146 PRINCIPLES OF PLANT CULTURE 



good crops of tobacco, cabbage, celery, lettuce or potatoes. 

 But with a sufficient restriction of food, the seed product 

 will suffer diminution or be wholly cut off. 



252. Reducing plant-food. Crop-growing tends to 

 reduce plant-food in the soil in proportion as the fertilizing 

 components of the crops are removed from the land and 

 are not returned to it, directly or in equivalent. Fortu- 

 nately, considerable plant-food is constantly being liber- 

 ated by the disintegration and decay of rock or soil 

 materials, or is being deposited from the atmosphere in 

 rain or 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 continued crop-removal invariably 

 reduces below the limit of profitable yields. 



253. Deficient food elements. The food elements 

 most likely to be deficient, when plants are properly sup- 

 plied with water, are nitrogen, phosphorus and potassium. 

 These are all liberated in greater or less quantities, when 

 vegetable or animal material (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 (259), and compounds of nitro- 

 gen, phosphorus and potassium may often be purchased 

 in artificial fertilizers at prices that place them within the 

 reach of the cultivator. 



254. Importance of nitrogen. 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 combination with certain other 



