144 



SOIL PHYSICS AND MANAGEMENT 



Arid soils are naturally low in organic matter because the moist- 

 ure is not sufficient to produce a large growth of vegetation. 



(b) Vegetation. The upland timber soils contain much less 

 organic matter than the adjacent prairies. It is safe to assume that 

 the prairies were much more extensive formerly than now. Newly 

 formed lands were originally treeless and covered by smaller plants, 

 but more especially grasses. This was particularly true in the glaci- 

 ated area. The prairies were covered with grasses whose network 

 of roots extended to a depth of 8 to 20 inches or more. A sample of 

 virgin blue stem prairie sod on brown silt loam contained roots at 

 the rate of IS 1 /^ tons per acre to the depth of 6% inches. Part of 

 these roots died each year, and the partially decayed material 

 accumulated in the soil, forming the black prairie soils of the corn 

 belt. In Illinois the analyses of 302 samples show the surface soil to 

 a depth of 6% inches to contain 4.53 per cent, or about 45 tons of 

 organic matter per acre. This includes the rolling and flat prairie 

 soils, but not the swamps. The subsurface, 6% to 20 inches in depth, 

 showed an organic-matter content of 2.8 per cent. 



That this is probably true as to the origin of the black earth soil 

 or chernozem of Russia is well shown by the following table which 

 gives the relative amount of roots and percentage of humus in six- 

 inch depths: 



Roots and Humus in Three Chernozem Soils at Different Depths. The Roots 

 in the Surface Six Inches is Taken as 100 Per cent 



These determinations are rather typical for semi-arid or rather 

 sub-humid prairie soils where there is a greater tendency for the 

 roots to penetrate deeply. A similar condition exists in humid soils, 

 with this difference, that the great mass of roots is nearer the surface. 



The invasion of the prairies bv forests has been goiner on very 

 slowly. The first trees to spread over the prairies were wild cherry, 



