324 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



value of this plant food would be $9.50. Wheat at $1.00 per 

 bushel would bring $33.33 per ton. The value of plant food in 

 the grain is thus nearly y$ the selling price of the crop. Economy 

 in the application of fertilizers is essential to profit with such crops. 

 Nitrogen should be secured as much as possible from the air by 

 legumes. Application of fertilizers should be based largely upon 

 the needs of the soil. 



Onions, asparagus, melons, cabbage, and tomatoes, are ex- 

 amples of crops which have a high value per acre. Such plants 

 may be fertilized liberally, since the cost of even large applications 

 of fertilizer is in small proportion to the value of the crop. 

 Manure or legumes turned under should also be used on account 

 of their beneficial effect on the soil. 



Character of the Crop. Plants vary in the quantity of plant 

 food needed and in their ability to secure it. The season of 

 growth is also of significance, since plants growing during the 

 cooler period of the year are supplied with less nitrogen by the 

 soil than those growing during the warm season. 



While each plant possesses individual characteristics which 

 distinguish it from others, they may be divided into groups which 

 have somewhat similar characteristics, particularly as regards 

 method and time of growth and their capacity for acquiring food 

 from soil sources. 



The cereals have a wide root growth and are able to acquire 

 food from insoluble phosphates and potash readily. As, with the 

 exception of Indian corn, their development takes place 

 early in the summer before conditions are favorable for rapid 

 nitrification, they are particularly benefited by nitrates. Corn 

 does not usually require as large proportions of nitrogen as of 

 mineral constituents, as its growth is made in the summer while 

 the conditions are very favorable for nitrates. 



The grasses resemble the cereals in their power of acquiring 

 mineral food and are also benefited by application of nitrogen. 



The clovers readily acquire mineral food, and also take nitro- 

 gen from the air. 



