MATEBIALS EXISTING IN THE SOIL. 



49 



the practice of agriculture in every part of the world. It 

 has demonstrated that, not only does every plant require 

 that the substances above described should be present in 

 the soil for its use, but that the different families into which 

 we are accustomed for convenience to divide our crops, are 

 distinguished by a remarkable difference in the proportions 

 in which these substances are found to exist ui their incom- 

 bustible remains, and that different plants, like wheat and 

 clover, though growing upon soils of every variety of compo- 

 sition, invariably select different proportions of particular 

 kinds of matter for their nourishment, some plants being found 

 to contain in their ashes, and, consequently, to take up from 

 the soil chiefly potash and soda, and others again silica or 

 phosphorus, or sulphur. The value of this information will 

 be fully illustrated in a subsequent chapter. 



5(3. The proportions in which the materials that have just 

 been described enter into the constitution of plants are sub- 

 ject to considerable variations. The following tables, however, 

 constructed from the analyses of Boussingault will give you 

 an idea of the composition of the organic portion of your 

 ordinary crops, and also of the amount of matters derived 

 from the soil, which that distinguished chemist found in the 

 plants grown upon his farm in the east of France. 



1 00 lbs. of the following plants in the fresh state usually 

 contain — 



