Important nutrient cycles

The phosphorus cycle, though simpler than some of the other global cycles discussed, is a particularly important one. There is little phosphorus in the atmosphere and the cycle is mainly between the lithosphere and the biosphere (soils and vegetation). Most phosphorus comes into the biosphere from the breakdown of rocks which contain the major supply of phosphorus. The phosphorus enters into solution from which form it can be taken in by plant roots and soil organisms. Mycorrhizae, a fungi, plays an important part in helping the roots take in phosphorus, and through the roots phosphorus can be transmitted to the plants and animals above ground. Generally in natural systems there are only small amounts of phosphorus circulating. In agricultural systems the use of fertilizers has led to relatively large amounts of phosphorus entering the soil system sometimes creating problems for the wider environment.

Potassium, like phosphorus, is an element than can be limiting to plant productivity. The source of potassium is mainly in rocks and sediments. As these break down during soil formation so the potassium they contain becomes more accessible, and eventually can come into solution and become available to plant roots. Once it enters into the biological part of the cycle, it will be cycled and re-cycled. Plant roots take it up and it then supports plant growth. As the plants die, so their remains return to the soil and the potassium they contain is cycled within the organic matter, eventually becoming available to the plant roots again. In agricultural systems, fertiliser inputs of potassium also become part of the cycle.

The Sulphur cycle is more of a global cycle than are those of phosphorus and potassium and is similar to the nitrogen cycle in many ways. In the sulphur cycle, the atmosphere receives sulphur from the burning of fossil fuels, from volcanic activity and as part of gases released from the land and oceans. Even so the atmosphere contains less sulphur than is in the soil and land plants, much less than is in seawater, and a tiny fraction of what is in rocks and sediments. Sulphur compounds move from the ocean to the atmosphere, be picked up by the rain and moved in the rain or in gaseous form to the land surface. In the soil most of the sulphur is in organic form as part of soil organic matter. Here it undergoes, as nitrogen does, oxidation and reduction reactions which transform it into states from which it can be taken up by plant roots or by soil animals. Eventually a part will be returned to the atmosphere.