PHOSPHORUS (P) 30.79 Atomic weight
(Primary Nutrient)
Phosphorus is not found free in nature, except in a few meteorites. It does occur abundantly in compounds that are widely distributed in rocks, minerals, plants and animals. It ranks twelfth in abundance among the elements in the earth’s crust, representing 0.1%.
Phosphorus is absorbed by plants as H2PO4-, HPO4--, or PO4--, depending on soil pH. Most of the total soil phosphorus is tied up chemically in compounds of limited solubility. In neutral to alkaline soils, Calcium phosphate is formed, while in acid soils, Iron and Aluminum Phosphates are produced. Phosphorus is present in all living cells.
Phosphorus is utilized by plants as an integral element of nucleic acids (DNA and RNA). It is used in the storage and transfer of energy through energy-rich linkages (ATP and ADP). Phosphorus stimulates early growth and root formation. It hastens maturity and promotes seed production. It also helps in the formation of all sugars, oils, and starches. It helps in the transformation of solar energy into chemical energy, and in plant maturation. It is beneficial in the plant’s ability to resist stress.
Phosphorus supplementation is required most by crops under these circumstances: (1) growth in cold weather, (2) limited root growth, and (3) fast top growth.
Phosphates in Soils
Several factors influence the availability of phosphates to the plant. Soil pH influences the ionic character of phosphorus. At a low pH, P is principally in the H2PO4 form. At intermediate soil pH levels, H2PO4 is predominant and at higher values P3O4 is present. Plants can absorb all of these forms, but this ionic nature of phosphorus also influences the way it binds to soil colloids. Anions are strongly attracted to and absorbed by soil particles, Phosphates, at low soil pH, can form insoluble and unavailable salts with iron and aluminum cations.
In soils with increased Calcium and a high pH, CaCO3 can greatly reduce the plants available phosphate.
Organic matter contains large amounts of the total soil phosphate but these are not readily taken up by plants unless there is first an enzymatic cleavage of the Phosphate bond. This cleavage is achieved by the enzyme Phosphatase which is produced by microorganisms.
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