XIII.] MEAT AND FISH MANURES 259 



Somewhat similar in their composition and in their 

 action to the Peruvian guanos are a number of manures 

 which are manufactured out of residues of meat and 

 fish that accumulate in various processes of preserving, 

 canning, and packing these articles for food. It is 

 customary to extract as much as possible of the fat of 

 these materials, and then reduce them to a very fine 

 powder, which will contain nitrogen varying from 4 to 

 ro per cent, and phosphates which lie between 10 and 50 

 per cent. These fertilisers decay in the soil, and yield 

 ammonia in the same steady, continuous fashion as the 

 nitrogen compounds of the Peruvian guano, and we 

 may take it as a rule that the richer they are in 

 nitrogen the more active will that nitrogen be. Meat 

 and fish guanos, as they are called, form valuable 

 fertilisers for perennial crops like fruit and hops, and 

 may also be mixed in small quantities with purely 

 mineral manures to form fertilisers for root crops. 

 Rougher manures of this class are sometimes known as 

 tankage, or as greaves, and are comparatively slow 

 acting, some of them approach closely the bone meal 

 described above. The nitrogen they contain should be 

 reckoned as of less value than in the more concentrated 

 fertilisers. Naturally all these classes of meat and fish 

 residues can only be valued at the basis of their analysis. 

 A meat guano, for instance, may be a highly nitrogenous 

 fertiliser worth ;^8 or £g a ton, or on the other hand, 

 little better than a bone meal costing half the price. 



Very similar in their actions to meat and fish 

 residues are certain vegetable residues which from time 

 to time can be purchased as fertilisers. The best 

 known of them is rape dust, which consists of ground- 

 up residues of impure rape seed from which the oil has 

 been extracted by chemical processes. As a rule, such 

 seed residues from which oil has been extracted are 



