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River flows through and erodes soils which nurtured gr-eat <br />grassland plains. Figure 1 shows that only two, true grassland <br />- biomes exist in this hemisphere, and the Missouri River drainage <br />is almost entirely grassland (Van Riper 1962). <br />True prairie composed 87 percent of the Missouri River basin. <br />Soils subtending this prairie have high organic mat-ter content <br />resulting. largely from root decomposition. The belowground <br />biomass of prairie plants may be up to four times the aboveground <br />biomass (Risser et al. 1981). The multitude of finely branched <br />roots of grasses elongate rapidly in the well-drained, arable <br />soils of the Missouri River basin in order to make new soil <br />contact to avoid moisture stress. Since the extension rootlets <br />are short-lived, their death contributes to a large amount of <br />humus in these soils. The humus content is often the <br />accumulation of many generations of plant material (Daubenmire <br />1967). Leaf litter in this particular grassland decayed rapidly <br />due to the warm, humid conditions for part of the year and this <br />added much to the surface horizon's organic matter content. <br />These soils were readily converted to fertile cropland with <br />the advent of farm machinery. Under intensive .farm management <br />about 25 percent of the organic matter was lost from the surface <br />horizons of three soils in the northern part of this basin after <br />an average of 43 years of cropping (Brady 1974). Continuous corn <br />production from comparable soil plots in Ohio showed a reduction <br />- 6 - <br />