Phosphorus is a highly reactive element, bonding quickly with a wide variety of other elements. In the soil, it is usually combined with oxygen to form phosphate. Phosphate combines with calcium in alkaline soils and with iron and aluminum in acid soils to form almost insoluble compounds. Because phosphorus compounds are usually insoluble, they are very stable in the soil. Unlike nitrogen and potassium fertilizers, phosphorus fertilizers can be applied in large quantities every few years; they leach very slowly from the soil.
Because of their low solubility, phosphates are present in the soil water in extremely low concentrations, only two or three parts per million. But their presence is constant; as soon as the dissolved phosphate is removed by plants, more dissolves to replace it. As a result, plants with large root systems, such as trees, are able to absorb enough for their needs. But plants with small root systems, especially seedlings, don’t have enough root area to absorb such dilute chemicals.
Mature trees seldom need added phosphorus, even in regions that have phosphorus-deficient soils. But seedlings almost always benefit from extra phosphorus. Without large amounts of phosphorus, top growth slows until the root system has grown large enough to absorb the needed amounts. Since absorption by roots slows in cold soils, phosphorus often needs to be added under those conditions too. Winter and early spring crops grow better with added phosphorus.
A phosphorus deficiency shows up first as slowed growth. The leaves become dull and dark green or grayish green. Some plants (such as corn and tomatoes) develop magenta areas on their leaves.