28 



FUNDAMENTALS OF AGRICULTURE. 



in sufficient quantity in ordinary soils, but certain ones 

 are usually rather limited in amount, so that several 

 large crops in succession exhaust the available supply. 

 So it becomes necessary to add these to the soil, or in 

 other words, we have to apply fertilizers. The sub- 



TUBERCLES OF VELVET BEAN PRODUCED BY INOCULATION. 



stances most generally applied are potash, phosphoric 

 acid, nitrogen (i. e., ammonia), and, for many soils, 

 lime. 



Some Plants Gather Nitrogen from the Air. Al- 

 though almost four-fifths of the air is composed of 

 nitrogen, most plants cannot make use of this supply, 

 and would die for want of it unless compounds con- 



