SOIL. 323 SOIL. soils are classed as sedentary or transported. When a soil is found resting on the parent rock from whose decay it has originated it is spoken of as scdcntari/ soil. It may show a gradual transition from the fully formed soil at the sur- face to the solid rock beneath. With this class may be grouped the humus or peaty soils due to accumulations of organic matters in bogs, swamps, and marshes. In many cases the residu- al products have been removed from the place of their formation by the action of water, ice (glaciers), and wind and deposited elsewhere in the form of clayey, sandy, or loamy soils, often representing the mingling of material from sev- eral diiTcrent sources. This type is termed Ininsportcd soil, and, though naturally very vari- able in character, includes some of the most productive soil in the world. The most important soils of this class are the allvivial soils, which often form a broad flood-plain (q.v.) bordering a river or a delta (q.v.) at the mouth, as in the case of the Nile and the Mississippi rivers. In the northern half of the United States much of the soil is of the glacial drift ttipc and repre- sents the debris of decayed rocks of various kinds brought down from the north during the glacial period (q.v.). .HoUan- soils are formed by wind action. They include: (1) Sand dunes, those shifting, sandy soils heaped up by wind action upon many ocean coasts and the shores of inland seas. ( See Dune; Dune Vegetation.) (2) Ash soils, the accumulations of ashes ejected by volcanoes. The deposits are often of considerable extent and are frequently very fertile, iluch of the highly productive region around Jlount Vesuvius, in Italy, is of this kind. Such soils are found in Nebraska, Colorado, and ilontana. Soils derived from disintegration of volcanic lava are of fre- quent occurrence, as, for instance, in the Hawaiian Islands, in Idaho and other Northwestern States. The loess soils of China and other countries are of leolian origin, although the so-called loess soils of America are believed to be for the most part of alluvial origin. Soils containing an ex- cess of soluble salts are found scattered through- out regions of deficient or irregular rainfall and are known as alkali soils (q.v.). Humous, peatv, or moor soils are composed largely of organic matter. The purest types are represented by the accumulations of peat (q.v.) formed in ponds and swamps; marine marshes, and muck soils represent a less pure variety. 'hen projterly drained and aerated and. in the case of marine marshes, freed from excess of solu- ble salts, they often prove very jiroductive. In practice soils are classified as gravelly, sandy, loamy, calcareous, humus, or peaty, etc., distinctions based on the fineness of the soil par- ticles and the relative proportions of sand, clay, lime, and humus, which they contain. Soils are also frequently classed as light and heavy, ac- cording as they are easy or difficult to till. In this sense a sandy soil is termed 'light' (easy to till ) , although actually having greater weight than a cl.ayey soil, which is termed 'heavy' (dif- ficult to till). The productiveness of a soil de- pends chiefly upon its chemical composition and its physical properties. Chemical and pliysical or mechanical analysis separates soil constituents into two general classes: (1) food constituents, and (2) physical constituents. The food constit- uents necessary to plant growth are nitro- gen, silicon, sulphur, phosphorus, chlorine, alu- minum, calcium, magnesium, potassium, sodium, and iron in various forms of chemical combina- tion. The mechanical constituents incKulc clay, silt, sand, humus, etc., which act as a physical support to plants and have an indirect fertilizing value. They form as a rule the large proportion of the soil mass, usually 90.95 per cent. Chemical PRorERTiEs. The average chemical composition of soils of humid and arid regions is shown in the following table prepared by Kil- gard: Average Chemical Composition of .Soils of Humid AND Arid Keqions CONBTITTENTe Humid region (averapre of 46fi Hoils) ■Arid rej^ioD (avertlKC of 31S 80ils) Insoluble matter Per cent. ^:0?l}87.087 .210 .091 .108 .225 3.131 4.296 .113 .052 PtT Cfnt. 70.505 1 7:266 h^- 135 .729 .264 Soda Iron oxid 5 75'> 7.888 117 riiosphoric acid 041 Carbonic acid 1 316 Water aud organic matter 3.G44 4.945 2.700 5.460 .122 .750 IG 870 Nitrogen in Inimns The proportions of actual fertilizing constitu- ents in soils, viz. potash, phosphoric acid, nitro- gen, lime, etc., are relatively small, arid soils showing somewhat larger proportions tlian humid soils. Other mineral constituents are usually present in sufficient quantity to supply the needs of plants. Humus (q.v.) is of special importance as a soil constituent not only on account of its beneficial effect on the physical properties of soils, but because it is an important source of nitrogen, as well as of phosphoric acid, potash, lime, etc. The proportions of the latter constitu- ents found in liumus in the form of humates represent to a large extent the amounts available in the soil for plant food. The nitrogen of hu- mus is converted into a form (nitrate) available for plants by the process of nitrification (q.v.) . Physical Properties. The physical proper- ties of soils which are of special importance are color, weight, fineness of division or texture, structure or arrangement of particles, adhesive- ness, and relations to gases, heat, moisture, and dissolved solids. Variations in these properties determine to a large extent the productiveness of soils. Ciood tilth and texture with their ac- companiments of good water conditions, aeration, and temperature are fully as essential to plant growth as an adequate supply of plant food. Physical properties of soils are, however, so largely dependent upon their natural character, and can be modified to such a limited extent by man, that it is of the greatest importance in practice carefully to select soils with special reference to the suitability of their physical characteristics to the crop to be gro'wn. The physical properties of soils are determined to a large extent by the proportions they con-
