Variability in Soil Test Potassium and Crop Yield

The goal of this research is to improve K fertilizer management for corn and soybean in Iowa and to generate results useful for other regions. Several sub-projects are based on short-term or long-term interrelated trials. This summary highlights results of two sub-projects.

Research that included 33 corn site-years and 14 soybean site-years investigated K uptake and recycling to the soil. Dry matter yield and K accumulation were measured in vegetative tissue at physiological maturity, in grain and residue at harvest, and in residue placed on the soil surface until spring. On average, K accumulation by harvest in grain and residue was 68 and 34 kg K/ha in soybean and 29 and 52 kg K/ha in corn, and the increase from fertilization was greater in residue than in grain (60% and 9% for soybean, and 57% and 7% for corn). Two months after harvest, K remaining in residue was 50% in corn and 19% in soybean and by April decreased to 31% and 12%, respectively. The K loss decreased exponentially as precipitation increased, and the effect was more clear in soybean (R² = 0.64) than in corn (R² = 0.38). Soil test K often increased from fall to spring and the increase was related to the K lost from residue (r² = 0.56 in soybean and 0.16 in corn). The study demonstrated that precipitation strongly influences K recycling and affects soil test K temporal variation. Results were summarized for extension conferences and a paper was published in the Soil Science Society of America Journal.

Research conducted during four years at two continuous corn trial sites evaluated interactions among N and K fertilization and corn hybrids susceptible or resistant to rootworm. Completed grain yield analyses showed little response to K and no interactions at a northern Iowa site, because soil-test K of non-fertilized plots was near optimum. At a southeast Iowa site, there was a yield response to added K. Reduced rootworm incidence due to genetic resistance did not affect the K application rate needed to attain maximum corn yield; however, the resistant hybrid showed higher yield, greater increases from N and K additions, and a higher N rate was needed to maximize yield. The study showed that a healthier root system increases yield and optimizes N and K use. We collected leaf samples at the R1 growth stage and grain samples at harvest for N and K analyses. Results are being summarized at this time.