Forestry and agriculture
There is often a very close relation ship between the natural vegetation and the nutrient status of the soil. Nutrient cycles under forest are a case of this. Nutrients in forest soils arise from a number of sources, including weathering of rocks in and below the soil and removal of some nutrients from the atmosphere. These nutrients are integrated into the plant-soil interactions that form the basis of plant growth. An excellent example of this is the Amazon rainforest where despite the luxurious vegetation system above ground, there is only a thin layer of organic matter beneath it; but this contains virtually all the nutrients that are needed to support the forest. The cycle between this thin organic-rich layer and this amazing vegetation cover above it that it helps to sustain is one of the marvels of nature. If this precious organic layer is interfered with, as in deforestation, this marvellous cycle is broken creating then problems for the forest and its life.
Agriculture is an example of a land use system which interrupts the natural nutrient cycles in that instead of the nutrients being returned to the soil from the plant when it dies, most of the plant is removed from the field in which it is grown. This means that most of the nutrients are not returned to the soil but exported to other situations, for example to provide food for humans. Growing crops thus alters the natural nutrient cycle. If farm crops continued to be grown without putting back nutrients into the soil, the soil would become depleted of nutrients and yields of crops would decline. The farmer therefore has to compensate for the loss of nutrients by adding supplies of fertiliser which contain a range of nutrients, i.e. farming enters the nutrient cycle.
Humans have had important effects on the nutrient cycles, especially since the Industrial Revolution in the 18th century. In order to grow sufficient food to meet the growing population fertilisers and manures have been added to the soil, often in large amounts. Only some parts of these are used by the crop, the surplus is often washed out and moves into the water courses where it can create numerous environmental problems. It is important to have a good understanding of the natural nutrient cycles, to learn from them, and to know the principles of these in soils as far as possible.