This time of year I still get questions about Phosphorus and P containing fertilizers. It is a complex issue and a very important one as it is a costly input that is needed for top yields of most everything we grow around here. The big questions are how much and where to apply, what kind. Answers to these questions affect your bottom line. First thing that needs to be understood is how plant take up P. Basically the plant uses phosphate ions H2PO4 and HPO4-2. There are a few organic P compounds that can be absorbed with water moving into the plant. Phosphate ions do not readily remain soluble. pH is the major factor in P availability. In our area of the state the pH levels tend to run higher. In a perfect world a range of 6.5 to 7.0 is preferable. However our heavier soils often run 7.2 plus. Fortunately, our soil test levels tend to run on the higher side for most fields that I pull samples, from years of P fertilization. The form of P that plants utilize is also readily tied up in soils especially on either end of the pH scale. For us it is calcium, it goes from mono-calcium to di-calcium to tri-calcium phosphate the latter being the most insoluble.
Now some claim that a particular P fertilizer cannot be readily tied up in soil because of the formulation. Poly-phosphates will not tie up, nor are they plant available. As time goes by they are converted to ortho-phosphate which can be tied up within hours depending upon pH of any particular soil. I’ve yet to find or hear of a “magic bullet” when it comes to P fertilizers. A pound of P is a pound of P. As we consider overall fertility management, depending upon current levels, we can add, replace, or draw down P levels in our fields. If we need to maintain then what we take out in grain or root or forage needs to be replaced at some time. And that time can fluctuate depending upon those current levels. In general I like to see at least 40 ppm of P on a soil test. With that level, the probability of response to applied P is very low. Therefore P will not be a limiting factor for yield. In the spring we need adequate moisture and good soil temperatures for the life in our soils to mineralize P for our plants. This is a real simple explanation to a much more complex topic.