Variability in Soil Test Potassium and Crop Yield

Four conventional plot trials with corn were established at four research farms in 2006, five additional trials were established in 2007, and evaluations continued in 2008 for a total of 22 site-years. Treatments replicated four times were two hybrids and five K fertilizer rates (0 to 180 lb K₂O/A). All sites had histories of rootworm infestation and soil test K ranged from the upper range of the Very Low class to a value between Optimum and High. No root insecticide was applied. Ear leaves and grain were sampled from all plots. At silking time, three contrasting K treatments were selected to sample whole plants (except roots) and to assess rootworm injury. Grain yield and rootworm injury data were summarized from 2006 to 2008, while plant analysis data were summarized for 2006 and 2007.

Average rootworm injury for the susceptible hybrid at each site-year ranged from 0.1 to 2.4 (0.9 on average) on a scale from 0 to 3, while for the resistant hybrid, injury ranged from 0 to 0.4 (0.1 on average). Potassium increased grain yield at three of the nine locations. The rootworm resistant hybrid yielded higher than the susceptible hybrid in most site-years, but the difference was significant only in 6 site-years (where root injury ranged from 0.2 to 2.2). Preliminary statistical analyses showed no significant interaction between hybrid and K rate. Yield trends suggested, however, that in one site-year the rootworm resistant hybrid yielded higher, but also needed a higher K rate to achieve maximum yield while in 3 site-years the susceptible hybrid needed a higher K rate to achieve the maximum yield. Results from the available tissue tests for 13 site-years showed that K fertilization almost always increased leaf K concentration, but the increase was significant at 8 site-years and the increases appeared consistent across hybrids. Tissue tests for above-ground plant parts also showed K uptake increases at these sites. However, effects of hybrid and K rates on plant tissue N or P were small and inconsistent. Work during 2009 will continue by evaluating the last five field trials, analyzing plant tissue samples, and summarizing data from the previous years. IA-09F