Variability in Soil Test Potassium and Crop Yield

The goal of this long-term research effort has been to improve the potassium (K) management of corn and soybean. Extensive research in the North Central region of the U.S. has investigated, separately, both nitrogen (N) and K fertilizer management for corn. However, there is scarce information available about how N and K interactions affect corn grain yield and nutrient uptake. The objective of this study was to evaluate the effects of various combinations of N and K rates on corn yield, N and K tissue concentrations, and N and K removed with grain harvest. Two trials with continuous corn were conducted from 2013 to 2016 at fields with Webster (northern Iowa) or Mahaska (southeast Iowa) soils, which were managed with chisel-plow/disk tillage. The annual combinations consisted of five N rates (0 to 300 lb N/acre) and four K rates (0 to 72 lb K₂O/acre). Granulated urea was broadcast and incorporated into the soil in the spring at one site and a urea ammonium nitrate solution was injected at the V4 to V5 growth stage at the other site. Potassium chloride (KCl) was broadcast and incorporated into the soil in the spring at both sites. Leaves opposite and below the primary ear were sampled at the R1 growth stage and grain was sampled at harvest.

Fertilization with N increased leaf and grain tissue N concentrations, and fertilization with K increased the K concentration of both tissues. Potassium fertilization had no clear effect on leaf and grain tissue N concentrations. Nitrogen fertilization decreased or did not affect leaf and grain K concentrations when K was not applied and the soil K supply limited yield. Potassium fertilization alleviated or offset the effect of N application that reduced tissue K concentrations.

There was a large grain yield response to N fertilizer and a smaller response to K fertilizer at both sites, and there were significant N by K interactions. Corn yield and the yield response to N were higher with adequate K supply compared with K deficient treatments. Similar interactions were observed for N and K removed with grain harvest. The study demonstrated that K deficiency not only limits corn yield, but also limits its capacity to respond to N fertilization.