Relation of Root Distribution to Organic Matter in Prairie Soil

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Abstract
A typical square meter of vegetation was selected in the upland Andropogon scoparius prairie on Lancaster loam soil, near Lincoln, Nebraska, and another in the lowland A. furcatus prairie on Wabash clay loam of the flood plain of the Missouri River. Surface soil was removed in 6-in. layers; the deeper soil in foot sections. Roots and rhizomes were removed by washing, and dry weight, N content, and organic matter determined. Approximately 60% of the underground parts in the upland type occurred in the first 6 in. The Andropogon roots were distributed to a depth of 4 ft. In the lowland sod, 60% of the entire root system, by weight, occurred in the surface 6 in. of soil; the grass roots extended to a depth of 7 ft. Volume weight (weight of dry soil per unit volume, i.e., grams per cc.) of the soil in the undisturbed field condition was determined. From the volume weight, specific gravity, and field moisture content at time of sampling, pore space, space occupied by water, and air space were calculated. Volume weight increased gradually in Lancaster loam from 1.12 in the surface 6 in. to 1.64 in the 4th foot, where both living roots and dead organic matter were relatively sparse. In Wabash clay loam it increased gradually from 1.05 in the first 6 in. to 1.33 in the 7th foot. This was accompanied by a gradual decrease in both living root materials and dead organic matter. Pore space in the surface 6 in. of Lancaster loam constituted 57% of the volume of the soil, where approximately (during 1932) 25% was occupied by water and 32% by air. It decreased to 39% at 4 ft. Pore space in the surface 6 in. of Wabash clay loam occupied 60% of soil volume, decreasing with depth to 51% in the 7th foot. Organic matter and N content of roots of upland were somewhat higher than those of lowland, especially in the surface 6 in. The ratio of soil organic matter to N varied in Lancaster loam from 20 in the surface soil to 8.3 in the 4th foot, a variation typical of upland prairie soils. In Wabash clay loam the ratio of organic matter to N showed variations corresponding to the variations of organic matter at the different depths. This indicated that the strata of high organic content had accumulated a surface type of organic matter during intervals in the silting process by which the present soil has been built up. Except in the surface 6 in of soil, there is an approximately linear relation between the amount of root material and the amount of soil organic matter in the various soil horizons. Roots and rhizomes constitute about 1/10 of the total organic matter in the surface 6 in. of soil; in the deeper sections the proportion decreases gradually from 3 to 4% in the 2nd 6 in. to 1% in the 4th foot of Lancaster loam, and to 0.25% in the 7th foot of the Wabash soil.