6 THE FERTILITY OF THE SOIL [CH. 



United States, however, the limits are '005 mm. 

 (Wfrrr m -} an d "05 mm. (^ in.) respectively. 



A second group of soil constituents includes those 

 derived from the organisms deposited along with the 

 mineral particles while they lay under water. Of 

 these the most important is calcium carbonate, the 

 substance of which chalk and limestone are composed, 

 and into which lime rapidly changes when added to 

 the soil. As we shall see later on it plays an extremely 

 important part in the soil, and profoundly influences 

 the fertility and the vegetation relationships. It 

 differs from the substances already described in that 

 it dissolves somewhat in the soil water, and notable 

 quantities are washed out, amounting at Rothamsted 

 to about 800 Ibs. per acre per annum. 



Calcium phosphate also belongs to this group, 

 although some is derived from rock. It commonly 

 occurs only to a small extent, but it is an indis- 

 pensable food for plants and therefore essential to 

 fertility. 



These two groups of inorganic substances the 

 silica and complex silicates derived from the rocks, 

 and the calcium carbonate and phosphate derived 

 in part from organisms that once have lived do not 

 form the complete soil. A third constituent is present, 

 the so-called organic matter, derived from previous 

 generations of plants. It is a familiar observation 

 that no ordinary soil remains long without a covering 



