CROP RESIDUE MANAGEMENT TO CONSERVE SOIL ORGANIC-MATTER CONTENT IN SUGARCANE-BASED CROP ROTATIONS

In rice-plant cane-ratoon cane and cowpea-plant cane-ratoon cane rotat ions, rice produced a significantly greater root biomass (1 . 5 t/ha) than cowpea (0 . 7 t/ha), but, overall, the cowpea-plant cane-ratoon c ane rotation added little more residues (7 . 9 t/ha) to the soil than the rice-plant cane-ratoon cane rotation (7 ..5 t/ha). Organic matter (OM) and total N in the soil not only increased more after cowpea (OM = 0 . 78%, N = 0 . 048%) compared with rice (OM = 0 . 66%, N = 0 . 024 % but also remained at a higher level throughout the crop sequence. Ti me of N application to sugarcane did not influence significantly the O M and N contents of the soil. However, placement of N at 15 cm depth a long stubble rows in the ratoon crop improved the OM and N contents of the soil more than the top dressing of N. Sugarcane yields were signi ficantly higher in cowpea-plant cane-ratoon cane rotation than those i n the rice-plant cane-ratoon cane rotation. Split applications of N at 45 and 90 days after planting produced a significantly higher plant-c ane yield than other split applications, but did not influence ratoon- cane yield significantly.