VI. 



MANUEING AND GENERAL MANURES 



131 



from a coal from Porthmawr in Wales, which, according to an analysis 

 by Phillips, 1 contained 6*6 per cent of phosphorus pentoxide. 



Though in such coal ash the phosphorus is probably locked up in 

 a somewhat insoluble form, it might furnish a useful source of plant 

 food in districts where phosphates are difficult or expensive to procure. 



Oil-cakes. These, the residue of husks, etc., left after the oil is 

 expressed from certain seeds, are often highly nitrogenous and 

 generally contain considerable quantities of phosphates and potash ; 

 indeed, of all forms of plant food. Usually they are used as food for 

 animals ; but in some cases, owing to the presence in the seed of 

 poisonous or unpalatable substances, they are only fit for manurial 

 purposes. 



Kape, mustard, cotton and castor cakes are the principal ones used 

 for manure, and of these, cotton cake but rarely. The percentages of 

 the important constituents in such cakes are given in the following 

 table : 



These substances decompose slowly in the soil, so that they are not 

 quick-acting manures. They give better results as a rule on clay soils 

 than on light sandy ones. 



When the oil has been extracted by solvents the cake is of greater 

 value as a manure, since not only is it richer in the proportion of 

 valuable ingredients, but the absence of oily matter permits of more 

 ready access of water and thus favours oxidation. 



1 Quoted in Muspratt's Dictionary of Chemistry. 



