CHAPTER VI 



SOIL AND SUBSOIL 



THE soil may be conveniently divided into two strata: (1) the 

 top soil, consisting of (a) surface to 6% inches and (b) subsur- 

 face 6% to 20 inches, and (2) subsoil, which extends to an indefi- 

 nite depth, but is sampled from, 20 to 40 inches. The difference 

 between the two divisions, the top soil and subsoil, is mainly due to 

 the action of organisms, both plants and animals, although physical 

 and chemical agencies have played no inconsiderable part in pro- 

 ducing these differences. 



1. The Top Soil. (a) Surface. The surface soil is confined 

 to the part usually turned by the plow and is the stratum with 

 which the farmer is most familiar. Organic matter and fertilizers 

 are incorporated in this stratum and for this reason the roots of 

 our common crops are largely confined here. The most obvious dis- 

 tinction in nxost soils between this and any other layers is the 

 darker color produced by the larger content of organic matter or 

 humus. This brings about decided color changes, sucli as darkening 

 when moistened. Hydrated ferric oxide, if very abundant, may 

 obscure the dark color of organic matter. 



The surface soil frequently differs from the other strata, and 

 more particularly the subsoil, in being made up of slightly coarser 

 material. This difference is not found in arid regions. It is due 

 to the washing downward of the fine particles by percolating water, 

 as well as by their removal through surface run-off during heavy 

 showers. This stratum contains the largest amount of fertility but 

 generally the least of lime. Organisms of all kinds, usually found 

 in soils, are more abundant in this layer. Here are found the most 

 favorable condition's for bacterial growth and activity. The germs 

 of fungous diseases, if present in the soil, are more abundant in 

 this stratum. It is the only part of the soil that we can change 

 materially and hence its importance. 



(b) Subsurface. The subsurface stratum lies between the sur- 

 face and the subsoil, but usually resembles the surface more closely 

 than it does the subsoil. The stratum is a natural one, extending 

 from the plowed soil to the line where the change in color, physical 

 composition and structure indicates the beginning of the subsoil. 



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