EFFECT OF AGGREGATE SIZE AND SOIL-WATER CONTENT ON EMERGENCE OF SOYBEAN (GLYCINE-MAX, L MERR) AND MAIZE (ZEA-MAYS, L)

Rapid wetting of irrigated soils often leads to slaking and slumping, and on drying a surface crust and hard-set conditions may occur. This results in reduced crop emergence unless the surface is kept moist. The effect of aggregate size and water content on the emergence of soybean and maize from an Entic chromustert (heavy cracking clay) was determined using pots of sieved aggregates with size ranges less than 1, 1-2, 2-5 and 5-15 mm at soil water contents of 15, 20 and 25 g (100 g)(-1). Unsieved soil was used as a control. Greatest emergence tended to occur from fine ( 1-2 mm) seedbeds compared with coarse (5-15 mm) seedbeds for both crops. A covered treatment, simulating a stubble mulch, resulted in greater emergence than an uncovered treatment for all water contents and aggregate sizes. Earlier emergence occurred from finer (less than 1 mm and 1-2 mm) seedbeds than from coarse (5-15 mm) seedbeds, and at the greatest water content used. Soil strength, measured with a shear vane, decreased with increasing water content and tended to be less on fine (1-2 mm) seedbeds compared with very fine (less than 1 mm) or coarse (5-15 mm) seedbeds. It is recommended that, for good emergence from this Entic chromustert, seedbeds be brought to a water content of 25 g (100 g)(-1) by capillary wetting to prevent hardsetting and consist of 1-2 or 2-5 mm aggregates for soybean and maize, respectively, and have a stubble mulch on the surface. This corresponds to an equivalent depth of water of 15 mm and 9 mm for soybean and maize, respectively, in the top 50 mm of the profile.