Soil is a mixture of minerals, organic matter, living organisms, water and air. The minerals (about 50 per cent of soil) are particles of weathered rock, which include tiny grains of clay and silt as well as larger sand particles. The living organisms include fungi, bacteria and insects, and the organic matter incorporates dead plants and microbes, plus other animals in various stages of decomposition. The final product of decomposition is a dark, organic material called humus, which contains relatively high amounts of nitrogen – a nutrient that’s vital to plant health.
Initially, soils form from a ‘parent material’ on the Earth’s surface. This could be bedrock that’s been eroded, or material that’s been transported by glaciers, rivers or wind. Over time, more layers, or ‘horizons’, are built up. This process takes tens of thousands of years as rocks are slowly weathered and organic matter accumulates. The properties of the soil depend on the local conditions. Sandy areas have gritty soils that drain rapidly; boggy areas have dark, spongy peat; clay-rich areas have sticky soils that crack when dry.
We usually take soil for granted, but it’s estimated that a quarter of all species live in or on soil, and we need soil to grow 95 per cent of our food. Soil also plays a wider role in absorbing water (and thus reducing flooding), and locking away carbon. However, soils are under growing pressure from intensive farming and tilling, overharvesting and the use of chemicals. As a result, we lose the equivalent of 30 football pitches of fertile soil every minute.