This post has been jointly written by Daniel. L. Evans, Jim Harris, and Sacha Mooney.

When we visit the doctors for a regular health check, we rarely depart holding a single-digit scorecard. For years, however, that’s exactly how we’ve attempted to assess soil health. As pressures on global soils and their services intensify, we need to radically change how we monitor these vital resources.

Recently, there has been increasing recognition about the fundamental roles that soils play in supporting environments, ecosystems, and society. Civilisations have always depended on soil but growing pressures on soils and the key services they provide over the past century have demonstrated how important they are in tackling the pressing issues society now faces. For securing nutritious food and providing fresh water, tackling the biodiversity crisis, and addressing the climate emergency, the ground beneath our feet represents one of our biggest allies. Motivated by the urgent necessity to protect these services for future generations, scientists, policymakers, and stakeholders have channelled their collective energies in developing tools to measure ‘soil health’.  We have critically examined this concept and offered a way forward with a new theory for soil health (Harris, Evans and Mooney, 2022).

Soil health – powerful metaphor, powerless insights

‘Soil health’ has ascended into a powerful and popular metaphor since the 1990s. As an easy-to-understand term, it has been deployed in environmental action plans, guidance notes, legislative documents, and even the name of a new institute. Problematically, though, soils are not living organisms where health can be readily measured. Instead, they represent a complex accumulation of living and non-living solids, liquids, and gases, that perform multiple functions across different spatial and temporal scales. For the same reason that a doctor cannot assess ones cardiac, respiratory, muscular, and mental health from one simple swab of the mouth, it is implausible to push a dipstick into a soil and obtain a holistic overview of soil health. What is more, we can’t be sure that soil is even a system where such measurements make sense.

Until now, though, these simple, in-situ, atomized measurements of soil properties have been common practice. Some soil properties, such as pH, bulk density, and soil organic carbon, have become proxies for soil health. As critical as these properties are, taking readings of these at singular timepoints does not allow us to understand the soil as a continuously evolving and multi-scalar system, subject to above-ground and below-ground volatilities. Taking a standalone reading of soil organic carbon cannot serve as an indicator for how soils change and transform, respond to stimuli, and recover from disturbance. Just as you may record your heart rate before, during, and after exercise, we must also monitor the soil’s response and recovery (e.g., its resilience) to stresses and disturbance. So, soil health is a powerful metaphor for communication, but offers fairly limited insights into the whole soil system – unless we can prove the soil has systematic properties, and at what scale these emerge (Figure 1).

A paradigm shift for soil research

We argue that we need a radical shift in how we monitor soils and offer a new manifesto for future soil research programmes – a transformative step forward in our ability to provide governments and land users the metrics essential for the sustainable management of soil resources around the world. Our ‘New Theory for Soil Health’ comprises four key measures.

• Signs of Life – what organisms exist in soil? We need to identify soil biology, assess a soil’s biodiversity, and determine whether this biology is simple or complex.
• Signs of Function – to what extent do soils carry out processes? We need to identify how soils transform energy and materials across different scales, how efficient are these processes, and how soil systems respond to new living and non-living inputs.
• Signs of Complexity – to what extent are the components of soils connected and interdependent? We need to characterize the complex relationships and feedbacks between living and non-living parts of the soil systems, such as the intersections between soils and plants, fauna, rocks, water, and air.
• Signs of Emergence – to what extent are soils resilient to disturbance? We need to assess how soils respond and recover in the face of short- (e.g. drought) and long-term stresses (e.g. global climate change).

Our new theory of soil health lays out the foundations of a new framework to fuel enriching and exciting soils research programmes in the future. Framing our research in terms of the signs of life, function, complexity, and emergence is critical because, we argue, these are the most vital indicators of a healthy soil system – that is, a soil which can deliver the goods and services to meet the demands of the present and future generations (Figure 2). 

The intensifying pressures being placed on soils, as well as the impacts of global environmental change, suggest that soils will need to become reactive, adaptive, and provident to continue delivering goods and services into the future. We suggest that as soils follow an enriching trajectory of life, functions, and connectivity, they become increasingly resilient. Managing soils in such a way that enables them to become enriched with life, function, and connectivity should therefore be a guiding principle for sustainable soil management. However, measuring whether soils are moving towards a trajectory of resilience is complex. Key to this is the need for interdisciplinary research:  biologists, biochemists, physicists, chemists, mathematicians all have a role to play in unravelling the rules and interdependencies in soils. It commands us to think innovatively about how, when, and how often soils are measured and monitored. After all, to misquote Heraclitus, “nothing is permanent except soil change”.

Reference:

Harris JA, Evans DL & Mooney SJ (2022) A new theory for soil health [Opinion], European Journal of Soil Science, Article No. e1329. DOI: https://doi.org/10.1111/ejss.13292

Photo by Karsten Winegeart on Unsplash