Fertilisers (also spelled fertilizers) are chemical compounds given to plants to promote growth; they are usually applied either through the soil, for uptake by plant roots, or by foliar feeding, for uptake through leaves. Fertilizers can be organic (composed of organic matter), or inorganic (made of simple, inorganic chemicals or minerals). They can be naturally occurring compounds such as peat or mineral deposits, or manufactured through natural processes (such as composting) or chemical processes (such as the Haber process). These chemical compounds leave lawns, gardens, and soils looking beautiful as they are given different essential nutrients that encourage plant growth.
They typically provide, in varying proportions, the three major plant nutrients (nitrogen, phosphorus, potassium: N-P-K), the secondary plant nutrients (calcium, sulfur, magnesium) and sometimes trace elements (or micronutrients) with a role in plant or animal nutrition: boron, chlorine, manganese, iron, zinc, copper, molybdenum and (in some countries) selenium.
Both organic and inorganic fertilizers were called “manures” derived from the French expression for manual tillage, but this term is now mostly restricted to organic manure.
Though nitrogen is plentiful in the earth’s atmosphere, relatively few plants engage in nitrogen fixation (conversion of atmospheric nitrogen to a biologically useful form). Most plants thus require nitrogen compounds to be present in the soil in which they grow.