There are eighteen essential elements needed by plants and animals on earth and because life needs to continue from year to year so must the availability of these nutrients. Of the 18 essential nutrients some are classed as macronutrients because the plants and animals need them in fairly large quantities, others are classed as micronutrients and are needed in much smaller quantities. The macronutrients are carbon, nitrogen, oxygen (see the separate linked sections on these three major cycles), phosphorus, potassium, calcium, magnesium, sulphur and the micronutrients are boron, chlorine, cobalt, copper, iron, manganese, molybdenum, nickel and zinc. In addition to those listed above, some elements such as selenium and iodine, though not required by plants, are essential nutrients for humans and for animals. There are a few other elements which whilst important for growth are not necessarily essential.
Over the millions of years that plants, animals and man have been on the planet, there has been ongoing adjustment by plants and animals in a particular period to changing soil and climatic conditions. In each climatic period there has been generally been a balance created between the nutrient needs of the vegetation and animals and the ability of the soil to supply it. At the heart of this is nutrient cycling. The cycling of nutrients is essential for maintaining plant growth, and ultimately the sustenance of plants and animals.
Each nutrient has its own cycle but many elements appear in more than one cycle. Some of these cycles, such as those of nitrogen, carbon, oxygen and sulphur, involve movements and transfers between atmosphere, biosphere, lithosphere and hydrosphere. Others are more restricted and mainly involve movements between the rocks and soils below ground and the plants and animals above ground. The most important parts of the nutrient cycle relate to the exchange of nutrients between three main pools (See the diagram): (i) that in the above ground plants and animals; (ii) that within the soil, specifically within the organic matter; (iii) and that in inorganic form in the soil consisting of inorganic ions derived from various sources including weathering of minerals in the soil, ions in solution in the soil and ions absorbed onto the surfaces of minerals in the soil. The cycling of nutrients between these main pools constitutes a major part of the nutrient cycles and is one of the main reasons why plants, animals and man are able to live on earth.