14<S Principles of Plant Culture. 



crops are removed from the land and are not returned to 

 it, directly or in equivalent. Fortunately, considerable 

 plant food is constantly being liberated by the disinte- 

 gration and decay of rock or soil materials, or is being 

 deposited from the atmosphere in rain and snow, so that 

 it is impossible to exhaust the soil of plant food, even 

 with the most improvident culture. But the cultivator 

 should aim at the largest returns from his soil, and these are 

 impossible ■without restoring certain materials that con- 

 tinued crop-removal invariably reduces below the limit 

 of profitable yields. 



254. The Food Elements Most Likely to be Deficient, 

 ■when plants are properly supplied with water, are nitro- 

 gen, phosphorus and potassium. These are all liberated in 

 greater or less quantities, when vegetable or animal ma- 

 terial (organic matter) decays in the soil; hence all such 

 material has more or less value as fertilizers. But we 

 need not wholly depend upon refuse organic matter for 

 fertilizers, since the leguminous plants add nitrogen to 

 the soil (260), and compounds of nitrogen, phosphorus 

 and potassium may often be purchased in artificial fertil- 

 izers at prices that place them within the reach of the 

 cultivator. 



255. Nitrogen is the Most important Fertilizing Element 

 because it is liberated in smallest amount by rock decay 

 and is most expensive in the market. Nitrogen is chiefly 

 used by plants in the form of nitrates, i. e., in combina- 

 tion with certain other substances as soda, potash, lime, 

 magnesia and iron. Ammonia, which is a gaseous com- 

 pound of nitrogen and hydrogen, is also used to some 

 extent by plants. Free nitrogen, the most abundant 



