CLIMATIC Am) GEOCHBMICAL EELATIONSHIFS 81 



mates there are usually certain well-defined diif erences be- 

 tween the surface soil and the subsoil, besides the organic 

 content already spoken of. The subsoil is usually of a finer 

 and heavier character than the surface soil, due to the 

 downward movement of the small particles. This tends 

 to give the subsoil high retentive power, and may make 

 it rather impervious to water. Poor drainage conditions 

 may result. A certain amount of retentive power in a 

 subsoil is of considerable advantage, in that it aids in 

 the storage of water and prevents the excessive leaching 

 away of soluble plant-food. Moreover, almost all the 

 bacterial activities so important in the simplification of 

 compounds carrying food constituents are restricted 

 to the surface soil. The subsoil, being protected by the 

 layers above, has not been subjected to such vigorous 

 weathering, and as a consequence its mineral constituents 

 are not so available for the use of the crop. The deepen- 

 ing of the plow line and a consequent turning-up of the 

 subsoil must be carried out very cautiously for the above 

 reason. The cropping power of a soil may be markedly 

 reduced by the presence of too much of such material on 

 the surface at one time. 



The root distribution is restricted largely to the surface 

 soil, and this condition determines to some extent the 

 larger accumulation of humus therein and also its better 

 aeration and drainage. Experiments conducted in Utah ^ 

 show that with .barley, corn, and clover, from 90 to 96 

 per cent of the roots grow in the upper seven inches of 

 soil. From experiments made in Kansas ^ and in North 



1 Sanborn, J. W. Roots of Farm Crops. Utah Agr. Exp. 

 Sta., Bnl. 32. 1894. 



2 Ten Eyek, A. M. The Roots of Plants. Kansas Agr. Exp. 

 Sta., Bui. 127. 1904. 



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