Fertilisers can help the soil, but they can also cause damage. The natural ecosystems of the world have never needed additional fertilisers, so why do humans need it now?

In very primitive societies, the production of food was motivated by hunger. The search for food stopped when the tummy was full. Overharvesting was unknown. As societies became more sophisticated, the reasons for producing food changed. Agricultural societies started to produce food for others. Today's farmers do it for money. They are able to do so because of world trade, a monetary system, transport, subsidies and means of preservation. Let's be honest: food is not produced because someone somewhere else on the planet is hungry. It is not produced 'to feed the world'. The free market system just happens to distribute it efficiently to those who can afford to pay, making it seem so.

Everywhere in the world and over many thousands of years, farming has been a hit-and-miss affair. Land was cleared and farmed. If it failed, the land reverted back to scrub and forest or was lost altogether, leaving the bones of the land, the naked rock, behind. Today's farming is very much the same, but in the meantime we hope to have learnt from some of our mistakes.

Five principal reasons for applying fertiliser

• balancing the soil: to bring the composition of nutrients in a soil up to the mix required by crops, or to add nutrients that are in short supply. By analysing the base rocks from which the soil is weathered, and knowing the requirements of the standing crop or mix of crops over a few seasons, the nutrients in shortest supply can be determined and these can be added in the from of artificial fertiliser. In traditional soil testing, a sample of the A horizon is analysed, rather than the C or B horizons. This is done because the composition of the A horizon, the plough  and root zone, is of immediate interest for this season's production. By analysing the C or B horizons as well, the original nutrients in shortest supply can be detected and added to improve future soil composition. 
• replacing: to replace the nutrients that have been harvested. In small-scale primitive societies, human and animal wastes were returned to the cropland where they originated from, but in large-scale agriculture where the produce is sold and consumed very far away, this can no longer be achieved. Artificial fertiliser is then necessary to maintain the soil's natural fertility. See also the note on traditional farming and waste recycling below.
• rapid response: quick release fertiliser is applied to meet the sudden need of a fast growing monoculture.
• optimising: artificial fertiliser is added to optimise some economic parameter, usually the amount of profit from the operation. Many fertiliser companies define optimal yields without mentioning increased risk of soil and water pollution, and down-slope degradation such as poisonous plankton blooms in the sea. 
• feeding soil organisms: a most important aspect often overlooked is the use of fertiliser to feed soil organisms from the woody substances in it. In the 19th century, Dutch 'mixed farmers' applied up to 20t/ha of manure. Nitrogen can escape from manure in the form of ammonia NH3, a highly volatile form of nitrogen.