8 



for hard and dry soils. It is a general fertilizer ; it furnishes a cer- 

 tain amount of the three principal elements of plant-food : nitrogen, 

 pliosphoric acid and potash. Commercial fertilizers are often special 

 manures. The term " phosphates," which applies only to fertilizers 

 whose chief constituent of fertility is phosphoric acid, is often 

 erroneously taken by farmei's to mean a complete manure. 



Tlie supply of harnyard mamire is lirtiited / it cannot alioays 

 he ohtained. ^YJlat shall %oe substitute f — The orchardist may not 

 be able to secure barnyard manure but he can always make use of 

 "green manures." By this term ''.is meant a crop which is pri- 

 marily grown for the purpose of improving the soil and not for the 

 harvested product." (Yoorhees, Fertilizers, p. 118.) 



These green tnanures are of two classes : nitrogen collectors and 

 nitrogen consumers. — The members of the first class are able to 

 gather nitrogen from the air in addition to that which they take 

 from the soil. Those of the second class can only obtain it from 

 tlie soil. When we grow the nitrogen collector and plow it under, 

 we save the nitrogen taken from the air as well as that from the 

 soil. When we grow^ the nitrogen-consumer and plow it under, we 

 have only given back to the soil in a slightly changed form the 

 nitrogen originally taken from it. 



Commercial fertilizers core of two classes, standard high-grade 

 fertilizers and those which are variahle in composition and ccvail- 

 ahility, called low-grade. (Review Lesson 3.) — Among commer- 

 cial fertilizers there are certain materials wdiose composition and 

 availability are fairly cotistant. Nitrate of soda, sulphate of ammonia 

 and dried blood are called high-grade standards, because their com- 

 position may be depended upon to vary but little. These are nitro- 

 genous fertilizers, and the nitrogen is usually in a form of ammonia 

 which is immediately available. 



In the phosphates the phosphoric acid is not directly available. 

 Because of this fact the standard supplies of phosphoric acid are 

 derived from these materials after they are manufactured into 

 superphosphates. There are many kinds of these superphosphates. 

 They may be considered standards, as they always contain a high 

 percentage of available phosphoric acid. South Carolina and Ten- 



396 



