Plant Nutrients and Fertilizers in Soil

Plant biomass is composed largely of carbon, hydrogen, and oxygen, which plants extract from water, and atmospheric carbon dioxide. Other nutrients that plants require in relatively large quantities are calcium, magnesium, and sulfur, which are usually present in sufficient abundance in soU, and nitrogen, phosphorus, and potassium, which are commonly added to soil as fertilizers. 

Natural processes usually do not produce sufficient nitrogen to allow maximum plant growth so that artificial means are used to extract nitrogen in a chemically combined form from the atmosphere. This is done by the Haber process, which combines elemental Nj and Hj over a catalyst at very high pressures of about 1000 times atmospheric pressure and an elevated temperature of 500°C. The reaction is 

Anhydrous ammonia is a corrosive toxic substance. It can cause frostbite when it evaporates from exposed skin. Ammonia is a potent skin corrosive and harms exposed eye tissue. Because of its high water solubility, inhaled ammonia is absorbed by the moist tissues of the upper respiratory tract. Inhalation of ammonia causes constriction of the bronchioles and can cause lung edema (fluid accumulation) and changes in lung permeability. Anhydrous liquid ammonia stored in tanks has been a favorite target of operators of illicit meth labs in rural areas who use it in the synthesis of methamphetamines. Certainly, many of the thieves are harmed by exposure to ammonia, but they rarely seek treatment for the injury. 

Phosphorus is an essential plant nutrient required for cellular DNA and other biomolecules. It is utilized by plants as H2PO4 and HPO ions. Phosphate minerals that can be used to manufacture phosphorus-containing fertilizers occur in a number of places throughout the world. In the United States, Florida has especially abundant phosphate resources, largely as fluorapatite, Ca5(P04)3F, as well as hydroxyapatite, Ca5(P04)30H. These phosphate minerals are too insoluble to serve directly as fertilizers and are treated with phosphoric acid and sulfuric acid to make superphosphates that are much more soluble and available to plants  

Plants require several micronutrients, largely elements that occur only at trace levels, for their growth. These include boron, chlorine, copper, iron, manganese, molybdenum (for N fixation), and zinc. Some of these are toxic at levels above those required for optimum plant growth. Most of the micronutrients are required for adequate function of essential enzymes. Photosynthetic processes use manganese, iron, chlorine, and zinc. Since the micronutrients are required at such low levels, soil normally provides sufficient amounts. 

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