Variability in Soil Test Potassium and Crop Yield

The goal of this research is to improve K fertilizer management for corn and soybean. This year we finished statistical analyses and began summarizing for publication the results reported last year for a study that assessed N and K fertilization for corn hybrids susceptible or resistant to rootworm. The study documented that a healthy root system not only increased yield, but also optimized both N and K use.

We also continued the third year of two field studies with continuous corn to evaluate how N and K applications interact to optimize grain yield and nutrient use. At a northern Iowa site, there was a very large yield response to N and a small response to K because initial soil-test K was near optimum. At a southeast Iowa site, which also included a S treatment, there was a very large yield response to N, a moderate response to K because initial soil-test K was lower, and no response to S. The most relevant result from yield, leaf tissue testing, and nutrient removal assessments for both sites was a very significant N by K interaction. Corn yield was higher with adequate rates of both nutrients, the yield response to increasing N application rates was greater when K was applied, and a higher N rate was needed to maximize yield with added K. However, more K than currently recommended in Iowa did not affect the response to either nutrient. This result shows the importance of balanced nutrition but that excessive levels of one nutrient do not imply a need for excessive levels of the other. With current unfavorable crop prices, farmers tend to reduce K applications proportionally more than N applications. This study provides compelling evidence of the impact of K deficiency.

We continued to evaluate five long-term trials in different soils that compare broadcast and planter-band K fertilizer placement methods for corn-soybean rotations managed with no-till or chisel-plow/disk tillage. There were significant grain yield responses to K fertilization at four of the locations by both crops. These responses have increased in recent years, but the K application rate that maximized yield has not differed between placement methods at any site for rates ranging from 35 to 70 lb K₂O/A/yr. However, a very important result for both crops that was not observed before was that K deficiency reduced yield more under no-tillage than with tillage (15 to 20%). Therefore, no-till producers should be much more aware of the risk of under-applying K fertilizer.