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Soils - Part 6: Phosphorus and Potassium in the Soil

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Summary

Soil phosphorus is relatively stable in soil. It moves very little when compared to nitrogen. In fact, this lack of mobility is due to the rather limited solubility of soil phosphorus compounds. Because of the limited solubility of these compounds, fertilizer phosphorus will become much less available as it reverts back to soil phosphorus compounds. Fertilizer phosphorus that reverts back to soil phosphorus compounds is not lost completely, but becomes slowly available to crops over several years. The rate depends greatly on soil type. For most Nebraska soils, applying more fertilizer phosphorus than needed for optimum yields is probably not economically justified.

Phosphorus availability is controlled by three factors: soil pH, amount of organic matter, and proper placement of fertilizer phosphorus. Acid soils should be limed to bring soil pH up to nearly 6.5. The pH of alkaline soils (over 7.0) probably cannot be practically lowered for better phosphorus availability.

Organic matter maintenance is an important factor in controlling phosphorus availability. Mineralization of organic matter provides a steady supply of available phosphorus. Organic soil phosphorus may represent 30-40 percent of the phosphorus available to Nebraska crops, and may be a major factor affecting phosphorus availability during wet, cold springs.

Placement of phosphorus is the best practical control of phosphorus availability. Placing phosphorus with seed wheat has given much better results than broadcast applications. Banding phosphorus two inches to the side and two inches below the seed of row crops provides a ready source of phosphorus for the young seedling; however, soil phosphorus must be deficient before yields can be expected to be increased.

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