Phosphorus and potassium fertilization tips
Fabian Fernandez, University of Illinois, offers tips to help answer some of the common questions about fall phosphorus and potassium applications.
Placement. Should P and K be incorporated in the soil with tillage, left on the surface, banded on the surface, or banded deep (4-8 inches) below the surface? All the research I and others have done in the Corn Belt indicates that, for the most part, how you apply P and K does not matter; what is important is that you do apply it, or that you confirm that test levels are adequate for crop production. Deep banding is sometimes suggested because "it makes the nutrient more available," but we have not seen evidence for that being the case. The only time researchers have seen an advantage to deep placement is when soil test levels are low. Concentrating the fertilizer in a band could allow the plant to take P and K from a "hot spot," and the fertilizer has less chance to be fixed in the soil.
However, with adequate test levels, and due to the characteristics of most soils in Illinois, nutrient fixation is not a big problem in the state. Sometimes matching the fertilizer band with the planter provides a crop effect similar to starter fertilizer. Of course, two factors are important to keep in mind: starter effects are not always seen (they normally occur with early planting or when soil conditions are cool and/or wet), and a starter fertilizer effect is more often aesthetic--the crop "looks good" early on, but often that doesn't translate to greater yield.
The use of strip-tillage is increasing in many parts of Illinois. While it is very easy to apply P and K during the strip-till operation, it is not a "requirement" to deep-band the fertilizer then. Some people find it faster and more convenient to broadcast the fertilizer beforehand. One advantage we have seen with deep placement of P is being able to, over time, lower P test levels in the surface layer of the soil without reducing overall fertility of the field.
This reduction can help in minimizing potential P runoff from fields and possibly reducing the negative impact of P loading into bodies of water. Something to keep in mind, though, is that with deep placement of fertilizer, traditional sampling approaches to determine soil fertility might not work as well. I would say that while we have not seen yield advantages to deep placement of fertilizer with strip-till, we have seen a yield advantage compared to a strict no-till system due to the tillage effect. One final point is that if you plan to strip-till this fall, it is better to wait until at least mid-October. Normally by then the heavy rains have passed, and there is less chance for the berm created during the tillage operation to become too mellow by spring.