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Microbiology => Nitrogen Fixation => Fertilizer
Fertilizer
Fertilizer, natural or synthetic chemical substance or mixture used to enrich soil so as to promote plant growth. Plants do not require complex chemical compounds analogous to the vitamins and amino acids required for human nutrition, because plants are able to synthesize whatever compounds they need. They do require more than a dozen different chemical elements and these elements must be present in such forms as to allow an adequate availability for plant use. Within this restriction, nitrogen, for example, can be supplied with equal effectiveness in the form of urea, nitrates, ammonium compounds, or pure ammonia.
Virgin soil usually contains adequate amounts of all the elements required for proper plant nutrition. When a particular crop is grown on the same parcel of land year after year, however, the land may become exhausted of one or more specific nutrients. If such exhaustion occurs, nutrients in the form of fertilizers must be added to the soil. Plants can also be made to grow more lushly with suitable fertilizers.
Of the required nutrients, hydrogen, oxygen, and carbon are supplied in inexhaustible form by air and water. Sulfur, calcium, and iron are necessary nutrients that usually are present in soil in ample quantities. Lime (calcium) is often added to soil, but its function is primarily to reduce acidity and not, in the strict sense, to act as a fertilizer. Nitrogen is present in enormous quantities in the atmosphere, but plants are not able to use nitrogen in this form; bacteria provide nitrogen from the air to plants of the legume family through a process called nitrogen fixation. The three elements that most commonly must be supplied in fertilizers are nitrogen, phosphorus, and potassium. Certain other elements, such as boron, copper, and manganese, sometimes need to be included in small quantities.
Many fertilizers used since ancient times contain one or more of the three elements important to the soil. For example, manure and guano contain nitrogen. Bones contain small quantities of nitrogen and larger quantities of phosphorus. Wood ash contains appreciable quantities of potassium (depending considerably on the type of wood). Clover, alfalfa, and other legumes are grown as rotating crops and then plowed under, enriching the soil with nitrogen.
The term complete fertilizer often refers to any mixture containing all three important elements; such fertilizers are described by a set of three numbers. For example, 5-8-7 designates a fertilizer (usually in powder or granular form) containing 5 percent nitrogen, 8 percent phosphorus (calculated as phosphorus pentoxide), and 7 percent potassium (calculated as potassium oxide).
While fertilizers are essential to modern agriculture, their overuse can have harmful effects on plants and crops and on soil quality. In addition, the leaching of nutrients into bodies of water can lead to water pollution problems such as eutrophication, by causing excessive growth of vegetation.
The use of industrial waste materials in commercial fertilizers has been encouraged in the United States as a means of recycling waste products. The safety of this practice has recently been called into question. Its opponents argue that industrial wastes often contain elements that poison the soil and can introduce toxic chemicals into the food chain.
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