This month, a Missouri farmer emailed us a burning question he had about P & K fertility, so we asked a few experts.
Regarding phosphorus (P) and potassium (K) availability, how much more available is fall versus spring applied? Secondly, if you apply your soybean fertilizer with corn, and beans are planted following the corn, is that P & K more available than fall applied before bean planting?
Paul E. Fixen, Ph.D., Senior Vice President, Americas and Oceania Group and Director of Research, International Plant Nutrition Institute, replies:
Many studies address aspects that pertain to a specific situation but none offer a broad conclusion. Based on what weknow about nutrient behavior, we would not expect much difference between fall and spring application of P and K.
In the past we have speculated that tillage may be a factor. In no-till where P and K are topdressed, some extra time for the P and K to move downward, as occurs with a fall application, may be advantageous. However, where tillage incorporates the fertilizer, I think you can expect minor differences.
Another factor is that certain soybean varieties have increased sensitivity to the chloride in muriate of potash. A fall application offers some time for chloride to dissipate from the fertilized zone, which would not occur with spring application to soybeans. This is a bigger issue with high application rates and where soil internal drainage is limited.
T. Scott Murrell, Ph.D., Director, North Central U.S. International Plant Nutrition Institute (IPNI), replies:
This is a very complex question. So much depends on soil mineral composition and acidity when it comes to P and K reactions in soils after application. Co-application of other materials, like lime, can also affect availability.
The form of fertilizer applied can be important, too. For instance, P applied as an ammonium form can increase the availability of P while the ammonium is present in the same band at the time the plant root is actively taking up nutrients, as in a spring application of starter fertilizer.
The science would not support a definitive answer that fits all situations. The answers and factors at play will be different for the two nutrients.
Dan Froehlich, Manager New Product Development, Mosaic, replies:
There are really no issues around spring versus fall applied K as long as you are not on soils that are extremely sandy (CEC less than 10). K can get tied up between the clay layers when the soil dries, but most of it is released when moisture causes them to swell again.
P is a different animal. It can be tied up very quickly in soils that have high pH. This fixation is difficult to avoid and is one of the reasons people band their P when they have high-pH soils and their soil P level is close to the critical level. We usually like to put the P on closer to when the plant will take it up from the soil. The longer it sits in the soil, the better chance it has to be fixed.
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