Now how about Phosphorus
From: Karl Danneberger
Phosphorus fertilization has garnered considerable press lately. In Minnesota for example phosphorus applications are banned because of levels found in surface water (whether this is due to lawn fertilization or not is irrelevant at this point). In a few other communities phosphorus can only be applied if soil test results show phosphorus to be low. On golf courses, phosphorus levels are monitored through soil test reports, however for the lawn care market this is much more difficult to do on a large geographical area.
Phosphorus is a major element that is especially critical during turf/seedling establishment. In mature stands phosphorus is critical component important in the energy (ATP) required for growth. Deficiencies show as a slowing of growth and eventually phosphorus deficient symptoms can appear (bluish color leaf blades). A proper balance between soil phosphorus levels and that needed for growth is both environmentally sound and necessary for maintaining quality turf.
Dr. Wayne Kussow at the University of Wisconsin wrote a recent article (Phosphorus fact or fiction Landscape Management 42:56-58,60,62) where he mentions 2 general hints that might help you plan your phosphorus program. The first is major nutrient analysis of leaf clippings remains rather constant for turfgrasses. The ratio of nitrogen (N), phosphorus (P2O5) and potassium (K2O) roughly falls in a 4:1:3 ratios. By using a fertilizer with an N: P2O5 ratio of 4:1 you will roughly maintain the current soil test levels of phosphorus. If you use a ratio less than 4:1 such as 2:1 or 3:1 you will contribute to the phosphorus levels in the soil. On the other hand if you use a fertilizer ratio greater than 4:1 like 8:1 or 10:1 you will slowly deplete the phosphorus in the soil. You can either build-up, maintain, or deplete phosphorus levels over time by the fertilizer you use.
In addition, from research at the University of Wisconsin, it takes roughly 1 lb. P2O5/1000 sq.ft. /year to maintain soil test levels when clippings are removed. If clippings are returned that amount drops in half to 0.5 lb. P2O5/1000 sq.ft. /year. Phosphorus levels in the soil are impacted by climate, growing season, and soil texture to name just a few. However, the 4:1 ratio and 1 lb rule for clippings removed is a good base to start designing a phosphorus fertility program.