THE PHYSICAL PROPERTIES OF THE SOIL. 773 



THE STRATIFICATION OF THE SOIL. 



The soil, as a wliole, is by no means uniform, and this is especially 

 true of sedimentary soils. Difterent soil layers have a different mechan- 

 ical constitution, the difference being- sometimes very great. Since the 

 individual layers behave differently toward water, air, and heat, the 

 determination of the arrangement of these layers is of particular 

 importance. The stratification may be ascertained by the examination 

 of vertical sections of soil exposed in pits dug at various suiiable places 

 or on the surface of columns of soil taken by means of a sampling tube. 

 Information can also be obtained as to stratification of the soil by examin- 

 ing existing exposures, such as railroad and other cuts and excavations. 



The upper layer of the soil, containing the roots of plants and altered 

 by vegetable growth, is called "surface soil," while the layer below this 

 is termed "subsoil." The surface soil includes two layers, an upper 

 one or tilled soil, which is artificially altered by tillage, and a lower 

 one influenced only by the growth of plants, and otherwise in its 

 natural condition. We may therefore distinguish between an upper 

 and a lower surface soil. In practical agriculture the term surface soil 

 generally indicates onlj^ the ui^per layer or tilled soil, while the subsoil 

 means the part of the soil below this. The subsoil here begins imme- 

 diately below the tilled soil, no matter how thick or how thin this may 

 be. The subsoil is sometimes regarded as that layer on whose texture 

 the moisture of the upper layer depends (i)ermeab]e or impermeable sub- 

 soil). A consistent and general terminology does not exist at present, 

 but that suggested above corresponds best with natural conditions.^ 



THE DEPTH OF THE SOIL. 



The depth of the soil means the thickness of that part which can be 

 penetrated by the roots of plants. This is of great importance to crops, 

 inasmuch as, within certain limits, they develop proijortionately to the 

 thickness of the layer in which they can extend their roots. The depth 

 of the soil, therefore, is determined by the distance below the surface of 

 the layer which prevents the growth of the subterranean organs of the 

 plant. In mountainous districts the depth of the rock below the sur- 

 face determines the depth of the soil. In level lands the dej^th of the 

 soil is determined by the distance below the surface of layers of closely 

 compacted materials (clay, rock, bog-iron ore) or of coarse gravel or 

 sand, which prevent more or less completely the growth of the roots on 

 account of their impenetrability, excess of water, or deficiency in the 

 substance necessary to the life of the organs. With regard to the depth 

 of different soils the following gradations maybe noticed: 



Thin soil 15 to 30 cm. 



Medium soil .30 to 60 cm. 



Thick soil 60 to 100 cm. 



Very thick soil above 1 meter. 



' L. von Libuman, Die geologischen Verhaltnisse von Grnnd und Boden, 1883. p. 223, 

 A. Nowacki, Practische Bodenkunde, 1892, p. 39. F. Dafert, Kleines Lehibuch der 

 Bodenkunde, 1885, p. 162. W. Schumacher, 1. c, p. 326. 



