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Phosphorus (P) is an essential macronutrient that all plants need in relatively large quantities. The amount of P fertilizer needed by turfgrass is usually significantly less than nitrogen or potassium. However, P is particularly important during early grass seedling growth and development stages.  Phosphorus plays a role in establishment, rooting, maturation, growth, and reproduction of plants. Plants can extract the relatively immobile soil P as dihydrogen phosphate (H₂PO₄^-) or hydrogen phosphate (HPO4^-2). The terms available phosphate, available phosphorus, available phosphoric acid, and P₂O₅may be used to refer to phosphorus fertilization.

While P is an important nutrient for grasses and other plants, it is also a vital nutrient for algae and weeds in our lake systems. Phosphorus is usually the least abundant nutrient in freshwater lakes, and is often a limiting factor for the growth of algae and weeds. Lake enrichment of P can cause undesirable algal blooms and increased aquatic weed pressure, a process termed eutrophication. A result of eutrophication is an environment unsuitable for many fish and wildlife inhabitants. 

Turfgrass P deficiencies are usually first recognized by stunted growth and reduced seedling vigor. It is unusual to see a P deficiency in a mature plant. In addition to the reduced growth, leaf blades can turn a purple to reddish color. The turf stand will begin to decline in quality, if the deficiency is not addressed.

Most soils in Iowa contain adequate amounts of phosphorus and no additional phosphorus should be used in a fertilizer program unless indicated by a low soil test. A 1.0 lb. of P₂0₅ per 1000sq. ft. is permitted for establishment purposes; however, it is still strongly recommended that this application follow a low soil phosphorus determination.

Recent regulations in Minnesota and Wisconsin restrict residential landscapes phosphorus applications in an effort to minimize environmental threats. While there are no phosphorus restrictions in Iowa, phosphorus should only be applied when a soil test has indicated a need for additional amounts. The Iowa Professional Lawn Care Association (IPLCA) has placed a self-enforced restriction on the use of P fertilizers on lawns surrounding lakes and other waterways. They will use P containing fertilizers in these areas only at the time of establishment. They are also careful to remove all fertilizer from hard surfaces to prevent movement into sanitary sewer systems.

The entire extension publication is attached in pdf form.  To download the publication, click on the following link Phosphorus Publication.