Soil Health Test­ing: What? Why? How?

In Octo­ber 2107, many of LEAF’s great and good met at Cran­field Uni­ver­si­ty to dis­cuss the ten-year plan for LEAF Mar­que. The meet­ing focused on what best prac­tice will look like in the future and how LEAF Mar­que needs to adapt to dri­ve change. Soil health was iden­ti­fied as a pri­or­i­ty area and the poten­tial of offer­ing farm­ers a suite of mon­i­tor­ing tech­niques to evi­dence their soil health improve­ments. Here, Sam Brook from the Uni­ver­si­ty of Read­ing, out­lines the scop­ing study he is car­ry­ing out for LEAF exam­in­ing the what, why and how of soil health testing.

In October 2107, many of LEAF's great and good met at Cranfield University to discuss the ten-year plan for LEAF Marque. The meeting focused on what best practice will look like in the future and how LEAF Marque needs to adapt to drive change. Soil health was identified as a priority area and the potential of offering farmers a suite of monitoring techniques to evidence their soil health improvements. Here, Sam Brook from the University of Reading, outlines the scoping study he is carrying out for LEAF examining the what, why and how of soil health testing.

Measuring soil health is complicated. Some might say wonderfully complicated, others awfully complicated. Having carried out an extensive, albeit not exhaustive, literature review on soil testing, I can offer the following morsels of information on the three tests that I am investigating with LEAF:

Earthworm counts

Scientific studies have established worms as a keystone species for soil health; playing important roles in the physical structural, biological diversity and chemical make-up of soils. These ‘ecosystem engineers’ and ‘bioturbators’ of soils are sensitive to contamination and land management practices meaning that earthworm counts can provide a dynamic and reactive measure of soil health in ways that perhaps other indicators, such as organic matter content, cannot.

However, the method is impeded by differing methodology and difficulty in reliably establishing worm morphology and species – itself a key part of the utility and relevance of earthworm counts – meaning lack of standardisation undermines comparability of results. Adopting a standard methodology, such as #60minuteworms, developed by Jackie Stroud of Rothamsted Research, would help address this. Variability in conditions and seasonal fluctuations are other key limitations to earthworm counts as a comprehensive, year-round test for soil health.

Visual Soil Assessment (VSA)

Visual Soil Assessment (VSA) is designed specifically to assess soil health by capturing visually observable aspects of chemical, biological, morphological, and physical make-up of soil. It is easy, quick and cheap to perform but some argue that the price for these advantages sacrifices scientific rigour to achieve this.

VSA is widely regarded to be semi-quantitative, and thus partly reliant on subjective value-judgements for determining the health of any given soil. For some this is a fundamental weakness, however, the element of subjectivity is reduced by scorecards and visual material to aid with classification. There are also studies that show strong correlations between VSA soil health verdicts and those findings produced by laboratories on the same sample. Substituting handling of aggregates for scoring for a drop-shatter test also helps reduce the unwelcome intrusion of subjectivity into the testing process.

Studies showing high levels of correlation both with more sophisticated soil tests, and across different practitioners suggest that it is highly suitable for in-field soil health testing. The same caveat as for earthworm counts applies: consistency in prevailing conditions when re-testing is imperative if repeatability is to be relied on.

Soil erosion assessments

There are various types of soil erosion: wind, water, tillage and co-extraction. Scientific research in some of them is not yet developed enough for us to understand their implications for soil health. Naturally soil erosion, meaning the loss of soil, is detrimental to soil health, but most work focuses on soil erosion quantity, with no consideration of the impact on soil quality. In summary, there are significant blanks to fill in when it comes to soil erosion and soil health.

What has been established by scientific study is the difficulty in quantifying soil erosion. Modelling approaches are reasonably successful, but are extremely data hungry, and findings can be undermined if any of these inputs are erroneous. There are also significant issues with scaling – typically erosion rates are established at a plot level, and then extrapolated – with more recent research showing that erosion rates do not extrapolate linearly, and often studies over-estimate soil erosion rates at a landscape level by a factor of between two and ten.

General consensus amongst the scientific community is that erosion assessments of soil loss are a 'work in progress' and cannot be considered a reliable proxy for soil health. Naturally soil erosion should be avoided as far as possible, as it is a negative externality, but at the moment there is no reliable connection between testing soil erosion and establishing soil health.

Engaging with LEAF Members and Demonstration Farmers

Another key part of the scoping study is engaging with LEAF farmer members. After a crash-course in survey design (it’s harder than you think!), I sent out a survey to assess the popularity (in terms of current use), user-friendliness, reliability, relevance and time and cost efficiency of earthworm counts, Visual Soil Assessment and soil erosion assessments for capturing soil health.

In addition to the literature review and on-line survey, I also interviewed a number of LEAF Demonstration Farmers. What emerged from my discussions was that although soil health is a very variable and intangible concept, there was a heartening belief in the importance of soil health, and the value to farmers in monitoring it. They all felt there is no ‘perfect test’ or ‘silver bullet’ when it comes to soil health and that LEAF's Integrated Farm Management (IFM) was an important catalyst for encouraging greater engagement with soil health issues. I came away from all my meetings feeling impressed and enthused by the respective efforts of each farmer: whether that’s the commitment to making reduced tillage work in challenging conditions such as at Leckford Estate and Ragley Home Farm; Cambs Farms Growers’ commitment to a zero-bare earth policy; Elveden Farm’s work with CO2 burst and carbon cycle testing; or Barfoots fight against compaction and introduction of a 5m controlled traffic system. On the basis of the work and mindset of LEAF Demonstration Farms, the future is bright!

No quick fixes

There are no quick fixes or easy answers for capturing soil health. Each of the methods selected by LEAF are flawed, albeit in different ways. From wider reading around the challenges of soil health testing, there is no other test, whether simple and cheap, or expensive and lab-based, that is considered to comprehensively capture soil health. However, what is clear from my discussions with LEAF Demonstration Farmers and looking through the survey responses, is that LEAF's IFM, backed up by its suite of management tools and resources supports farmers in delivering a range of practices to improve the quality, performance and resilience of soils.

The intention is to produce another blog post at the end of the study, summarising findings and results from the survey, so please keep an eye on your inbox for the next one.