Page:The New International Encyclopædia 1st ed. v. 18.djvu/378

* SOIL. 324 tain of stones, gravel, sand, clay, lime, and or- ganic matter. A soil containing much sand is dry, warm, and easy to vorl<, but as a rule is naturally poor and has little absorptive power for water and fertilizing matter. A soil in which clay predominates is apt to be cold, wet, and difficult to till, but to have a high absorptive power not only for water, but for fertilizing matter as well. Clayey soils generally contain more plant food than sandy soils. Humus makes soils light in weight and dark in color and greatly increases tlicir alisorptive power. Lime not only has value as a plant food, but improves the structure of both clayey and sandy soils and corrects acidity. It also promotes the de- composition of organic matter and aids nitrifi- cation. Mechanical analy.^is, which separates the par- ticles of a soil into six or more grades of fine- ness ranging from stones and gravel through sand and silt to clay, furnislies a valuable means of securing data for judging of the physical properties of soils. The productiveness of a soil depends very largely upon its texture and struc- ture, i.e. the size of the particles and their ar- rangement. These determine ver,y largely the circulation of water and gases, the solution and retention of plant food, and the growth of plant roots. When the grains are single or separated the soil is said to have a puddled structure, while a compounding of the soil grains gives a floccu- lated structure. The latter is desirable in all good soils, as it increases the pore space and facilitates the circulation of air and water through the soil mass. Flocculation may at times be. caused by frost action, but more fre- quently is produced by the action of lime. Fer- tilizers vary in their action on soils, some, like nitrate of soda, producing puddling, while others produce flocculation. The finer the soil particles the greater the injurious efl'eets of puddling, clay soils sufl'cring from this cause more than sandy soils. Puddling increases the water-retaining power, and thus retards percolation, but may ac- celerate capillary rise of water in the soil layers, Flocculation of the particles decreases the re- tention of water, aids percolation, and may re- tard evaporation. Water passes more easily from a coarse to a fine layer than from a fine to a coarse one, a fact taken advantage of by the farmer when he firms the soil by rolling and then loosens the surface by harrowing, which de- stroys the capillary spaces and so checks the escape of water into the air. The water is thus held near the surface, where it is readily acces- sible to the roots of plants. The action of the mulch (q.v. ) depends upon this principle. In humid regions the clay particles of the soil are usually washed down to a layer .several inches below the surface, the surface layer being called the soil proper, and the lower one the subsoil. In arid regions this difference does not exist, but the fine clay particles are evenly distributed throughout the soil layers. Soils vary widely in tlveir absorption power for w.ater and for fertilizing matter, a property frequently due in clayey and humus soils to the presence of colloid substances. Of the three principal fertilizing constituents — nitrogen, phos- phoric acid, and potash — soils apparently have the least retentive power for nitrogen (in the form of nitrate) and the greatest for phosphoric acid. SOILING, SOILING CROPS. The temperature of soils is modified by a va- riety of conditions, e.g. a dark-colored 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 proliably 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 growtli is verj* large — from 250 to 500 pounds for each pound of dry matter produced by the plant. Bibliography. Shaler, Origin and Nature of Soils (Twelfth Annual Report United States Geological Survey, 1890-91, part i,, Geolfigy, pp. 213-245) ; Jlerrill, Ifocks. Rock Weathcrimj, and Hoils (New York, 1897); King, The Soil (ib., 1898) ; Stockbridge, Rocks and Soils (ib., 1895) ; Hall, The Soil (London, 1903) ; Fream, Soils and Their Properties (London. 1S95) ; Warington. J'hi/sical Properties of Soils (Oxford, Eiig.. 1900) : Raniann. Forstlichc Jiodenkunde and Standortslehre (Berlin. 1893) ; Brooks, Agricul- ture, vol. i. (Springfield. 1901): Roberts, The Fertility of the Land (Xew 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 (L'nited States Department of Agriculture, Weather Bureau Bulletin 3, Washington, 1892) ; Whitney, Some Physical Properties of Soils in Their Relation to Crop Production (l'nited States Department of Agriculture, Weather Bureau Bulletin 4, Washington, 1892); King, The Principles and Conditions of the Movements of (Iroundu-ater (L'nited States Geological Survey, Nineteenth Annual Report, pt. ii., Washington, 1899) ; the reports and bulletins of the Bureau of Soils of the L'nited 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. SOIL AMENDMENTS. Substances, such as lime, gypsum, salt, muck. etc.. which increase the productiveness of soils without directl.v supplying any constituent which the plant needs. They act mainly by improving the physi- cal condition of soils, collecting and conserving moisture, setting free latent plant food, and cor- recting certain faulty conditions, such as acidity and alkalinity. Some of them, like muck, con- tain considerable amounts of availalile fertilizing ingredients. In all cases, however, they are used mainly for their secondary or incidental efl'eets. SOILING. SOILING CROPS. Soiling con- sists in feeding graxing animals in inclosures or in barns with green forage grown especially for the purpose, instead of turning them out to pas- ture; and soiling crops are the crops grown for this purpose. Soiling is a feature of intensive farming and small holdings, but it is also prac- ticed with profit in regions where the agricultural resources are in process of development. Lender European conditions stock is frequently fed in barns the year round, but in the United States soiling is usually combined with pasturing, stock being less frequently fed in barns during the sum-