Soil fertility management after drought
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.
Residual soil nitrate
If the corn plant vegetation and/or grain yield was drastically affected by drought conditions, then N uptake would have been reduced and unused nitrate-N could be accounted for in determining the N fertilization rate for the 2013 corn crop. There are two methods to estimate carryover N. The direct method is to sample the soil profile (a minimum of 2 feet) after harvest and measure the nitrate-N concentration. Sampling would be by 1-foot increments. If dry conditions persist, most applied N should remain in the top 2 feet. Sampling to 3 feet would be preferable, especially where rainfall was enough to move nitrate deeper in the profile. To add up nitrate-N in the sampled profile, multiply the concentration in each foot by four to get the nitrate-N amount per foot and then add the amounts together. One would not want to account for all of the nitrate-N as a subtraction from the next crop N recommendation as there is always some nitrate in the profile at the end of the season. A suggestion from research conducted in Wisconsin (which should be appropriate for Iowa) is to only account for nitrate-N greater than 40 lb nitrate-N (2 foot depth) or 50 lb nitrate-N/acre (3 foot depth), with the remaining amount then subtracted from the normal rate recommendation. A second method to estimate carryover nitrate-N is to use the 2012 corn grain yield. Take the total N applied for the 2012 corn crop and subtract the 2012 grain yield in bu/acre. Then assume 50 percent of that amount will remain available to the 2013 crop if precipitation is normal or below normal for the fall/winter/early spring. The remaining nitrate-N amount will vary depending on the actual rainfall and potential losses from fall through spring. For example, if the total N application for the 2012 crop was 190 lb N/acre and the 2012 corn yield was 50 bu/acre, then the unused N would be 190 minus 50 = 140 lb N/acre. The 140 lb N/acre times 50 percent leaves 70 lb N/acre to subtract from the 2013 rate recommendation.