Phosphates Their Sources 79 



the phosphate in raw bone meal of the same fineness 

 showing rather a lower rate of solubility than the phos- 

 phates in steamed bone. The phosphate in the finest 

 steamed bone is much more soluble than that in the 

 coarser grades. This measure of the rate of solubility 

 of bone, while not, perhaps, showing the exact rate at 

 which the plants may obtain it, is a fairly safe guide in 

 its use for most crops, as compared with those mineral 

 phosphates which are not perceptibly soluble in this 

 medium. The range of solubility of different kinds and 

 grades of bone is from 20 to 75 per cent, and the average 

 of a large number show about 30 per cent soluble in 

 citrate of ammonia, which would be called "available" 

 if found in mixed fertilizers, and probably can be as safely 

 depended upon as the available shown in other products. 



In any case, animal bone, or finely ground mineral 

 phosphates, cannot be depended upon to meet fully the 

 needs of quick-growing crops for phosphoric acid, but 

 may answer an excellent purpose where the object is 

 to improve gradually the soil in its content of this con- 

 stituent, as well as to supply such crops as are continuous, 

 or that grow through long periods, as, for example, 

 meadows, pastures and orchard and vineyard crops. 



As to the specific substance, the iron phosphate, or 

 Thomas phosphate powder, experiments in Europe have 

 shown that it possesses a higher rate of availability than 

 other phosphates which are insoluble in water, but which 

 show the same rate of solubility in ammonium citrate, 

 though its solubility, or availability, is measured to some 

 extent by the degree of fineness to which it is ground; 

 and it is believed that its special form, the tetra-calcic, 

 also exercises a considerable influence upon the rate of 

 availability. 



