Phosphorus is one of three primary nutrients needed by turfgrasses as a regular fertilizer addition. Although it is present in small amounts in turfgrass tissues (0.3–0.55 percent on a dry weight basis), phosphorus is extremely important for rooting, seedling development, cell division, and the synthesis of various compounds used by plants. Phosphorus is available to turfgrasses as H 2 PO 4 - and HPO 4 = and is mobile in plants (meaning that it can move from one portion of the plant to another).
Phosphorus deficiencies in turf are usually expressed in the early stages of seedling development, appearing as a purple or red coloring of leaf blades and as reduced growth and tillering.
Phosphorus is present in inorganic and organic forms in mineral soils, and both are important sources for plants. Although the total amount of phosphorus in soils can be large, much is unavailable to turf because it forms insoluble complexes with other elements and/or because it is “fixed” to clay particles.
The most important factors affecting phosphorus availability to turfgrasses are soil pH and concentrations of iron, aluminum, manganese, and calcium in soils. In acid soils, the H 2 PO 4- form of phosphorus predominates and combines with iron, aluminum, or manganese to form insoluble compounds that are unavailable to turfgrasses. When the soil pH drops to 5.5 and below, enough phosphorus can be rendered unavailable to cause deficiencies in turf. Also, under acid conditions, some phosphorus can be “fixed” by silicate clays, resulting in reduced availability to plants.
In high-pH soils, HPO 4 = is the most common form of phosphorus. In these soils phosphorus combines with calcium to form insoluble calcium phosphates. As the soil pH approaches 8.0 or above, significant amounts of phosphorus are unavailable to turfgrasses. Maximum amounts of plant-available phosphorus (both inorganic and organic forms) are obtained by keeping the soil pH between 6.0 and 7.0.
Phosphorus can be supplied to turf as inorganic and/or natural organic fertilizers. Inorganic phosphorus fertilizers include superphosphates and ammonium phosphates and are manufactured by treating rock phosphate with various acids. Natural organic fertilizers typically contain phosphorus derived from plant or animal by-products. These fertilizers can contain as much as 13 percent phosphorus.
Phosphorus is largely immobile in soils—meaning that it takes a long time to move from the turf surface into the root zone. Phosphorus may take weeks or months to move just a few centimeters in soil. Because of its poor mobility, phosphorus should be incorporated into the soil prior to seeding or sodding at the amount recommended on your soil test report. Apply the phosphorus to the surface, then incorporate it 4–6 inches deep with a rototiller so that developing roots can use the fertilizer. On established turf, some phosphorus can be incorporated into soil either just before or just after cultivating with a core aerator. Perhaps the best approach to phosphorus fertilization of established turf is to soil test every three years to monitor your phosphorus levels and to use phosphorus-containing fertilizers periodically to maintain adequate levels.
Phosphorus, along with nitrogen, is one of the major nutrient sources contributing to surface- and ground-water pollution in the United States. Although phosphorus is not readily leached from turf soils into groundwater, recent studies of phosphorus fate on cropland have shown that this nutrient can enter surface waters via erosion and runoff. Avoid applying phosphorus fertilizer where runoff is likely—such as on frozen soils and paved surfaces.