Phosphorus (P) or Phosphorus Pentoxide (P2O5) - Among other important functions, phosphorus provides plants with a means of using the energy harnessed by photosynthesis to drive its metabolism. A deficiency of this nutrient can lead to impaired vegetative growth, weak root systems, and fruit and seed of poor quality and low yield. Soil phosphorus exists in a wide range of forms. Some is present as part of soil organic matter and becomes available to plants as the organic matter decomposes. Most inorganic soil Phosphorus is bound tightly to the surface of soil mineral particles. Warm, moist, well aerated soils at about pH 6.5 optimize the release of both these forms. Plants require fairly large quantities of phosphorus, but the levels of phosphorus available to plant roots at any one time is quite low. Soil tests attempt to assess the soil’s ability to supply phosphorus from bound forms during the growing season. Phosphorus is an important environmental problem when it is washed, usually with eroding soil, into lakes and streams, so excess applications should be avoided. (see Phosphorus and the Environment)
The soil test for phosphorus is called the Bray and Kurtz I or Weak Bray test. The test results are expressed in parts of elemental P per million. This test is a measure of the relative availability of phosphorus for plant growth. The test does not measure the total amount of phosphorus in the soil.
A "below optimum" test indicates that crops would very likely respond to the addition of phosphorus fertilizer. Under low soil test conditions, banding a portion of the recommended phosphate, or using a "starter" fertilizer in or near the row, may be advantageous to ensure enough is available to the small root systems of young plants, early in the season.
A soil testing "optimum" in phosphorus means that crops are likely to respond to phosphate, if growing conditions are favorable for high yields. Crop response means noticeable improvement in growth, health, appearance or productivity.
Soils testing "above optimum" in phosphorus are not likely to produce economic yield increases with an application of additional phosphorus. In fact, it would cost more to fertilize than you would get back in profits from increased yield. When soils test in the optimum range, applying only a small quantity of phosphorus is suggested to maintain the high fertility status.
There is no economic advantage to applying phosphate to soils testing "above optimum". The MSU Soil lab does not suggest applying phosphorus on soils testing very high.
In the spring, when soils remain wet and cold for long periods, crops often show symptoms of phosphorus deficiency. With the onset of warm weather, the crop may grow out of the deficiency. This deficiency is caused by the slow breakdown and release of the organic phosphorus, the slow rate of root growth and the reduced availability of oxygen, which may affect phosphorus uptake. Under these conditions, a small amount of row-applied phosphorus benefits early growth on corn and some other crops more than broadcast phosphorus only.
For annual flowers and vegetable transplants, soluble fertilizers high in phosphorus are often used in the water at transplanting time to make certain there is adequate phosphorus available to the small root systems right away. This would be especially important if soil levels tested below optimum.
This site was last updated 11/18/08