242 PERUVIAN GUANO, ETC. [chap. 



experiments, however, show that it is active enough 

 to produce nearly its full effect in the first season. 

 The organic matter rape dust supplies has a bene- 

 ficial effect upon the tilth of the soil ; on the Rotham- 

 sted mangold field, as has been pointed out earlier 

 (p. 218), the best results as regards the proportion of a 

 full plant obtained are yielded by the plot manured 

 with rape cake. In general farming rape cake has been 

 found a very suitable source of nitrogen for the barley 

 crop ; it is highly esteemed by hop growers, though 

 of late years its comparatively high price per unit of 

 nitrogen has much diminished its consumption in Kent 

 and Sussex. It is also valued by fruit growers, but it 

 is supposed to make a bad top dressing for grass, and, 

 like all manures of its class, it should not in its fresh 

 condition be put in contact with germinating seeds or 

 young plants, probably because of the fungi and moulds 

 with which it becomes permeated in the soil. 



Other cake residues of a similar character come on 

 the market from time to time in the shape of damaged 

 cargoes of cotton, linseed, or other cakes, that have been 

 spoilt for food by getting damp and heating or by the 

 access of sea water. They may be judged on the same 

 basis as rape cake and their value estimated from their 

 analysis. 



Castor cake or pomace, the residue left after castor 

 oil has been expressed from the seeds, has no value for 

 food, but makes a good fertiliser of the same class as 

 rape cake. It is not often available in this country, as 

 the castor oil is generally expressed before exportation ; 

 in India and other tropical countries, however, it forms 

 a very valuable source of manurial nitrogen, because 

 organic compounds of nitrogen are particularly desirable 

 in tropical soils, which so rapidly lose their humus under 

 cultivation. 



