Variability in Soil Test Potassium and Crop Yield

In 2007, this project evaluated the impact of rootworm resistance genetics on grain yield and nutrient uptake by corn. Nine conventional plot trials with corn were established at five Iowa

State University (ISU) research farms. The treatments included two corn hybrids (i.e., rootworm resistant and susceptible) and five K fertilizer treatments (0 to 180 lb K₂O/A). All sites had corn planted the two previous years, which showed evidence of rootworm infestation. Initial soil test K across sites ranged from values at the borderline between very low and low to optimum and high according to ISU soil test interpretations. No insecticide was applied. Plant samples taken for nutrient determination included ear leaves at silking, total above-ground plants near silking stage, and harvested grain. Rootworm damage evaluations were made as suggested by the ISU near silking stage.

Only grain yields and rootworm damage ratings are summarized at this time, and results should be considered preliminary because no detailed outlier or statistical analyses were conducted yet. Rootworm damage evaluations showed no damage for rootworm resistant corn at any site (0.1 or less in a 0 to 3 scale) while damage for susceptible corn ranged from 0.6 to 2.2 across sites. Across K rates, the rootworm resistant hybrid yielded significantly higher than the susceptible hybrid on three fields, which had medium to high rootworm infestation. The response to K fertilizer clearly differed between hybrids at one site, where the susceptible hybrid needed more K to achieve maximum yield. Similar but less defined trends were observed at two other sites. These results are in contrast with 2006 results, when at one site the rootworm resistant hybrid yielded higher, but also needed a higher K rate to achieve maximum yield. Plant analyses that are not completed at this time should provide information useful to better interpret these results. IA-09F