334 SOILS AND PLANT LIFE 



per acre; and to secure the best results, this is not all 

 applied at one time but two or three applications in smaller 

 amounts are made during the growing season. . 



Ammonium sulfate, which contains about twenty-one 

 per cent of nitrogen, is often used. It is quite effective ; 

 but a rather serious objection to it is found in the fact 

 that it apparently tends to leave the soil acid. 



Calcium nitrate, which is manufactured chiefly in 

 Norway, and which contains about twelve per cent of 

 nitrogen, is a highly satisfactory fertilizer save for the 

 fact that the nitrogen in it is rather more expensive than 

 that in the products named above. 



Fish fertilizers, consisting usually of the dried, ground 

 bodies of the menhaden, are often used with excellent 

 results. They contain from eight to eleven per cent of 

 nitrogen as a rule and also from one to two per cent of 

 phosphorus. 



Phosphorus. — Unlike nitrogen, this element can not 

 be restored to the soil by growing legumes or any other 

 special crops. Rather it can be added only in the form 

 of barnyard manure, as already stated, or in the form of 

 commercial fertilizers. 



The commercial fertilizers conmionly used for this 

 purpose are: 



Ground phosphate rock, or floats. This rock is quarried 

 in Florida, the Carolinas, Tennessee and some of the 

 western states. It is also called rock phosphate or in- 

 soluble phosphate of lime. It contains from eleven to 

 thirteen per cent of phosphorus. It is insoluble in water 

 and for this reason could not be used as a fertilizer at all 

 but for the fact that it is very gradually dissolved by the 

 acids which are formed by the decay of the humus in the 

 soil. It is usually applied with barnyard manure. Its 

 effects are not quickly seen as a rule but extend over a 



