FERTILIZER CONTROL STATION. 263 



and ammonia, are two of the most valuable nitrogenous materials 

 which are used to supply nitrogen to the farmer. Their nitrogen is 

 immediately available for use by the plant, the nitric acid of the one 

 and the ammonia of the other being the compounds of nitrogen which 

 largely serve as plant-food. The following materials furnish organic 

 nitrogen to fertilizers : 



Dried blood, dried and ground fish, azotin and ammonite (pre- 

 pared animal matter), fish scrap, meat scrap, cotton-seed meal, 

 castor pomace, horn, hair, wool, leather waste, etc. These sub- 

 stances must decompose and the nitrogen become changed into 

 compounds of nitric acid and ammonia before it is available to plants. 

 There is, therefore, a great difference in the value of organic nitro- 

 gen as found in the above-named materials. Dried blood, for 

 instance, decomposes in the soil rapidly, while horn, hair, wool and 

 leather scrap decay very slowly, and the nitrogen which they contain 

 becomes useful only after a long period of time. These latter sub- 

 stances are not only less useful to the farmer than blood, fish and 

 meat, but they are also much less costly, and their presence in a 

 fertilizer supposed to be manufactured of the best materials is good 

 evidence of fraud. 



The phosphoric acid of superphosphates is determined in three 

 forms, according to its solubility in various liquids, viz : soluble^ re- 

 verted, and insoluble. 



Soluble phosphoric acid is that which exists in fertilizers in a form 

 freely soluble in water. It is obtained by treating certain phosphatic 

 materials, such as bone and South Carolina rock, with sulphuric acid 

 (oil of vitriol) . 



In the chemical changes caused by the sulphuric acid, hydrated 

 calcium sulphate (gypsum) is formed if sufficient water be present, 

 which is the same compound as land plaster. The advantage of 

 having the phosphoric acid of fertilizers rendered soluble is not that 

 it remains so in the soil, for it becomes insoluble in water very 

 shortly after application, but in the fact that when the compounds of 

 the soil change it back to an insoluble condition it becomes deposited 

 in pai tides so minute that they are easily appropriated by the roots 

 of plants. 



Eeve7^ted phospJioric acid is a term originally signifying phosphoric 

 acid that had once been "soluble," but which from some cause had 

 ''reverted" or "gone back" to forms insoluble in water. Now it is 

 used to designate that which is dissolved by a solution of ammonium 



