sync: date=May 2007

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 "manure" 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.

It is believed that organic agricultural methods are more environmentally friendly and better maintain soil organic matter levels. However, there are no generally accepted scientific studies that support this supposition. Regardless the source, fertilization results in increased unharvested plant biomass left on the soil surface and crop residues remaining in the soil. Too much of a vital nutrient can be as detrimental as not enough. "Fertilizer burn" can occur when too much fertilizer is applied, resulting in a drying out of the roots and damage or even death of the plant. Organic fertilizers are just as likely to burn as inorganic fertilizers. If excess nitrogen is present the plants will begin to exude nitrogen from the leafy areas. This is called guttation.

History

While manure, cinder and ironmaking slag have been used to improve crops for centuries, the use of fertilizers is arguably one of the great innovations of the Agricultural Revolution of the 19th Century.

Key people

In the 1730s, Viscount Charles Townshend (16741738) first studied the improving effects of the four crop rotation system that he had observed in use in Flanders. For this he gained the nickname of Turnip Townshend.

Chemist Justus von Liebig (18031883) contributed greatly to the advancement in the understanding of plant nutrition. His influential works first denounced the vitalist theory of humus, arguing first the importance of ammonia, and later the importance of inorganic minerals. Primarily his work succeeded in setting out questions for agricultural science to address over the next 50 years. In England he attempted to implement his theories commercially through a fertilizer created by treating phosphate of lime in bone meal with sulfuric acid. Although it was much less expensive than the guano that was used at the time, it failed because it was not able to be properly absorbed by crops.