Field Evaluation of "Enhanced" Phosphorus Fertilizers.

Phosphorus fertilizer trials began centuries ago shortly after it was discovered that this element was essential for plant growth. Heavy debate regarding the proper experimental approach began immediately. Through time, scientists discovered proper principles of experimental design, conducting trials, and statistical analysis. Recent so-called meta-analysis evaluations have highlighted fundamental problems in how some current research is being conducted and analyzed. A review of recent phosphorus fertilization studies for six relatively new products with claims of enhanced efficiency for plant phosphorus nutrition will be presented. The most widely studied of these products is a polymer additive with claims of binding cations known to form poorly soluble complexes with phosphorus. A recently published meta-analysis of a wide variety of studies showed an average of no response or benefit for this product. However, further analysis of nearly 500 field comparisons showed an average yield increase of 2.1% when averaged over all crops and conditions. Interestingly, a large percentage of these studies were conducted under conditions which violate the tenets of basic soil fertility experimentation. A majority of the studies were conducted in soils with high to very high residual soil phosphorus or at rates of fertilization far above prudent levels or under other conditions where additive response to increased P solubility is unlikely. Further evaluation including only sites likely to show response to phosphorus fertilizer resulted in an average yield increase of 5.5%. Evaluation of other specialty phosphorus fertilizers shows similar results—that yield increases are not likely when plants already have ample phosphorus. Responses are much more likely when soil pH is not conducive to phosphorus solubility, when soil test phosphorus is low, and when the rate of phosphorus fertilizer applied is relatively low.