[" 540 ] 



&c., contain about 5 per cent, of N and 5 to 30 per cent, of 

 phosphates, and is called fish-guano. Soot is top-dressed as 

 an insecticidal manure chiefly on cereals. Its manurial value 

 depends on the proportion of NH 3 , it contains, which varies 

 from i to 4 per cent., the average being about 2 per cent. 

 Soot consists mainly of finely divided carbon with from 16 to 

 40 per cent, of mineral matters. 



866. Guano is another nitrogenous manure which is 

 largely used in England and America, but which we are not 

 likely ever to make use of. It is applied at the rate of 2 to 3 

 cwts. per acre (-50 to 60 Ibs. of NH 3 , and 100 Ibs. of P 2 O 5 ). 

 Two. classes of guano are distinguished. Of these the nitro- 

 genous guanos obtained from the dry regions of Peru, contain 

 as much as 2 per cent, of N and 12 per cent, of P 2 O r , and the 

 Phosphatic guanos obtained from west regions -9 per cent, of 

 N, 32*5 per cent, of PaC^ and 3 to 4 of K 2 O. Being very 

 variable in composition it is usually purchased on analysis. 

 Bird's dung and bat's dung containing more moisture about 

 (25 per cent.) are, weight for weight less valuable. But 

 dried, they are of equal value to guano (i.\ to 10 percent. NH-j 

 and 6 to 30 per cent. Phosphates). 



867. While on this subject of nitrogenous manures we 

 may once more revert to the question of utilization of 

 sewage and study the methods that have been devised for 

 making town-sewage inoffensive and less objectionable for 

 use as manure. We have already recommended the use 

 of dry earth, powdered charcoal and trenching with the addi- 

 tion of lime. Green vitriol (FeSOi), alum and lime are in use 

 in Europe for rendering human excreta inoffensive. In the 

 case of urine, calcium and magnesium phosphates are used, 

 Attempts to purify sewage by precipitation and filtration 

 cannot be said to have passed the experimental stage. One 

 of the processes adopted for agricultural purposes is called the 



' A, B, C process', the constituents used for precipitating 

 being alum, blood and clay, The precipitate is sold as 



