Management of Irrigated Soybeans on a Moderately Coarse-textured Soil in the Upper Midwest

Increased availability of water for supplemental irrigation in the upper Midwest has enhanced an interest in growing soybeans [Glycine max (L.) Merr.] on moderately coarse-textured soils where they have not previously been introduced. The objectives of this study were to determine the yield potential of supplementally irrigated soybeans on these soils and to develop an irrigation schedule for efficient water management. The study, conducted on Maddock sandy loam from 1972 to 1974, employed a splitsplit plot design with water level as the main block and N fertility rates (0, 56, 112, and 224 kg/ha) as sub-plots. Water levels were Wl (dryland), W2 (under-irrigation), W3 (optimum) and W4 (over-irrigation). Sub-sub plots were planting dates in 1972 and cultivars in 1973 and 1974. Soil samples were collected from 15 to 30 cm intervals to depths of 90, 150, or 270 cm throughout the study and analyzed for NO:,-N in the soil profile under irrigated conditions. Grain yields were significantly different for water levels during each of the 3 years. Threeyear yield means were 543, 1,823, 2,428, and 2,164 kg/ha for Wl, W2, W3 and W4, respectively. The application of 24 kg/ha N fertilizer in 1972 increased grain yield at the 0.95 probability level. The seed was not inoculated that year but was inoculated in 1973 and 1974. Soybeans planted 10 May 1972, outyielded soybeans planted 6 June by 356 kg/ha. ‘Anoka’ outyielded the ‘SRF-100’ cultivar by 137 kg/ha in both 1973 and 1974. An irrigation schedule based upon tensiometers placed at the 30 cm depth until 15 July (growth stage R2) and at the 45 cm depth after 15 July proved effective in applying irrigation water. Analysis of NO3-N in the soil profile at different dates during the 3-year period showed that under heavy fertilizer NO₃ N applications, the NO₃-N moved below the crop rooting zone when excess water was applied. Based upon results of this study, soybeans can be seriously considered as a crop to be included in cropping systems for irrigated, moderately coarse-textured soils in the Upper Midwest.