38 THE CHEMISTRY OF THE FARM. 



Bones. These are largely employed as manure ; the 

 fat is usually first extracted by steaming. Commercial 

 bones contain about 3'6 per cent, of nitrogen, and 23 per 

 cent, of phosphoric acid, existing as phosphate of calcium. 

 Bones that have been boiled to extract the gelatin con- 

 tain about 1'4 per cent, of nitrogen, and 29 per cent, of 

 phosphoric acid. 



Bones decompose but slowly in the soil, especially on 

 heavy land ; their effect is thus spread over several years. 

 The finer the bones have been ground the more immedi- 

 ate is their effect. Bones are usually employed for pas- 

 ture, and for turnips. 



Oilcakes. Cakes of little or no value for feeding 

 purposes are used when ground as manure ; their value 

 is considerable, as they contain a good deal of nitrogen, 

 with phosphates and potash. See p. 139. 



Fhosphatic Slag, and Ground Phosphates. Some 

 phosphates when finely ground may be successfully em- 

 ployed as manure without previous conversion into super- 

 phosphate. The phosphates most suitable for this pur- 

 pose are Thomas' slag, phosphatic guanos and bone-ash. 

 Phosphatic slag contains generally 15 20 per cent, of 

 phosphoric acid ; it is now employed in enormous quanti- 

 ties. The soils most suitable for such manures are those 

 rich in humus, and poor in carbonate of calcium ; these 

 being the conditions (presence of humic and free carbonic 

 acid) most favourable to the solution of phosphate of 

 calcium. Moorland and pasture soils are especially suit- 

 able for such treatment. Thomas' slag is an effective 

 manure for swedes and early turnips. Undissolved phos- 

 phates must be employed in extremely fine powder. 



