

SOME CONCEPTIONS OF THE SOIL 



ship, reactions occurring not only within each group but 

 between the groups as well. Unless such interactions take 

 place it is unlikely that the mixture will ever be in a con- 

 dition either chemically, physically or biologically to sus- 

 tain plant growth. These reactions, although very complex, 

 take place with surprising ease and rapidity. As a con- 

 sequence the study of this complex, heterogeneous and 

 highly dynamic mass that 

 we call the soil is often be- 

 set with difficulties that 

 completely baffle our pres- 

 ent facilities for its study. 

 2. Soil-forming rocks. 1 

 — In any study of soil 

 origin or composition, how- 

 ever cursory, the geological 

 phases of the problem im- 

 mediately force attention. 

 This is due to the bearing 

 that certain geological phe- 

 nomena have on soil condi- 

 tions and crop growth. In 

 the soil we find that the 

 inorganic materials have 

 originated from the com- 

 mon rocks. The best known 

 country rocks are of course 

 involved because they present the greatest outcrop surface and 

 of necessity must contribute most to the mineral fabrication of 

 the soil. They are classified under three heads — igneous, sedi- 

 mentary and metamorphic. The most important types from 

 the standpoint of soil formation are the following : 



1 For excellent non-technical discussions of rocks and minerals: — 

 Pirsson, L. V., BocJcs and Bock Minerals; New York, 1915. Merrill, 

 G. P., Rocks, Bock Weathering and Soils; New York, 1906. 



ORGANIC- 

 10% 



Fig. 1. — Volume composition of a 

 loam soil when in good condition 

 for plant growth. The air and 

 water in a soil are variable and 

 their proportion determines to a 

 considerable degree the productiv- 

 ity. 



