Phosphorus fertilizer considerations for fall 2014

Farm Forum

The recent cool weather has made it clear that fall is fast approaching. The drop in commodity prices will likely cause a few conversations among farmers, consultants, and retailers on what fertilizer to apply for the 2015 cropping year. Many fields are currently being soil sampled for phosphorus (P), this fall is a good time to consider what is actually out in the field to best target P fertilizer applications.

If taken properly, a soil sample can aid in determination of the responsiveness of a crop to a given nutrient. Categories such as low, medium, and high, give a relative estimate of the soil’s ability to fully satisfy the needs of a given crop. For example, a soil testing low in P would have a very low probability of providing sufficient P while a soil testing high in P would have a high probability of supplying the full crops’ needs. Knowledge of the probability of response of a given crop is the first step in determining if it is economical to apply fertilizer in a particular field.

We are currently wrapping up a five year research project funded by the Minnesota Agricultural Fertilizer Research and Education Council (AFREC) that was established to begin to develop a database on P response for corn and soybean in Minnesota. An example of corn response to P is given in Figure 1. Included are two factors, first the probability of encountering a measurable yield response and second the average magnitude of response based on the current soil test P categories used in Minnesota. One important consideration when viewing these data is that a measurable yield response does not mean it is economically feasible to apply fertilizer. Taking into account the magnitude of response is also critical in factoring in where fertilizer should be applied for the best return.

The key point is that the probability of response is never zero, but the yield increase may not be large enough to pay for the cost of the fertilizer purchased. Phosphorus in starter fertilizer is a different story and must be a separate consideration. However, the potential response to starter is difficult to predict and is less likely with high soil test P values.

As we move forward with research, additional data will be added to the probability dataset. At this time the corn database is more advanced than for soybean, but additional soybean data will be completed this fall. Past research has shown that soybean is not as responsive to P as corn. However, research has shown that applying P to soybean in Low testing areas of fields can greatly increase the profitability of a corn-soybean rotation.