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Soils - Part 6: Phosphorus and Potassium in the Soil

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Importance of Phosphorus to Plants

Phosphorus is a component of the complex nucleic acid structure of plants, which regulates protein synthesis. Phosphorus is, therefore, important in cell division and development of new tissue. Phosphorus is also associated with complex energy transformations in the plant.

Adding phosphorus to soil low in available phosphorus promotes root growth and winter hardiness, stimulates tillering, and often hastens maturity.

Plants deficient in phosphorus are stunted in growth and often have an abnormal dark-green color. Sugars can accumulate and cause anthocyanin pigments to develop, producing a reddish-purple color. This can sometimes be seen in early spring on low phosphorus sites. These symptoms usually only persist on extremely low phosphorus soils. It should be noted that these are severe phosphorus deficiency symptoms and crops may respond well to phosphorus fertilization without showing characteristic deficiencies. In addition, the reddish-purple color does not always indicate phosphorus deficiency but may be a normal plant characteristic. Red coloring may be induced by other factors such as insect damage which causes interruption of sugar transport to the grain. Phosphorus deficiencies may even look somewhat similar to nitrogen deficiency when plants are small. Yellow, unthrifty plants may be phosphorus deficient due to cold temperatures which affect root extension and soil phosphorus uptake. When the soil warms, deficiencies may disappear. In wheat, a very typical deficiency symptom is delayed maturity, which is often observed on eroded hillsides where soil phosphorus is low.

Phosphorus is often recommended as a row-applied starter fertilizer for increasing early growth. University of Nebraska starter fertilizer studies conducted in the 1980s showed early growth response to phosphorus in less than 40 percent of the test fields (Penas, 1989). Starter applications may increase early growth even if phosphorus does not increase grain yield. Producers need to carefully evaluate cosmetic effects of fertilizer application versus increased profits from yield increases.

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