68 SOIL PHYSICS AND MANAGEMENT 



The thickness of this stratum varies from to 30 inches and even 

 more, as in the case of peat and other swamp soils. That of normal 

 upland loessial soils is from eight to ten inches. 



The amount of organic matter decreases with depth and varies 

 with that of the surface soil. Under normal conditions it is never as 

 abundant as in the surface because the root development is never 

 so great and the chances for the introduction of other vegetable 

 material are not so good. Exceptions sometimes occur in alluvial 

 land. The same downward movement of fine material has taken 

 place as in the surface soil, thus giving a slightly coarser texture 

 than in the subsoil. The subsurface may be made of distinct layers 

 that differ in color or texture or both. The color in prairie soil is 

 usually due to organic matter, while in timber soils it is principally 

 due to iron in some form. 



2. Subsoil. The subsoil extends to an indefinite depth, but is 

 sampled to 40 inches in humid climates. This stratum is of great 

 importance because drainage, capillary movement, root penetration 

 and resistance to drouth depend largely upon its character, and this 

 in turn depends largely upon its origin. If residual, its character 

 will vary with the parent rock from which it was derived. It will 

 be uniform if the parent rock was massive, and variable if the 

 parent rock was formed of strata of widely differing mineral and 

 physical composition. In cumulose, lacustrine, glacial and alluvial 

 deposits, the subsoil is likely to vary to almost any extent. There 

 may be substrata of gravel, sand, silt, clay and even peat with all 

 their variations. In loessial deposits two distinct layers usually 

 occur in the subsoil, the upper from 6 to 15 inches thick consisting 

 of a clayey silt or a silty clay, formed by the fine material carried 

 downward from the upper strata by water and deposited in the 

 upper subsoil, and the lower composed largely of silt and very fine 

 sand, the very pervious ordinary loess. Subsoils are usually less 

 pervious and more retentive of moisture than other strata. 



Tight Clay. All soils in humid climates permit more or less 

 water to percolate through them. When a rain falls water passes 

 into the soil through cracks, burrows, along roots and through the 

 pore spaces, carrying with it a small amount of very fine clay and 

 some iron oxide to the depth of percolation. In time the deposition 

 of this fine material between the coarser particles may produce a 

 very heavy, dense stratum, reducing the pore space to such an extent 

 as to make it almost impervious to air and water. This is especially 

 liable to take place in acid soils where no lime is present to precipi- 



