116 CONSERVATION AND IMPROVEMENT OF SOIL 



stimulate the growth of a green manure crop which will itself be 

 more permanent in its effects. Any commercial fertilizer is 

 available in proportion to the amount and kind of the three plant 

 foods which it contains. 



Commercial fertilizers are inspected by nearly all states for 

 the purpose of keeping up their standards, food content, availa- 

 bility, and to protect legitimate business from fraudulent sales. 

 The laws of some states require that the sources of the three ingre- 

 dients be published by manufacturers. By this means the user may 

 know more regarding the availability of the plant food he is buying. 



Nitrogen is the most expensive as well as the most important 

 element in commercial fertilizers. It usually costs three or four 

 times as much per pound as the other ingredients. Because of 

 this great cost the nitrogen must be carefully conserved by 

 applying it at a time when it can be readily taken up by plants, 

 particularly if it is all or nearly all in an available form. 



Nitrogen is derived from several sources: meat scraps, dried 

 blood, tankage, fish, cottonseed meal and such minerals as sulfate 

 of ammonia, nitrate of soda, and calcium cyanamid. Nitrogen in 

 the first four forms is sometimes called organic nitrogen. 



Nitrogen from dried blood is usually more quickly available 

 for the use of plants than from other forms of organic nitrogen. 

 It may contain from ten to fifteen per cent of nitrogen, but may 

 be as low as five or six per cent. When the price is satisfactory 

 this is a very suitable form in which to purchase nitrogen as plant 

 food. The danger is that the price per pound will be too great 

 when compared with other forms of nitrogen. Some phosphoric 

 acid is found in samples of dried blood. This is greater when meat 

 scraps have been added. 



Tankage is the waste from the slaughter of animals in the 

 packing houses. It varies widely in composition and the analysis 

 should always be known before a purchase is made. If much 

 meat scraps be used it may be rich in nitrogen and phosphoric 

 acid. The presence of ground bone in it increases the phosphoric 

 acid. Tankage is usually slow in its availability for plant growth. 



Fish is often a source of fertilizer ingredients. Waste fish 

 products from seine fishing along sea coasts are ground up and 

 used for fertilizer. They usually contain, besides a good supply 

 of nitrogen, some phosphoric acid. The price of these ingredients 

 in this form is usually high in spite of the fact that they are not 

 quickly available for plant growth. 



