The temperature of soils is modified by a va riety of conditions, e.g. a dark-eolored soil is usually warmer than a light-colored one; soils so exposed as to receive a large amount of the direct rays of the sun are warmer than those not thus exposed; dry soils are warmer than wet. The relation of soils to water probably more than any other one factor determines their productiveness. Water is not only necessary as a constituent of plant tissue, but it performs a most important function as a solvent and carrier of food in both soil and plant, and the amount required in plant growth is very large—from 250 to 500 pounds for each pound of dry matter produced by the plant.
BIBLIOGRAPHY. Stealer, Origin and 1Vatnrc of Bibliography. Stealer, Origin and 1Vatnrc of Soils (Twelfth Annual Report United States Geological Survey, 1890-91, part i., Geology, pp. 213-245) ; Merrill, Rocks, Rock Weathering, and soils (New York, 1897) ; King, The Soil (ib., 189S); Stockbridge. Rocks and Soils (ib., 1895) ; Hall, The (London, 1 P03) ; 1" ream, Soils and Their Properties (London. 1895) ; Warington,
Physical Properties of Soils (Oxford. Eng., 1900) ; Ramann. Forst fiche 1.10(1(ml:uncle und Standortslchre (Berlin. 1893) ; Brooks, Aglieni t nr e, vol. i. (Springfield, 1901) : Roberts, The Fertility of the Land (New York, 1897) ; :Mc Connell, Elements of Agricultural Geology (Lon don, 1902) ; Risler, Geologic agricole (Paris. 1884-95) ; Hilgard, The Relations of Soil to Cli mate (United States Department of Agriculture. Weather Bureau Bulletin 3, Washington. 1892) ; Whitney. Some Physical Properties of Soils in Their Relation to Crop Production (United States Department of Agriculture, Weather Bureau Bulletin 4, Washington. 1892) ; King, The Principles and Conditions of the Morements of Groundwater (United States Geological Survey, Nineteenth Annual Report. pt. ii., Washington, 1899) : the reports and bulletins of the Bureau of Soils of the United States Department of Agriculture. especially Bulletins 4, 10, 15, 17, 18, 19 and 22, and reports on Field Operations of the Bureau of Soils, beginning with the year 1899.