Nutrient Cycling is defined as the capacity of a soil to take up and recycle nutrients from different inputs (e.g., plant residues, manure) and to support the uptake of nutrients from soil minerals and organic matter, water and air by plants and the soil community. In this function model, the focus is on the cycling of plant essential macro-nutrients: nitrogen, phosphorus, potassium, sulphur, magnesium and calcium.
There are a numerous biological processes and actors involved in making this function work at is best capacity, these have been categorized as follows:
(1) Nutrient transformation – encompasses soil biological processes that lead to changes in the chemical or physical form of nutrient resources. Most chemical processes are supported by bacteria and fungi, while the physical reduction in size of nutrient resources is facilitated by soil fauna that feed on plant litter.
(2) Nutrient reallocation – refers to a change in location of and access to nutrient resources. Organic matter and soil can be mixed and/or moved around within the soil profile in different ways, for examples by earthworms that dig tunnels and pull dead leaves from the surface into the soil. In addition, the threads weaved by many species of fungi and other microbial residues glue together soil particles and organic matter. This reallocation of nutrient resources influences the processes grouped under nutrient transformation and nutrient assimilation.
(3) Nutrient assimilation – includes processes involved in the uptake of nutrients from soil by biota and plants and nutrient transfer through the soil food web. These processes are in part supported by symbiotic relationships between plants and microbes that lead to the fixation of nitrogen from air and more extensive exploration of soil for nutrient resources.
The reason why the Nutrient Cycling function is so important, specifically in the production of food, feed and fibre, is because it dictates the amount of nutrients available for the soil biota and the plants. With this function incapacitated, no crop could be grown on such soils. Other than providing nutrients to support plant production and soil life, the Nutrient cycling function also limits the leakage of nutrients into streaming water and gaseous losses to the atmosphere.