78 Fertilizers 



than one which is close and compact in texture and purely 

 mineral in its character, thus preventing the free access 

 of air and water, and in which no organic changes are 

 taking place. In the one case the conditions are such 

 as to favor the action of the natural agencies, and in the 

 other they are such as to retard their action. 



Influence of the kind of crop. 



In the fourth place, the value or usefulness of phos- 

 phates is measured to some extent by the characteristics 

 of the plant or crop to which they are applied. Plants 

 differ in their power of acquiring food. Certain plants 

 are able, because of their peculiar root system, or period 

 of growth, to appropriate food more readily from in- 

 soluble sources than others. 



General considerations. 



All these considerations must be observed in determin- 

 ing the usefulness of a phosphate. It is believed by 

 experienced farmers, though not absolutely confirmed 

 by experimental inquiry, that animal bone, for example, 

 is far superior, as a source of phosphoric acid, for most 

 crops, to the mineral phosphates, though both may be 

 ground to the same degree of fineness ; and also, that the 

 finer the bone is ground, the more rapidly will it give up 

 its phosphoric acid. 



Laboratory tests show that the phosphoric acid in 

 bone, while insoluble in water, may be partly dissolved 

 at a certain temperature by a neutral solution of am- 

 monium citrate. This medium is used to determine 

 what is called "available" in other phosphatic products. 

 The rate of solubility in this medium is measured by 

 the method of preparation of the bone and its fineness, 



