104 ELEMENTARY AGRICULTURAL CHEMISTRY 



utilisation then becomes very difficult, although highly desir- 

 able. Not only is valuable manure lost by the sewers dis- 

 charging into rivers, but the latter become so polluted as to be 

 a source of annoyance, and even danger. 



Many attempts to extract a portable manure from sewage 

 have been made, but without success. A very popular 

 method, known as the ABC process, consists in adding alum, 

 blood, and cla}^ when the coagulum formed carries down most 

 of the suspended matter, which, when drained and dried, is 

 sold as *' native guano." The metliod, however, fails to remove 

 the large proportion of the nitrogenous matter of the sewage 

 which exists in solution. 



Another plan is to utilise the sewage for irrigation purposes. 

 This is a better method, for a suitable soil extracts much 

 valuable fertilising matter from sewage, and will then yield 

 enormous crops. Great difficulty, however, is experienced in 

 obtaining a sufficient area of suitable land (light, sandy soils are 

 best) to deal efficiently with the enormous volume of sewage, 

 produced by a large town. Moreover, during frosty weather 

 difficulties arise in dealing with the outfall as rapidly as it is 

 delivered. The land, too, in time becomes so clogged with 

 matters derived from the sewage that it is rendered unfit for 

 further treatment — " sewage-sick." The composition of 

 sewage is naturally very variable, but it is always excessively 

 dilute, its manurial value, assuming that all its fertilising 

 ingredients are available, being about l^d. to 2d, per ton. 



Green Manuring*. — Soil deficient in humus may be greatly 

 enriched in that substance by growing any quick-growing crop 

 and ploughing it in. By this practice not only is the soil 

 enriched with carbonaceous material derived from the air, but 

 a considerable amount of nitrates which have been formed by 

 nitrification during the growth of the crop is assimilated, 

 converted again into complex organic compounds, and restored 

 to the soil. Without the crop these nitrates would have 

 been to a large extent lost by drainage. The planting of 



