Soil Fertility and Its Preservation 



43 



Natural processes that enrich the soil. There are Rain and 

 several ways in which Nature constantly adds to the weaiherin 9 

 soil some of the elements that it needs most. One of 

 these elements is nitrogen. The atmosphere contains a 

 small amount of ammonia, a compound that is rich in 

 nitrogen ; and every time it rains or snows, some of 

 this ammonia is brought down to the earth and added to 

 the soil. The weathering process that is continually 

 going on in loose, cultivated soil is always making soluble 

 new supplies of the food elements already in the ground. 

 Wind and running water carry dust and earth from place Wind and 

 to place, and these often add quite materially to the 

 fertility of the soil. But the most interesting process 



1700 



220Q Ib. 



No fertilizer 



1OOOO Ib. 



No fertilizer 



Nitrate 



Legume 



FIGS. 24, 25, and 26. An experiment station in California got the above yields 

 per acre from each of three similar plots. The first plot was not fertilized, 

 the second was fertilized with nitrate of soda, and the third was fertilized by 

 growing a legume crop (as clover) and plowing it under. 



