The dry growing season in 2012 has raised several soil fertility questions. In some cases, there has been relatively normal crop production and no need for management changes. In other situations with severely damaged crops, there is potential for adjustments for the 2013 corn crop.
Soybean yield and next year corn N rate
In Iowa we no longer use the soybean yield to adjust nitrogen (N) rate recommendations for the next-year corn crop. The reason is that there is no relationship between soybean yield and the rotation effect on corn N fertilization rate for the next year. Would the same hold for drought-damaged soybean in 2012? Yes, even with very low yields. What is important is that soybean was the previous crop. Soybean does not leave “extra” N behind at the end of the season. The reason for the difference in N rate requirement between corn following soybean and corn following corn is complicated, but important reasons deal with the difference in amount and quality of crop residue, and how that affects soil microbial processing, soil mineralization and N for crop residue decomposition. Also, there would not be carryover nitrate-N following soybean. So, just use the normal rate recommendation system (Corn N Rate Calculator, MRTN rate or profitable N rate range) for corn following soybean.
Corn yield and next year corn N rate
This is more complicated than the soybean crop question. In general, as long as plant vegetative growth and/or grain yield was not drastically affected by the dry weather, then use the normal rate recommendation (Corn N Rate Calculator, MRTN rate or profitable N rate range) to determine the needed fertilization rate for continuous corn in 2013. In that situation, corn production of vegetation/grain would use much of the N applied. If there is uncertainty about unused N, and if less than normal rainfall persists into next year, one could use the low end of the Corn N Rate Calculator profitable range for the 2013 rate recommendation.