t really provide additional information. So this one also requires further investigation.

The project therefore successfully screened a range of indicators and identified those with potential as early indicators of soil health. Further research is still required to verify the links between these tests and soil health, how sensitive they are to recent changes in management, and to build a database of results for development of interpretation systems e.g. a traffic light system similar to the existing AHDB soil health score card.

We expected biological tests to be good early indicators of soil health, but we identified a range of limitations that ultimately meant that most well-known biological tests were excluded from our shortlist. In most cases this was due to the high degree of uncertainty around interpretation of results from any biological testing including soil foodweb based methods e.g. microscopy to identify bacteria, fungi and some soil fauna, DNA and PLFA-based techniques that quantify different taxa in the soil, and enzyme assays that provide a measure of soil functions. In all of these cases, the extreme sensitivity of these measures to factors like recent weather events and tillage, means that changes in these indicators cannot be reliably attributed to a change in soil health. This limitation is coupled with the complete lack of any robust system to interpret results as ‘good’ or ‘poor’ in terms of soil health. The popular use of fungal:bacterial ratios has not been verified in academic literature as having any relationship with key soil functions that are essential in a healthy soil. So our original expectation that some of these popular measures of soil biological properties would be identified as an early indicator of soil health was not realised.

The next steps with this work should be to build the database of results for these soil health tests across a range of UK farming systems and soil types. The results for early indicators of soil health should be recorded along with recent land management and other standard, longer-term measures of soil health (e.g. soil organic matter, VESS, earthworms). A large dataset like this could then be used to verify that these early indicators are accurately foreshadowing longer-term changes in soil health and to start to develop interpretation frameworks that band results into an easily interpreted traffic light system (e.g. Poor (Investigate), Moderate (Review), Good (Monitor)).”

How to monitor soil health more frequently within the rotation is the subject of AHDB’s new evidence-based web pages, resulting from the project. AHDB’s Senior Environment Manager Amanda Bennett describes the approaches and how the results can support management decisions. New guidance to make sense of soil health indicators | AHDB

AHDB project webpage, including report:Early indicators to monitor changes in soil health: evidence to update guidance for UK farmers | AHDB

New AHDB webpages:

Measure aggregate stability to detect changes in soil health early | AHDB

Measure active carbon to detect early changes in soil health | AHDB

Using potentially mineralisable nitrogen to measure soil health | AHDB