What is Soil?

Associate Professor Rasmus Kjøller, University of Copenhagen, Department of Biology, Terrestrial Ecology

What is soil?
Soil, the biologically active part of the earth’s crust, is a complex mixture of basic mineral particles and organic matter, primarily from dead plant material. Globally, soil contains much more carbon than all living plants and trees. Soil is the natural medium for plant growth, and when a plant dies it provides nutrients for new plant growth – but how does this work?

What lives in soil – soil organisms?
In a teaspoon of soil, we find millions of living organisms, distributed across thousands of species, from microscopic fungi, bacteria and amoebae to small animals, worms, insects and mites, as well as the “giant” earthworms of course. But what do all the different sizes and types of living organisms live off in the soil?

What do soil organisms live off?
In principle, a dead plant contains all the nutrients that new plants need to grow, but the living plants cannot readily absorb the nutrients. First, the dead plant needs to be broken down to release the nutrients. Here the diversity of soil organisms comes into play. First, fungi and bacteria break down the complex substances of the dead plant. They excrete enzymes to attack the dead plant cells. The nutrients are then locked into the bacterial and fungal cells. However, the bacteria and fungi are excellent food for other soil organisms, the amoebae, worms, small insects etc. When they chew on the bacteria and fungi, plant nutrients are released and the plants can absorb them via their roots. So that’s what happens to the canvas: it is consumed by bacteria, fungi, amoebae, worms and insects. Some of it will enter the air as CO2, some of which can be absorbed by the parts of the plants that grow above ground, while some of it will be converted to nutrients that plants absorb via their roots. The discoloration of the canvas will in part be caused simply by fungal and bacterial colonies growing there, but the action of excreted oxidizing enzymes also tends to darken the substrate. The greener colours on the parts installed above ground will be microalgae and/or mould.

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What other important processes do soil organism do – ecosystem services?
Soil is an excellent matrix for holding both water and air, both of which are important for healthy plant growth. The soil organisms, especially the fungal hyphae, also bind the soil together in a web-like structure, which prevents soil erosion. A special group of fungi, the mycorrhizal fungi, forms direct links between the soil’s nutrient pockets and plant roots, thereby speeding up the return of nutrients to the plants. For their services, they are fed with sugar from the photosynthesis of the plants. Other soil organisms, bacteria in particular, are effective in breaking down potentially toxic substances stemming from human activities, including pesticides, oil spills and even plastics.

How will soil organisms and their processes respond to climate change?
Here in Denmark and in similar climates, we must expect that when it gets warmer, soil organisms will work faster. This may result in carbon being lost from the soil as CO2 at a faster rate than new carbon arrives. This is bad for soil quality as a substrate for plant growth, but also for the atmospheric CO2 concentration, which we must reduce to stop global warming. Therefore, it is important that we manage our soils in ways that maintain or even increase carbon storage, as well as soil biodiversity for the benefit of both sustainable plant growth and our climate.