Phosphorus

Phosphorus (P) is a critical nutrient for turfgrass growth and development, playing important roles in energy transformations in plant cells and root development. Therefore, P enhances turfgrass establishment and is the most important nutrient in ‘starter fertilizers’. On soils low in P, most of the enhanced establishment is from the (N). Phosphorus management is focused on maximizing plant response to supplemental phosphorus, when required as based on soil test results, while minimizing offsite movement.

In the soil, P is generally in complex with other elements and is an insoluble (plant unavailable) nutrient. Phosphorus is slowly made available to plants on an ‘as needed’ basis by chemical reactions in the soil that convert it to either of two anionic forms, hydrogen phosphate (HPO₄^2-) or dihydrogen phosphate (H₂PO_4-).

Phosphorus cycle.

 
A soil is considered to have a phosphorus deficiency if it is at or below the medium sufficiency level. Research has often found that turfgrass shows signs of distress at P levels of 5 to 11 ppm (Mehlich III), a range considered Low or Very Low. The medium sufficiency ratings for each test method are shown in the table below.

Medium sufficiency levels by test method. Source: http://nmsp.cals.cornell.edu/software/Morganequiv7.xls

 
Testing labs provide recommendations for the amount of phosphorus fertilizer needed to correct the deficiency. In the table below recommendations are made separately for fertilizing established turfgrass or for pre-plant fertilization to establish a new stand of turf with either seeded or sodded turfgrass.

Phosphorus fertilizer recommendations for turfgrass (Petrovic 2012)