Date of this Version
The importance of root extent and activities in relation to the supply of subsoil moisture has again been clearly demonstrated. In fact, it has been shown to be the chief problem in the continued production of alfalfa in a grassland climate.
Studies extending over a period of 15 years have shown that the native species of the tall-grass prairie never exhaust the water supply below that available for plant growth, except rarely in the shallower soil. The subsoil is always moist, and plants root deeply. Of 43 species typically representative of the flora, only 14 per cent absorb almost entirely in the first 2 feet of soil; 21 per cent have roots extending well below 2 feet but seldom beyond 5 feet; but 65 per cent have roots that reach depths quite below 5 feet, a penetration of 8 to 12 feet being common, and a maximum depth of over 20 feet sometimes being attained. There is sufficient rainfall to wet the soil very deeply, but it is not too wet for good aeration, while aerial conditions promote high transpiration. This results in deep root penetration.