Fertilizers and Fertility. 345 



those containing phosphoric acid ; (3) those containing potash. Follow- 

 ing are some of the materials most often used : 



Group I, containing nitrogen : 



Nitrate of soda contains 15 per cent nitrogen — all directly available. 



Dried blood 10-12 per cent ] Must undergo decay in the 



Tankage 4-10 per cent I soil to liberate the nitrogen 



Fish scrap 7-8 per cent J in available form. 



Group II, phosphoric acid: 



Acid phosphate consisting of phosphate rock or dissolved rock, 

 bones treated with sulphuric acid. The product is the same 

 whether bones or rock are used, and contains 14-16 per cent 

 available phosphoric acid, and about 50 per cent gypsum, or land 

 plaster. 



Group III, potash: 



Muriate of potash contains 50 per cent potash ] 



Kainit contains 12-15 per cent potash |. All available. 



Sulfate of potash contains 50 per cent potash J 



The substances named in Groups II and III are the principal ones 

 used to furnish phosphoric acid and potash in all grades of commercial 

 fertilizers. Those in Group I furnish nitrogen in a form that is either 

 directly available or may easily become so, and can be considered high 

 grade materials. It is in the nature of the materials used to furnish 

 nitrogen that one difference is found between high-grade and low-grade 

 fertilizers. Such products as tankage (low in nitrogen), ground-leather 

 scrap and hoof and horn meal, all contain nitrogen, but in such a form 

 that it undergoes the necessary changes in the soil very slowly. In 

 general, the value of a material furnishing nitrogen may be judged (i) 

 according to the readiness with which it decays in the soil, and (2) 

 as to its content of nitrogen. One product may contain 3 per cent nitro- 

 gen, decaying easily and be of more actual immediate value than another 

 containing 7 per cent nitrogen resisting decay. Of all substances used 

 to supply nitrogen, the nitrates, (as for example, nitrate of soda, or 

 nitrate of potash) are the only ones that supply nitrogen in a form 

 immediately available to plants. All other substances must undergo decay 

 or some change in the soil in order to liberate the nitrogen they contain. 

 As the nitrogen is liberated, it is changed into nitrates by the bacteria 

 in the soil. 



The nitrogen that is used by plants must be in the form of nitrates 

 before it becomes assimilated by them. The more easily a material 

 decays, the quicker will its nitrogen become available. 



