14 GENERAL CONSIDERATIONS 



chasing a pound of this most expensive of the ele- 

 ments of fertility. When crops are constantly taken 

 from the land, however, the potash and phosphoric 

 acid will at length be exhausted, no matter how great 

 its original fertility, and it will become necessary to 

 supply them by purchase. The important question 

 of methods for maintaining or improving the fertil- 

 ity of the soil will be more fully discussed in a sub- 

 sequent chapter. 



Besides the exceedingly useful organisms that have 

 been so briefly discussed in the preceding paragraphs 

 the soil frequently harbors others that may become 

 exceedingly injurious by attacking and causing dis- 

 eases in our cultivated plants or our domestic ani- 

 mals. As a general rule the richer a soil is in humus 

 and decaying vegetable matter, the more likely it is 

 to be able to support these noxious parasites, since 

 some parasites are able to support life, at least for 

 certain periods of their existence, on such decaying 

 materials. It frequently happens that when the 

 same crop is grown continuously on the same soil 

 for a series of years, the land becomes so filled with 

 some disease-producing organism that this crop can 

 no longer be produced and its cultivation has to be 

 abandoned. This again is too large a subject for 

 full discussion here, but enough has been said to con- 

 clusively show that the study of the biology of the 

 soil is fully as important as that of soil chemistry 

 and soil physics. In fact chemistry, soil physics, and 

 biology with its manifold branches, especially bacteri- 

 ology and vegetable physiology and pathology, are 

 the scientific foundation on which all rational farm 



