SOIL PHYSICAL CONSTRAINTS TO ESTABLISHMENT OF MUNGBEANS (VIGNA-RADIATA L WILCZEK) IN PADDY RICE (ORYZA-SATIVA L) SOILS

In several rice producing countries, production of upland crops, such as mungbeans (Vigna radiata L. Wilczek), is an important means to save foreign exchange on food imports and to exploit the residual soil water following paddy rice (Oryza sativa L.) harvest. Since the soil physical requirements for establishment and growth of upland crops are different from those for paddy rice, this study examined (1) soil physical properties that may limit mungbean emergence in rice soils, and (2) soil management techniques that will alleviate the soil physical constraints to mungbean establishment. Dry soil conditions and high soil resistance were the major soil physical constraints limiting mungbean seedling emergence in clay soils in a growth chamber experiment. Since seedzone water content is high after rice harvest, soil strength was viewed as the major constraint to mungbean emergence in paddy rice soils. This hypothesis was tested in a field experiment where tillage practices (moldboard plow, no-till), planting techniques (inverted-T, dibble, furrow), and delays to planting after rice harvest (1-3, 4-5, 7-8, 11-12, or 14-15 days after field drainage) were used to create varying soil strength conditions. The soil was Maahas silty clay (Vertic Tropaquept). Mungbean emergence and soil strength data showed that the maximum emergence corresponded to soil conditions where soil resistance to seedling emergence was lowest. There were few benefits from tillage on mungbean emergence. Inverted-T and furrow planting performed best in tilled and no-till plots, respectively, The optimum time for mungbean planting was from 1 to 8 days after field drainage for no-till, and from 3 to 5 days after field drainage for tilled soils.