Natural and Artificial Fertilisers

In a natural ecosystem, minerals taken up by plants are returned to the soil when organisms die or produce waste and saprobionts decompose them. On a farm this cycle is broken. Crops are harvested and livestock are removed for sale, so the nitrates and phosphates they contain are taken away. Without replacement, the soil gradually loses these ions and yields fall.Fertilisers are added to replace the nitrates and phosphates lost. There are two types.Natural (organic) fertilisers are made of dead and decaying remains or animal waste, for example manure, slurry, compost, crop residues and sewage sludge. The nutrients are mostly held in organic molecules, so saprobionts must first decompose them. This releases the ions slowly. Natural fertilisers are cheap on farms with livestock and add organic matter that improves soil structure, but their exact mineral content varies and is not known.Artificial (inorganic) fertilisers are mined from rocks or manufactured, for example ammonium nitrate. They usually contain nitrogen, phosphorus and potassium (NPK) in known proportions, so the farmer can supply exactly the ions a crop needs. The ions are soluble, so they are available to plants immediately, but they cost more to buy and are more easily washed out of the soil.Many farmers use a combination of both types.