Phosphorus (P) is essential for plant growth. It stimulates growth of young plants, giving them a good and vigorous start. Phosphorus management and nutrition has both economic and environmental implications.
Phosphorus Chemistry in Soils
Phosphorus exists in soils in organic and inorganic forms. Organic forms of P are found in humus and other organic material. Phosphorus in organic materials is released by a mineralization process involving soil organisms. The activity of these microbes is highly influenced by soil moisture and temperature. The process is most rapid in warm, well-drained soils. Research on Mississippi soils has shown that 1 percent of the total soil organic phosphorus is mineralized per year during cotton and soybean production. However, since initial levels are low, and plant uptake is only one possible fate of the mineralized phosphorus, the contribution by mineralization to plant available phosphorus is small.
Inorganic phosphorus is negatively charged in most soils. Because of its particular chemistry, phosphorus reacts readily with positively charged iron (Fe), aluminum (Al), and calcium (Ca) ions to form relatively insoluble substances. When this occurs, the phosphorus is considered fixed or tied up. In this regard, phosphorus does not behave like nitrate (NO3-), which also has a negative charge but does not form insoluble complexes.
The solubility of the various inorganic phosphorus compounds directly affects the availability of phosphorus for plant growth. The solubility is influenced by the soil pH. Soil phosphorus is most available for plant use at pH values of 6 to 7. When pH is less than 6, plant available phosphorus becomes increasingly tied up in aluminum phosphates. As soils become more acidic (pH below 5), phosphorus is fixed in iron phosphates. Some soils in the upland Prairie areas of Mississippi have pH values greater than 7. When pH values exceed 7.3, phosphorus is increasingly made unavailable by fixation in calcium phosphates.
Because most Mississippi soils are acidic, phosphorus fixation is dominated by Al and Fe compounds. A regular soil testing schedule, and liming program, should be followed to allow maximum agronomic availability of both native and fertilizer applied phosphorus. The efficiency of phosphate fertilizers is higher if lime is applied to fields prior to phosphorus fertilization.
Phosphorus uptake by plants
Almost all phosphorus is taken up by plants as either of two ions. Because the pH of most Mississippi soils is below 7, uptake is almost totally as the H2PO4- ion. On soils with higher pH, there will be some absorption of the HPO42- ion. Organic forms of phosphorus are not absorbed by plants.
The absolute quantity of these ions present in the soil and available for uptake at any one time is very small. The amount that is dissolved and accessible in the soil solution is in equilibrium with solid phase phosphorus. The solid phase consists of both the organic and inorganic forms in the soil. Crops need more phosphorus than is dissolved in the soil solution to grow economically, therefore this phosphorus `pool' must be replenished many times during the growing season. The ability of a soil to maintain adequate levels in the solution phase is key to the plant available phosphorus status of a soil.
Management of Phosphate Fertilizers
Phosphorus is not mobile within soils, therefore placement of phosphate fertilizers is a major management decision in crop production systems. No ideal special placement exists for all crops. Decisions about phosphate fertilizer placement depend on the intended crop, soil test P level, and environmental considerations.
Phosphorus movement off agricultural land to surface waters can accelerate eutrophication. This is the process in bodies of water of stimulating algal growth which ultimately die and decay in the water, and deplete available oxygen. The reduced oxygen levels ultimately result in reduced higher-order aquatic plant and animal populations. Animal manures and bedding materials contain significant amounts of phosphorus in organic forms.
After microbial mineralization from the organic forms, the phosphorus applied to soils is subject to the same fates as inorganic fertilizer phosphorus. Rates of land application of animal wastes such as poultry litter should be based on soil tests, nutrient content of the material, and crop needs. Directions on obtaining nutrient analysis of manures are available at your local Extension office.
Phosphorus movement in landscapes is intmately associated with soil erosion because P is closely attached to solid soil materials. Phosphorus addition to soils must be managed to lessen movement to surface waters.
Phosphorus needs of plants are most critical in the earliest growth stages. If the pH is between 6 and 7, and the soil has a low risk of erosion, P can be applied in the fall for cotton or grain production.
Frequently Asked Questions
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