DEVELOPMENT OF DOUBLE-CROP SOYBEAN UNDER DIFFERENT SOIL-WATER REGIMES

Double-crop soybean [Glycine max (L.) Merr.] comprises nearly one-thir d of the total soybean hectarage in the southeastern USA. This study w as initiated to investigate the water relations of soybean following w inter wheat [Triticum aestivum (L.) emend. Thell], with emphasis on th e role of stored soil water. A treatment scheme was devised in which w heat was killed at heading, mid-grain rill, and maturity, resulting in three different levels of stored soil water at soybean planting. In 1 986, soil water levels at planting were 26.0, 23.1 and 21.4 cm of wate r in the 0.9-m profile, respectively. A relatively dry early summer re sulted in poor soybean stands and slow early growth; however, rainfall was well distributed during the latter half of the growing season. So ybean yields in 1986 were 2.76, 1.60, and 0.44 Mg ha-1, respectively. Initial soil water levels in 1987 were 31.1, 27.5, and 25.1 cm of wate r in the 0.9-m profile. A wet early summer resulted in good stands and vigorous early growth, but a late summer drought caused severe stress during the reproductive growth period. Soybean yields recorded in 198 7 were 0.97, 0.54, and 0.30 Mg ha-1, respectively. In 1989, soil water levels at planting were 33.1, 34.3, and 31.6 cm and associated yields were 1.51, 1.81, and 1.11 Mg ha-1. Rainfall was above average and wel l distributed during the 1989 season. Within each of the years there w as a positive relationship between soil water level at planting and yi eld of double-crop soybean; however, between years the amount and dist ribution of rainfall had a more dominant influence on yield than did s oil water level at planting.