Variability in Soil Test Potassium and Crop Yield

A project studying impacts of rootworm incidence and genetic rootworm resistance on corn response to K fertilization was completed. Treatments were two hybrid isolines with or without rootworm resistance and five K fertilizer rates.

Data from 27 site-years showed that root injury by rootworms was much less for the resistant hybrid. The grain K concentration seldom differed between hybrids, but grain yield and K removal were greater for the resistant hybrid half the time. On average, the yield-maximizing K rate was slightly less for the resistant hybrid even though yield was higher. Potassium concentration and uptake in vegetative tissue was higher for the resistant hybrid when K supply was low. Results indicated better K use efficiency of hybrids with rootworm resistance, although higher grain yield and K removal resulted in higher K rates to maintain soil test K (STK).

There was progress in three other areas. Work continued at five long-term trials with corn-soybean rotations managed with and without tillage to understand STK temporal variability and relationships among fertilization rates, removal, and STK. We summarized 15 years of data for the no-till treatment. Potassium fertilization had a small effect on grain K concentration, but increased yield and K removal when STK was less than 150 to 180 mg/kg. Yield was poorly correlated with grain K concentration, but was linearly correlated with K removal. Iowa’s average grain K concentration corresponded with the highest concentrations observed. There was a large stratification of STK and non-exchangeable K. The non-exchangeable K partially explained large STK variation across K rates and years. We harvested and sampled the second year of two trials to evaluate interactions among hybrids, N fertilization, and K fertilization in corn. There were large yield responses to N, moderate response to K at one location, and a small positive NxK interaction at one location. Results of tissue tests for grain and leaves are being studied.

We also advanced on the study of K recycling with corn residue. This work is complemented by a similar study for soybean developed with other funds. There was significant K leaching to the soil from standing plants and residue during the period encompassing physiological maturity, harvest, and late fall (before the winter freeze). Study of this issue at different locations and years together with rainfall should explain a great deal of temporal variability in STK. IA-09F