4 THE FERTILITY OF THE SOIL [CH. 



been shielded from these actions, but it contains 

 markedly less of the finest material. 



These particles constitute the chief portion of the 

 soil and may be regarded as the framework round 

 which the soil is built. They show, however, certain 

 differences which are of fundamental importance to 

 the subject. The sands and fine silt, being formed of 

 the silica, are chemically inert and practically un- 

 alterable under natural conditions except that they 

 may very slowly be reduced in size by weathering 

 processes. The finer material, on the other hand, is 

 chemically active and may not only undergo chemical 

 changes, but may enter into reaction with various 

 substances ; it is not a single definite compound like 

 the silica, however, and cannot be represented by any 

 chemical formula. It possesses other properties that 

 mark it off very sharply from the coarse material. 

 It absorbs a considerable amount of water, swelling 

 up very much during the process ; conversely when 

 it dries it shrinks a good deal. In its wet state it is 

 very sticky, when dry it is very hard. It undergoes 

 a remarkable change after treatment with traces of 

 acids or of salts, notably calcium carbonate, and 

 becomes less sticky and more easily crumbled. All 

 these properties are readily observed in a clay field 

 where much of this fine material is present : the 

 persistent wetness of the soil, its stickiness, the large 

 gaping cracks that form during its contraction on 



