PLANT FOOD AND SOIL FERTILITY 33 



form of a commercial fertilizer may be sometimes justified 

 in spite of its expense. It may be used when nitrogen cannot 

 be secured through the use of manure and rotation of crops; 

 or when it is desirable to give a crop an especially good start, 

 as corn in early spring, or wheat in the fall. In such case the 

 nitrogen is added for its immediate effect. It is at once 

 available for plant use; whereas, in early spring, the natural 

 supply of available nitrogen is small due to the effect of cold 

 on bacterial action; and in the fall, because the preceding 

 crop has partially exhausted the supply. In such cases the 

 supply of phosphorus and potassium may also be limited, 

 and the use of a complete fertilizer one containing all three 

 elements would be justified. But whether nitrogen is 

 used alone or in a complete fertilizer, a light application is 

 generally more profitable than a heavy one. 



Nitrogen may be supplied from certain substances (called 

 carriers) that contain it. Nitrate of soda, sulfate of ammonia, 

 and dried blood are always safe to use and are really eco- 

 nomical even though they may seem expensive, since they 

 are readily available for plant use. 



PHOSPHORUS 



The supply of phosphorus. There is a limited supply of 

 phosphorus in most soils. A small amount occurs in manure 

 and in other organic material, but not in sufficient quantities, 

 as used in ordinary farm practice, to offset the amount re- 

 moved by crops. It must be bought and added to the soil 

 from time to time. Since it must be bought, economy in 

 buying should be considered. 



How to secure phosphorus. The chief source of 

 phosphorus is certain mineral deposits known as phosphate 



