A few years back, nitrogen rate trials were the most common on-farm experiment. That’s no longer true, maybe because we now have the late spring soil nitrate test for corn. At the Neely-Kinyon Research Farm, near Greenfield, Bernie Havlovic carried out a demonstration of nitrogen rates for corn following soybeans (Table 3). Four rates were compared: zero, 75, 110, and 150 pounds per acre spring-applied anhydrous ammonia N. The 110 pound rate, which was determined using the late spring soil nitrate test, yielded as well as the 150 pound rate, and both yielded significantly better than the check treatment. The corn yield in the 75 pound treatment was not significantly less than the two high rates. With more replications than the three that were used, the trial might have distinguished the 75 pound treatment as different too.
Tom and Irene Frantzen, New Hampton, tested the nitrogen contribution to corn from a previous crop of berseem clover (Table 1). There was no yield difference between the corn receiving 80 pounds N and that getting 20 pounds, suggesting that the berseem may have supplied a significant amount of N to the crop. The whole field had also received six tons of hog manure in October, 1993. The late spring soil nitrate test showed both treatments to be in the seventies (very high). However, both treatments gave late season cornstalk tests in the 600’s, suggesting the possibility of an N shortage.
In early 1994, there were dry and warm conditions that released soil nitrogen and led to the large number of high readings for the late spring test. Then the rains returned, leaching soil N out of the root zone – and conditions were also excellent for crop removal of nutrients. As a result, some PFI farmers were left wondering if they really did have enough nitrogen in 1994. Dr. Fred Blackmer, who adapted the late spring soil nitrate test for Iowa, recommends always including one field strip of a high nitrogen rate. This can be a very useful reference if questions arise in mid-season.