Effects of irrigation on the growth of soybean cultivated in a sand dune field

In 1999, 2000, 2001 and 2003, soybean plants were cultivated in a sand dune field with minimum irrigation Gust enough to prevent wilting) and in a red soil field without irrigation, and the relationship between the yield and precipitation during the cultivation period, from June to October, was examined (Exp. 1). The yield in the sand dune field was lower than that in red soil field every year, but in 2001 with a large amount of precipitation, it was similar to that in the red soil field without irrigation. The yield in the red soil field did not vary with the year, but that in the sand dune field highly correlated with the precipitation during the growth period (r = 0.80, P 0.20). In 1999, the growth and yield of soybean cultivated in the sand dune with minimum irrigation were compared with those under enough irrigation (Exp.2). The dry-matter production of soybean under minimum irrigation was less than that under enough irrigation from mid-July to mid-August when the amount of precipitation was small, but was similar to that under enough irrigation after mid-August when the amount of precipitation was not small. In the sand dune field with minimum irrigation, plant height, diameter of main stem and the number of branches were lower than those under enough irrigation, but the node number of main stem was similar to that under enough irrigation. Under minimum irrigation, the number of branching was low from early July to mid-August when the amount of precipitation was small. In addition, the pod setting rate was low resulting in fewer pod numbers. Although the amount of precipitation was not small after mid-August, the 100-seeds weight in the sand dune field with minimum irrigation was lighter than that in the field with enough irrigation. This is probably because there was no precipitation for three successive days in September, which resulted in deficiency of soil water.