International Soil Tillage Research Organization

the COM analysis. After the COM analysis was completed, subsamples ofthe soil from which the incorporated crop residue had been separatedwere used for additional analyses such as soil pH (Pikul and Allmaras,1985) and primary inoculum for root disease of peas and wheat (Wilkinset al., 1985).RESULTS AND DISCUSSIONSDepth distributions of dry bulk density and incorporated residue shownin Fig. 1, 2, and 3 are typical of those observed from use of themoldboard plow, sweep, and chisel, respectively, after wheat harvest.The standard errors given are means based upon disagreement betweenduplicate observations each from a composite of at least eight coreseach 20 mm long and 19 mm in diameter.Farm tractors of 30 kW size and associated machinery were used fordryland wheat at site 1 (Fig. I), whereas farm tractors of 80 kW sizeand larger were used for supplementary irrigated wheat at site 18 (Fig.1). Depth of crop residue incorporation at these sites was nearly equalto the mean curve (from 20 sites) shown in Fig. 4. Cumulative relativeCOM is a parameter normalized to a maximum value of unity using thetotal COM observed in the upper 0.3 m of the soil profile. A peakconcentration of COM greater than 8 g/kg as in site 18 was observed atu02 4 6 8 10COM (g/kg)Dry BVI~Density (h4g/rn3)Fig. 1. Typical profiles of dry bulk density and incorporated cropresidue (COM) produced by moldboard plowing after wheatharvest.