Redefining soil health
The Microbiology Society is undertaking a project entitled A Sustainable Future as part of our 75th Anniversary, which aims to highlight the Sustainable Development Goals (SDGs) to our members and empower them to use their research to evidence and impact the goals. Earlier this year, we put a call out to our members to submit case studies in the following three areas: antimicrobial resistance, soil health and the circular economy.
This case study is written by Dr Mark Pawlett, who is a Senior Research Fellow in Soil Biology, and Dr Jacqueline Hannam, who is a Senior Research Fellow in Pedology, at Cranfield University, UK. It focuses on Soil Health; maintaining the health of our soils has gained increasing prominence in recent years. Soils are essential for the global food system and regulate water, carbon and nitrogen cycles but are put under pressure from population growth and climate change.
Describing the health of complex systems requires the inclusion of interactions, both within the system and interactions with its environment. Definitions of human health incorporate the quality of life, physical, mental and social well-being, and the absence of disease (WHO). For soils, the term “soil quality” (Karlen et al., 1997) has largely been replaced by “soil health”; yet, these terms are often used interchangeably without clear context or definition. The main difference in definitions is that “soil quality” typically uses indicators of soil condition for specific individual traits or functions (such as yield), similar to a ‘quality of life’ indicator. In contrast, “soil health” encompasses benefits to wider ecosystem services within a more holistic approach, and as such considers multiple functions and their interactions in an ecological context. Many use variants of Doran’s (2002) definition, which describes soil health as “the capacity of a living soil to function, within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and promote plant and animal health”; thereby including multiple ecosystem functional traits, or “services” in addition to yield. Soil health is now a major environmental criterion for the sustainable management of soils used by policy makers in the UK. Providing greater clarity on definition is important, given that the term tries to explain complex environmental systems and interactions to different stakeholders with different perspectives relating to sustainable land and environmental management.
Soil functions are intrinsically dynamic processes, many of which necessitate biological interactions that are sensitive to management strategies. Functional traits include food crop production, carbon transformation and regulation processes, nutrient cycling processes, soil structure and stability, biodiversity and biological population regulation (soil food webs), soil water fluxes, and regulation of soil and water pollutants. A healthy soil encompassing multiple functional trails is required for a healthy ecosystem, and subsequently the delivery of ecosystem services and goods required by society. Healthy soils are biodiverse and resilient to perturbation, with efficient energy and nutrient flows that are often unique to ecosystems. Complexity, natural variability, and ecosystem boundaries can also change due to multiple anthropogenic stresses and disturbances (e.g. climate shocks and weather extremes). Definitions thus need to consider ecosystem complexity, whilst also reflecting emergent soil properties and adaptability in changing systems. As boundaries shift, management practices to maintain specific ecosystem services may not be sustainable or suitable.
Soil health can be defined as the capacity of soils to deliver multiple functional traits that are required to maintain ecosystem stability while allowing ecosystem development in a changing environment. Functional traits of specific ecosystems that are valued by society will vary between ecosystem and land use types. Not all soil functional traits can be delivered simultaneously; for example, the simultaneous provision of food crops and water storage is difficult. Assessments of soil health should thus consider multiple functional traits that are deliverable by the identified ecosystem type. The majority of commercially available soil health assessments focus solely on crop production (single functional trait: soil quality). Soil physical, chemical and biological parameters should also be represented to support improved decision-making. In particular, the soil biological component is nearly always inadequately represented or absent in many approaches or soil management tools. There is thus a need to better link specific soil characteristics with function for developing a more holistic approach to measuring soil health. Various soil health scorecard systems are currently under development, which attempt to combine multiple parameters. However, a knowledge gap in devising ecosystem specific indexes and scorecard systems that effectively describe either soil quality and/or soil health still remains. For these scorecard approaches to be applicable for practitioners, long-term field trials are required to provide the necessary data to identify the most effective indicators (Bünemann et al., 2018).
Soil health parameters measured should reflect soil functions, be sensitive to changes in land management, and measurable within reasonable time and resource constraints. Evidence suggests farmers often favour the term soil health as it incorporates in-field assessments that often includes expert qualitative judgements (e.g. visual soil assessment, earthworm counts, soil depth, infiltration rates) combined with quantitative descriptors (organic matter, pH, bioavailable nutrients, microbial biomass). This is fundamentally important as farmers embed valuable local experiential knowledge of their particular soil types into their land management practices. Describing healthy soil thus enables the land manager to engage with their soils, and to move beyond the view of soil existing purely to support primary production (yield) but to underpin multiple ecosystem services (functions). Better farmer engagement and co-production of soil health assessments will also facilitate longer-term in-depth analysis of soils. This improves understanding of changes related to natural and anthropogenic disturbance, and the benefits of farming innovations and risk prevention. Defining the critical triggers or ‘thresholds’ whereby soil becomes either “healthy” (or not) remains an on-going contested space both within academia and industry.
References
Bünemann, E.K. et al.,(2018). Soil quality-A critical review. Soil Biology Biochemistry, 210 105-125.
Doran, J. W. (2002). Soil health and global sustainability: translating science into practice. Agriculture Ecosystems & Environment, 88(2), 119–127.
Karlen, D.L., et al., (1997). Soil quality: A concept, definition, and framework for evaluation. Soil Sci. Soc. Am. J. 61:4-10.
About the authors
Dr Mark Pawlett is a Senior Research Fellow of Soil Biology, with a focus on developing innovative strategies for sustainable land management. Dr Jacqueline Hannam is a soil scientist specialising in soil data, digital soil mapping and soil health.
