154 PHOSPHATIC FERTILISERS [chap. 



land that bone meal answers best, for there the fine 

 roots of the grasses can come into very intimate contact 

 with the fragments of the fertiliser. 



The reason of the comparative ineffectiveness of 

 bone meal is not far to seek : owing to the toughness 

 of the cartilage structure of the bone, it is a matter of 

 difficulty to reduce it to a really fine state of division 

 — at any rate the bone meal of commerce is a com- 

 paratively coarse powder. Now, it has already been 

 pointed out that a tri-calcic phosphate, such as exists in 

 bones, is very far from insoluble in water charged with 

 carbon dioxide, but, as with all sparingly soluble salts, 

 the rate of solution will be proportional, other things 

 being equal, to the amount of surface the substance 

 exposes to attack, and for a given weight of material this 

 increases in the same proportion as the average diameter 

 of the particle decreases. In consequence, fineness of 

 grinding is perhaps the most important factor in deter- 

 mining the value of the insoluble phosphatic manures, 

 upon it more than even upon the chemical composition 

 depends the availability of the fertiliser to the plant. 

 Bone meal is slow-acting and ineffective, because it is 

 coarse ; nor is the phosphoric acid brought into solution 

 more readily in the second year than in the first, 

 because the coarse condition still persists. The availa- 

 bility of a phosphatic fertiliser might even be reckoned 

 as the product of two factors: a solubility factor depending 

 upon its chemical composition, and a second factor — 

 the area of surface of unit weight of the material. 



If bone meal has been overrated, on the other hand 

 steamed bone flour has not received the credit it 

 deserves. In the first place, analysts have rather warned 

 the farmer against steamed bone flour, as representing 

 in some way a spurious bone meal from which the 

 nitrogen had been illicitly extracted. The warning would 



