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Soil Health Series: The living soil - the engine of soil processes

Our soils are teeming with life. This life underpins the majority of processes within soil and is critical for delivering a whole range of vital ecosystem functions. Researcher Fiona Brennan, Johnstown Castle, talks about the organisms living within soil and how we assess soil biological health.

How do we define soil health?

Soil quality has been described as the soil’s ability to provide a range of different services through its capacity to perform soil functions under changing management and climatic conditions. Recently this term has been replaced by ‘soil health’, which emphasises the soil biological community as a key driver of soils capacity to deliver multiple functions. Soil health has been defined as the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals and humans. In light of this, developing knowledge and understanding of factors affecting soil health and monitoring the trends in soil health over time is required to better manage and protect our agricultural soils for future generations.

The life within soil

Soil is home to a staggering abundance and diversity of organisms, each of them occupying a specific habitat or niche within soil. This diversity encompasses an enormous range of organism types, sizes, shapes and lifestyles, which operate at many different scales. These include bacteria, archaea, protozoa, fungi, algae, tardigrades, rotifers, nematodes, mites, springtails, worms, beetles, ants, myriapods and a host of other soil-dwelling creatures. The diversity of life within soil is reflective of the highly diverse and dynamic nature of the habitat. Soils have a pronounced spatial structure and gradients in the availability of essential resources such as nutrients, water and organic materials exist - supporting a wide range of different niches. Soils are comprised of a mixture of minerals fractions (sand, silt, clay), organic materials and pore spaces, the latter of which can be filled with either air or water. Much of the life within soil exists within the pores, so the structure of the soil and in particular the nature of the pore network is important maintaining habitats that support life. Soils contain an enormous surface area, particularly pores associated with smaller soil particles such as clays where the surface area of the pore space can be as much as 24,000m2 within a gram of clay. When you consider that bacteria are typically only a few microns in length, you begin to appreciate that despite the abundance of organisms within soil, organisms even within the same handful of soil may never interact with each other such is the extent of the soil habitat. Interactions between organisms within soils can be either positive or negative - they can help one another or they can compete with each other, up to and including engaging in biological warfare or predation. The same is true of the relationship between soil organisms and plants - while the vast majority of interactions are positive, soil borne pathogens, pests and diseases can impact on plant health. As the conditions within soil change due to changes in the environment (for e.g. changes in temperature or rainfall) or management induced changes (nutrient inputs, liming, crops etc.) so too does the abundance, diversity and function of the communities of organisms within soil.

Photo by Matthias BacherEarthworm

How do we assess the biological health of our soils?

While we have long had a good understanding of the physical and chemical aspects of soil, our understanding of the soil biology has lagged well behind, with the biology often viewed as ‘a black box’. Given the importance of soil biology to the functioning of soils it is clear that to fully understand our soils, and how to manage them, we need to understand this biological aspect and how it interacts with the soil physical and chemical properties. The historical lack of knowledge pertaining to soil biology was largely due technological constraints, combined with the complexity of the habitat and that of the biological communities. Much of the life within soil, and the engine of soil function, is microscopic and traditional methods of exploring these communities by culturing organisms in the laboratory or placing a soil suspension under the microscope could only provide limited insights that were inadequate at assessing overall soil biological health. New technological advances (particularly molecular and sequencing based approaches) have largely overcome these limitations and are offering revolutionary advances in our understanding of the microbial communities within soil. There has also been much effort employed in the international community in developing a range of biological indicators that can be utilised to assess soil biological health. This is an exciting time for soil biology but many questions and challenges remain. Assessing the biological health of our soils directly is not an easy task due to the diversity and enormity of biological communities within soil. Further it is important to establish not only what communities of organisms are present in soils, and how these are impacted by management, but also what those organisms are doing and how this relates to ecosystem function. The communities of organisms present differ from soil to soil and from environment to environment so what is healthy for one soil may not be healthy for another, requiring bespoke management and assessment. Non specialist tools are required that can be employed at farm level. These include simple visual assessment tools that can provide much useful information to assess the biological health of soil.

What next?

Going forward we will need a long term monitoring of soils to detect changes in soil health over time and to act as an early warning system before problems arise on farms. Simultaneously we need practical management solutions for protecting the health and quality of our agricultural soil or for remediating soils which have been previously damaged. This knowledge can be integrated with knowledge transfer services to provide advice to farmers and farm advisors. We have made a good start but we have a long way to go!