Irrigation and Planting Date Effects on Soybean Grown on Clay Soil 1

In the southern Mississippi River alluvial plain, adverse weather conditions and the time required for seedbed preparation can often cause delays of 2 to 3 weeks in planting soybean [ Glycine max (L.) Merr.] on fine‐textured soils. Because about 2.6 million ha of these soils are planted to soybean, we conducted field experiments in 1980, 1981, and 1982 on Sharkey clay (Vertic Haplaquept, very‐fine, montmorillonitic, thermic) to determine if yields and yield components of mid May (early) and late May or early June (late) plantings would be similarly affected by irrigation. Plantings of ‘Bedford’ and ‘Bragg’ or ‘Braxton’ were made each year and furrow irrigation was initiated at growth stage R1 (I) and continued through stage R6 on designated plots. A nonirrigated (NI) treatment received only rainfall. Planting date did not significantly affect yield in the NI plots in any year, although the trend each year was toward higher yields from the late plantings. With the I plots, seed yields from the early plantings of 1980,1981, and 1982 averaged 65,370, and 475 kg ha −1 more than from the later plantings, respectively, with the increase in 1982 being significant ( P = 0.02). Irrigation of the early planting increased average yields by 1970,2020, and 1490 kg ha −1 in 1980,1981, and 1982, respectively; with the late planting, yield increases averaged 1730,1285, and 975 kg ha −1 for the same years. Increased number of seed was responsible for most or all of the yield increases resulting from irrigation. Irrigation of both plantings essentially ensured canopy closure of all cultivars. These results indicate that maximum benefit derived from irrigation applied to soybean grown on a clay soil can be realized if plantings are made in mid‐May.