MANURES 103 



It is largely used for hops, and as a constituent in many mixed 

 manures. Hair, horn, and feathers resemble wool in com- 

 position, and are occasionally employed as manure. 



7. Bones consist of about 70 per cent, of mineral matter, 

 chiefly calcium phosphate, and 30 per cent, of organic matter, 

 containing 3 or 4 per cent, of nitrogen and a variable quantity 

 of fat. Coaroe bones decay very slowly; in some soils they 

 can be found almost unchanged several years after their 

 application. They are therefore now reduced to small frag- 

 ments, and are graded, according to their size, as " half -inch 

 bones," ** bone dust," "bone meal," and " bone flour." They 

 are often heated with steam under pressure before grinding, so 

 as to remove the fat and some of the nitrogenous matter. 

 They are then more easily ground and decay more quickly 

 when applied. Bone ash is sometimes used; it is free from 

 nitrogen and organic matter, and valuable only for its phos- 

 phates. 



8. Soot consists largely of carbon, but that from house 

 chimneys contains about 3 per cent, of nitrogen, in the form 

 of ammonium salts and organic compounds. 



9. Oil cakes consist of the husks and residue left after 

 expressing oil from certain seeds, and are rich in all the con- 

 stituents of plant food. Usually these residues are employed 

 as food, but in some cases they are unpalatable or poisonous, 

 They then form valuable manures, though somewhat slow in 

 action. * If the oil has been extracted by solvents the product 

 is improved. Rape seed and castor oil seed cakes are the chief 

 examples. They contain from 5 to 6 per cent, nitrogen, 

 about 1 per cent, of potash and 1*5 per cent, of phosphorus 

 pentoxide. 



10. Human Excreta. — Where earth-closets are used the 

 night soil has considerable value as a manure, provided it can 

 be used locally. So, too, cess-pools yield a liquid manure rich 

 in fertilising materials. In large towns, however, the excreta 

 of the inhabitants usually passes into the sewers and becomes 

 mixed and diluted with much water and trade effluent. Its 



