Nutrients
Plant roots need to take up nutrient elements from the soil solution usually as positively or negatively charged ions, which must be present in the soil.
Nitrogen is taken up by most plants, other than legumes, in only two forms, as nitrate (NO3-) and ammonium (NH4+) ions. Under conditions in the UK, ammonium ions are rapidly oxidized via microbes in the soil to nitrate ions. These are absorbed by the roots of the plant, eventually forming amino acids and hence, proteins.
In very wet conditions, where oxygen is in limited supply, as in rice paddy fields, most of the nitrogen is taken up as ammonium ions.
The most commonly used nitrogen-containing fertilizers in Western Europe are ammonium nitrate and calcium ammonium nitrate (a mixture of ca 80% ammonium nitrate and 20% calcium carbonate). However, worldwide, about one half of the nitrogen applied as fertilizer is as carbamide (urea) It is the most concentrated solid nitrogen fertilizer (46%N) and is converted rapidly into ammonium ions by an enzyme, urease, which occurs in many soils. All nitrogen-containing fertilizers are made via ammonia and over 150 million tonnes are produced annually.
Phosphorus is usually taken up by most plant roots as the dihydrogenphosphate ion, H2PO4-. Phosphorus can be applied as triple superphosphate, which is water soluble calcium dihydrogenphosphate, Ca(H2PO4)2, or as an ammonium phosphate. The manufacture of these water-soluble phosphates is from phosphate rock (apatite), which contains various forms of tricalcium phosphate Ca3(PO4)2, which is itself insoluble in water. 180 million tonnes of the ore are mined annually, of which 73 comes from Asia and 43 from Africa (mainly Morocco).
Two ammonium phosphates are produced, ammonium dihydrogenphosphate and diammonium hydrogenphosphate. They are particularly useful as they contain both nitrogen and phosphorus. The one chosen for use depends on which ratio of nitrogen to phosphorus is most effective for growth of the plant. All are derived from phosphoric acid which is produced on a massive scale, some 38 million tonnes a year.
Potassium is taken up as the positively charged potassium ion K+ and is usually applied as potassium chloride. Very large amounts, over 50 million tonnes a year, of potash are mined, of which 19 million tonnes are mined in the FSU and 17 in North America.
Sulfur deposition from the atmosphere has decreased rapidly as emissions from industrial processes have declined in response to the requirement for 'clean' air. This is leading to a sulfur deficiency in the soils. Thus many companies now produce fertilizers containing sulfur, added usually in the form of ammonium sulfate or other sulfate salts. However, elemental sulfur is being increasingly used in some fertilizrs.
Solid fertilizers are available in various forms. Fertilizers such as ammonium sulfate and potassium chloride will be as crystals. Some, like urea, may be as small spheres (prills) produced when the molten material is sprayed through a perforated disc and the resulting droplets rapidly cooled and solidified as they fall down a tall tower (a prilling tower). The granules are usually 2-4 mm diameter, a form which is easy to handle and spread.
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