EFFECT OF PLANTING DATE AND TILLAGE SYSTEM ON SOYBEAN ROOT-GROWTH

Soil erosion and moisture retention are major concerns of soybean grow ers. Conservation tillage provides residue cover to reduce soil loss a nd water evaporation. This study was conducted on a Tiptonville silt l oam near Portageville, MO, USA. to determine the effect of tillage sys tem and planting date on soybean [Glycine max (L.) Merrill] root growt h and distribution. Tillage systems were conventional (clean) tillage, ridge tillage, and no-tillage. 'Essex' soybean was planted on 14 May, 15 June, and 7 July in 1992 and 12 May, 2 June, and 21 June in 1993. Roots were observed 30 and 60 days after emergence (DAE) using a minir hizotron system. Stand density was not affected by tillage in either y ear or by planting date in 1992. Tillage did not effect rooting depth in either year. In 1992, rooting depth 30 DAE was greater for the 14 M ay planting date than for either of the other two planting dates. No o ther planting date effects on rooting depth were found. Among soil dep ths, root length density (RLD) was greatest for the 14 to 26 cm depth in 1992 and for the 0 to 13 cm depth in 1993. Neither tillage system n or planting date affected RLD in either year and there was no interact ion between these main effects and soil depths. The largest changes in RLD (CRLD) were observed in the 14 to 26 cm and 27 to 39 cm depths in 1992 and the 0 to 13 cm depth in 1993. Tillage did not planting date in 1992 and the 12 May and 2 June planting dates in 1993 produced the highest yields. Tillage did not affect yield and there was no interact ion between tillage and planting date.