THE CHEMISTRY OF THE SOIL. 93 



MANURES CONTAINING PHOSPHORIC ACID. 



Phosphoric acid is applied to the soil almost exclusively in the form of 

 phosphate <>t' lime, and its sources arc I es, rock phosphates, and guanos. 



Bones and Bone-products. 



Hums vary very slightly in composition from whatever source they are 

 drawn, or from whatever part of the animal they are taken, though as a rule 

 the thigh-bones and the bones that have to bear the greatest meetianical 

 strain are the richest in phosphate of lime. Bones are composed chemically 

 of water, ossein or collagen, Eat, and mineral salts. The last named, which 

 are left behind as ash when the bones are burnt, consist principally of phos- 

 phate of lime. Bone-ash contains about 83 per cent, of this substance, 

 together with about 10 per cent, carbonate of lime, and in much smaller 

 quantities magnesium compounds, and fluoride and chloride of calcium. 



The ossein of the bones is the substance which is converted into gelatine 

 by boiling with water, and is an albuminoid containing about 16 per cent, 

 nitrogen. Bones are, therefore, a nitrogenous, as well as a phosphatic, 

 manure. An average sample, reduced to powder as bone-meal, contains about 

 45 per cent, phosphate of lime, and a trifle under 4 per cent, nitrogen. 



Bones are used in a variety of ways. They may be used whole or broken, 

 or reduced to powder (as bone-meal, bonedust, or ground bones) ; or they may 

 be boiled or steamed, fermented, calcined (bone-ash), charred (bone-black), 

 or converted into superphosphate. 



We shall now see how these different methods of treatment affect their 

 composition and action. 



Bone-meal. 



When simply broken or crushed their chemical composition is, of course, 

 unaltered, and the principal advantage derived from their finer mechanical 

 condition is the greater rapidity of their action. Whole bones resist decom- 

 position within the soil for a considerable length of time, and it is very 

 doubtful if their use is in any sense economical ; in fact, as the object of 

 artificial manuring is to feed the crop rather than the soil, it is doubtful 

 whether slow-acting manures are in any case economical. The case of lime, 

 which we have just considered, stands on a different footing. Lime is seldom 

 applied as a direct plant-food. Its action is practically confined to the soil. 

 The substances we have now to consider are valuable only when they are 

 available as plant-food. They produce little or no permanent benefit to the 

 soil, and if their decomposition is slow the plant receives its nourishment in 

 small driblets, inadequate to its needs. 



Bones are, therefore, most efficacious when crushed, and, within certain 

 limits, the finer the powder the better the product as a manure. An addi- 

 tional advantage of fineness of division lies in the ease and evenness with 

 which it can be distributed on the land or mixed with other manures. 



Bone-meal is decomposed in the earth, the nitrogen in the ossein being 

 converted by putrefaction into ammonia, and the phosphate of lime rendered 

 oluble by the action of carbonic acid and the vegetable acids. It is particu- 

 larly suited to curnips and root crops generally, grass, tobacco, fruit-trees, 

 and in fact is a manure of almost universal application. It is applied in some 

 countries at 3 to 5 cwt. per acre, and if mixed with a manure containing 

 potash, forms a complete manure, and an excellent substitute for stable 



