As with nitrogen, using the right product, at the right time, and at measured rates of application maximizes plant use of the fertilizer and minimizes the risk of phosphorus leaching or runoff. This requires considerations of soil and plant issues, as well as other sources of phosphorus that may need to be considered.
- Phosphorus fixation increases with increasing clay content in the soil. The larger amount of surface area associated with clayey soils and the Al-Fe minerals in the lattice help adsorb more P than other soils. In calcareous soils, the adsoption is associated with calcium carbonate (CaCO3).
- Larger fertilizer additions are required to maintain a level of plant available P in finer soils compared to that in coarser, sandy soils. The risk of leaching P is highest in sandy soils.
- The rate of biological activity, and therefore P mineralization, increases with increasing temperatures. Fertilizer applications should only be applied to active soils when soil temperatures are above 50°F.
- Liming acid soils increases the P solubility in acid soils, but over-liming can reduce P solubility. Sorption also occurs to calcium cations (Ca2+) but only at pHs up to 6.5. At higher pH values, Ca-P precipitates form.
- Incorporating P into the soil when possible increases adsorption and reduces the amount of plant available P. Broadcasting P fertilizer on the surface leaves the fertilizer susceptible to runoff.
- Returning clippings to the turf is a practical method of returning organic P back to the soil. Clippings may account for 0.10 to 0.35 lbs P per 1000 sq ft. If clippings are removed, the loss of P depletes available P for plant uptake.
Other Sources Issues
- Foliar applications at light rates may increase plant uptake. Unabsorbed foliar P, however, remains at risk for episodic losses due to runoff caused by heavy precipitation or excessive irrigation. A light irrigation after P fertilizer application has been shown to reduce P runoff.
- Phosphanate fungicides are chemically different from phosphanate fertilizers in that the fungicide provides a phosphite ion (H2PO3–) having one less oxygen atom. Potassium phosphite, also labeled as mono and di-potassium salts of phosphorus acid, Aliette, and Chipco Signature are the most common examples of a phosphanate fungicides. No evidence suggests that the phosphite ion is used in the plants metabolism. Regardless, the amount of P supplied in any fungicide application is negligible.