Tillage and potassium placement effects on potassium utilization by corn and soybean

Farmers in the lower corn belt region of the United States need to know how to fertilize soils in order to optimize the efficiency of potassium (K) use by plants under different tillage systems and with high yield management. A field study was conducted from 1994 to 1996 at the Dixon Springs Agricultural Center (DSAC) on a Bonnie silt loam (fine-silty, mixed, mesic Typic Fluvaquent) soil and at the SIU Belleville Research Center (BRC) on a Herrick silt loam (fine, montmorillonitic, mesic Aquic Argiudoll) soil. No-till (NT) and spring chisel tillage (CT) systems were compared in a corn (Zea mays L.)-soybean (Glycine max L.) rotation. Within each tillage system, K rates of 56, 112 and 168 kg K ha-1 as potassium chloride were combined with four placement methods that included surface broadcast, 25-cm surface band over the intended row, surface dribble 15 cm to the side of the intended row, and starter placement of 28 kg K ha-1 5 cm to the side and 5 cm below the seed at planting (with the remainder of the K surface broadcast). The broadcast K placement most often produced the lowest whole plant K composition of corn or soybean tissue when sampled within a month after emergence. This indicated that the broadcast treatment was less efficient than other placement methods in providing K in the early stages of plant development. However, by the reproductive stage of development, there were no differences in leaf K concentrations among the various placement methods for either crop. Increasing K rates almost always increased leaf K concentrations and usually increased early plant growth and yields. Early plant growth and grain yield were affected to a greater degree in corn than soybean. Soybean yields appeared to maximize at 56 kg K ha-1, whereas corn often showed higher yields (although not always significant) with K rates of 112 and 168 kg ha-1. Corn usually requires more K than soybean, which would explain its greater responsiveness to K. For corn, the placement method of K was not as critical as rate, even for no-till, across the six site-years of this study. Lower yields occurred in 1996 at both locations with the long term banding of K fertilizers over the intended rows of soybean. This is in contrast to a yield benefit from surface banding at other locations or crops as reported in the literature. Weather problems related to a wet spring followed by dry conditions late in the season may have contributed to the atypical results in 1996. These weather problems probably either lead to larger plants early which ran out of water late in the season, or higher accumulated salt concentrations in the root volume which reduced water uptake late in the season.