Variability in Soil Test Potassium and Crop Yield

This project studied early growth, early K uptake, and corn grain yield response to in-furrow fluid starter K and broadcast K across the landscape using eight replicated strip trials and precision agriculture technologies. A low-salt liquid starter fertilizer (0-0-30) was applied at 15 to 22 lb K₂O/A alone and after broadcasting 120 lb K₂O/A (a common removal-based rate used by farmers for corn of corn-soybean rotations). Soils were sampled using a dense grid sampling approach and yield was harvested with yield monitors. Two trials were managed with no-till and the others used chisel-plow/disk tillage.

Mean field soil test K ranged from 102 (low) to 223 ppm (very high), but each field had values ranging from very low or low to high or very high. Averages from the entire length of the strips showed that K applied as either source: (1) did not affect early plant dry weight or K uptake consistently; (2) often increased early plant K concentration; and (3) increased grain yield at three sites. At two sites, broadcast K increased yield more than starter K and at one site both fertilizers increased yield similarly. Analyses of data for within-field areas with different soil series or with soil test K in different interpretation classes showed no consistent differences in response to starter or broadcast K. An interesting result was that a yield response to K sometimes was observed for soil test K levels higher than those for which K application is recommended in Iowa and neighboring states. The few instances with a early plant growth or K uptake response did not show a yield response. Starter K applied in addition to broadcast K never increased yield further.

Comparisons of these results with others using N-P or N-P-K starters indicate that application of in-furrow K has little or no true starter effect, which confirms previous Iowa research conducted with granulated fertilizer applied beside and below the seed row. However, the low liquid starter K rate applied often increased corn yield as much as the higher broadcast rate, which shows that farmers have management options concerning use of these K fertilizers sources for production of corn. IA-09F