Relation of Root Distribution to Organic Matter in Prairie Soil

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 the Wabash clay loam soil of the floodplain of the Missouri River. 2. The surface soil was removed in 6-inch layers and the deeper soil in foot sections to the depth of root penetration; the roots and rhizomes were carefully removed by washing, and their dry weights, nitrogen contents, and organic contents determined. 3. Water content of soil and atmospheric factors affecting plant development were measured and rate of growth of the vegetation was determined. Root habits of the plants excavated were noted. 4. The soils varied so uniformly in texture, structure, and fertility with depth, that (except in buried surface layers) root distribution was of the usual type found for these species over a wide range of prairie soils. 5. In the Lancaster loam 60 per cent of the underground parts of the dominant grass was found in the surface 6 inches; the remainder of the root system was distributed to a depth of 4 feet. In the Wabash clay loam 68 per cent of the underground parts (60 per cent excluding rhizomes) was found in the surface 6 inches; the remainder of the root system extended to the depth of 7 feet and nearly to the water table. 6. The hygroscopic coefficient and moisture equivalent of each section of the soil profile were determined as indexes of the texture. The volume-weight of the soil in the undisturbed field condition was determined. From the volume-weight, specific gravity, and field moisture content at the time of sampling, it was possible to calculate the pore space, space occupied by water, and air space in each section of the soil. 7. Volume-weight in the Lancaster loam increased gradually from 1.12 in the surface 6 inches, where an abundance of roots, rhizomes, and dead organic matter filled the soil, to 1.64 in the fourth foot, where both living roots and dead organic matter were relatively sparse. 8. Volume-weight in the Wabash clay loam increased gradually (except in buried surface soils) from 1.05 in the first 6 inches to 1.33 in the seventh foot. This was accompanied by a gradual decrease in both living root materials and dead organic matter. 9. Pore space in the surface 6 inches of Lancaster loam constituted 57 per cent of the volume of the soil. On an average 25 per cent was occupied by water and 32 per cent by air. It decreased with depth to 39 per cent at 4 feet, where 19 per cent was occupied by water and 20 per cent by air. 10. Pore space in the surface 6 inches of Wabash clay loam occupied 60 per cent of the soil volume. On an average 32 per cent was filled with water and 28 per cent with air. It decreased regularly with depth (except for buried surface layers) to about 51 per cent in the seventh foot, where water occupied 41 per cent and air only 10 per cent. 11. The organic matter and nitrogen content of the roots in the upland soil were somewhat higher than in the lowland, especially in the surface 6 inches. 12. The composition of the rhizomes in the lowland was determined separately; their organic matter and nitrogen content were greater than in the roots, owing to the storage of food reserves. 13. The percentage of organic matter and nitrogen in each layer of the soil was determined, and the weight of each per square meter of soil calculated. 14. The ratio of soil organic matter to nitrogen varied in the Lancaster loam from 20 in the surface soil to 8.3 in the fourth foot, a variation typical of upland prairie soils. 15. The ratio of organic matter to nitrogen in the Wabash clay loam showed variations corresponding to the variations in the organic content at different depths. This indicates 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. 16. Except in the surface 6 inches 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. 17. In the surface soil the presence of a large amount of living rhizome and root material and the favorable conditions for the decomposition of dead organic matter increase the proportion of roots and rhizomes to soil organic matter. 18. Roots and rhizomes constitute about one-tenth of the total organic matter in the surface 6 inches of soil; in the deeper sections the proportion decreases gradually from 3 to 4 per cent in the second 6 inches to 1 per cent in the fourth foot of the Lancaster soil, and 0.25 per cent in the seventh foot of the Wabash soil.