Evaluation of soybean in saturated soil culture related to its adaptation and nitrogen fixation.

Soybean (Glycine max (L.) Merrill) under saturated soil culture (SSC) can acclimate to the imposition of high water table which stimulates soybean growth and nodulation and increases yield. This study attempted to evaluate effects of saturated soil culture on soybean adaptation and nitrogen fixation in Chiang Mai, Northern Thailand. Two soybean varieties, SJ5 and NW1, were cultivated under high water table and with conventional irrigation method in sandy loam soil during rainy season in 1989 in the experimental station of the Multiple Cropping Centre. Meanwhile, effects of starter nitrogen fertilizer with two levels, nil and 50 kg/ha were observed. On eight occasions during growth, plant dry matter, nitrogen status, nodulation, and nitrogen fixation were measured and after harvest, yields and yield components were recorded. SSC increased the amount of nitrogen fixed by 44 percent over the conventionally irrigated method. This effect was closely related to the depression of uptake of soil nitrogen, shoot nitrogen concentration, and increases in nodulation. The crop in SSC derived more of its nitrogen from fixation throughout the growing season. SSC treatment removed less soil nitrogen from the soil than CI. SJ5 under SSC without starter nitrogen derived 74 percent of its nitrogen from the air compared with 54 percent when irrigated conventionally. With the removal of 2 t/ha of soybean seed from the field, there was a positive balance of 25 kg N/ha after the SSC treatment compared with a depletion of -13 kg N/ha from the soil after a CI crop. NW1 responded in the same way. The application of 50 kg N/ha depressed nodulation and nitrogen fixation at early stages, but the effects disappeared later. Plants under saturated soil and conventional irrigation method responded to starter nitrogen in the same way. There was no significant dry matter and yield response to SSC for both soybean cultivars. However, as an alternative crop in water logging areas, SSC technique may produce soybean yield as high as that of well irrigated soybean.