Phosphorus is one of the three primary plant nutrients. It is used in all phases of plant growth, and in all parts of the plant. In the soil, phosphorus is taken up by plants in the form of phosphate. Phosphate compounds have two qualities that govern phosphate use by the plant. One of these qualities is that only a tiny amount of phosphate dissolves at any time in the soil water. Because the part that does not dissolve cannot be washed from the soil, phosphorus lasts for a long time in the soil. One application lasts for months or years. This also means that plants must have an extensive root system to get enough phosphate from this dilute soil solution. The other quality of phosphate is that it is very active chemically, and quickly forms compounds with many chemicals. Because of this quality, phosphate does not move through the soil. Therefore, phosphorus fertilizers should be placed in the root zone of the plant. Southern and eastern soils are often deficient in phosphorus. Rock phosphate: Most phosphorus fertilizers begin as phosphate rock. This material is almost insoluble, and only slightly available to plants.
Superphosphate: Superphosphate is phosphate rock that has been treated with acid to make it soluble. In this form, the phosphate remains soluble for a few weeks or months in the soil.
Bone meal: This organic form of phosphate is made of ground bones. Chemically, it is similar to rock phosphate, and is almost insoluble.
