Phosphorus concentration in plant leaves ranges between 0.2 and 0.4 percent (Walsh & Beaton 1972). Phosphorus is important for plant growth because of its role in ribonucleic acid (RNA), the plant cells genetic material, and its function in energy transfer with adenosine triphosphate (ATP).
Phosphorus is available for absorption by plants from the soil as the orthophosphate ion (H2PO4 and HPO4). These ions react quickly with other compounds in the soil to become much less available for plant uptake. The presence of aluminum, iron, calcium, and organic matter links phosphorus in highly insoluble compounds. The concentration of orthophosphate ion in the soil solution is very low, less than 0.05 mg/L, so an equilibrium is established between the soluble ion and the adsorbed form in the soil.
Phosphorus immobility in soils is caused by several factors: presence of hydrous oxides of aluminum and iron; soils that have a high clay content, especially ones high in kaolin; soils high in volcanic ash or allo-phane; low or high soil pH; and high exchangeable aluminum. Of these factors, the one most easily manipulated is soil pH. Maintaining a soil pH between 6.0 and 6.5 achieves the most plant available phosphorus in a majority of soils. Knowing the extent each of the factors are at work in a particular soil gives the upper limit at which phosphorus loading can occur in the soil before soluble phosphorus leaching from the soil becomes a serious water quality concern.
The relative immobility of phosphorus in the soil profile allows some agricultural waste to be applied in excess of the crop's nutrient needs, resulting in a soil phosphorus residual. Building a soil phosphorus residual can be beneficial in soils that readily fix phosphorus into an insoluble, unavailable form for plant uptake. This phosphorus reservoir, if allowed to rise, gives a corresponding rise in the soluble phosphorus content in the soil. This addition of total phosphorus has to be tempered with some restraint.
Manure applications can actually increase phosphorus leaching because organic phosphorus is more mobile through the soil profile than its inorganic counterparts. This would be particularly true on coarse textured soils that have a low cation-exchange capacity and low content of iron, aluminum, and calcium.
High phosphorus application rates appreciably increase the phosphorus concentration in the soil solution and availability for plant uptake into plant tissue, but this phosphorus rarely becomes toxic to the plant. Phosphorus toxicity depends on the plant species, phosphorus status of the plant, concentration of micronutrients, and soil salinity. Poor growth in plants that have high phosphorus levels can cause reduced nodulation in legumes, inhibition of the growth of root hairs, and a decrease in the shoot to root ratio (Kirkham 1985).
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