Excess Moisture Injury of Soybeans Cultivated in an Upland Field Converted from Paddy. V. Supplemental nitrogen application as a countermeasure against excess moisture injury.

Abstract

The purpose of this experiment was to examine whether the application of nitrogen (N) fertilizer as ammonium sulfate is an effective countermeasure against excess moisture injury of soybeans [Glycine maz (L.) Merr.]. Soybeans (cv. Tamahomare) were grown in an upland field converted from paddy. Flooding treatment was conducted in furrows to a depth of 5 to 8 cm for 8∼11 days at flower-bud differentiation stage and subsequently fertilizer was applied at the rate of 12 g N per m2. In the flooding plot without supplemental N, seed yield decreased by 20% ; however, in the flooding plot with supplemental N, seed yield decreased only by 6%, since the decreases in pod number were low. The reasons for the low reduction in pod munber were examined in a pot-cultured experiment. Plants under excessive soil moisture and without supplemental N had low N content in the leaf blade which brought about a marked decrease in the photosynthetic rate. In contrast, supplemental N increased the N content in the leaf blade and photosynthetic rate. It was assumed that the increased photosynthetic rate by supplemental N inhibited bud, flower and pod shedding.