56 PLANT PRODUCTS 



Unfortunately the really valuable and important fertilizing 

 ingredients remain in solution, whilst the sludge is of 

 inferior composition. A very large tank space is necessary, 

 and the materials obtained are of small value. 



The Septic Tank Method. — Instead of trying to 

 divert the normal course of events, a system of facilitating 

 the natural decomposition of sewage has been introduced 

 with very considerable success. In the decomposition of 

 sewage there are roughly two stages. The first is due to the 

 decomposition by anaerobic bacteria, much in the same way 

 as in the fermentation of farmyard manure, described above. 

 During this process the insoluble matter goes into solution, 

 even cellulose becoming very largely decomposed during this 

 stage. Subsequently, the action of aerobic bacteria will 

 oxidize the materials in solution, and convert them into 

 inoffensive materials. In practice it has often been found 

 unnecessary to adopt any elaborate plant to separate the 

 two stages, since a preliminary depositing tank of small 

 dimensions, to remove gravel and grits, followed by larger 

 tanks, for the bacterial digestion suffices. Coke beds with 

 sprinklers form a favourite modern oxidizing part of the 

 system. The resulting liquors contain practically everything 

 of value, and can either be run on to a farm, or be run into 

 a river without harm. The sludge from the septic tanks 

 is usually quite inoffensive, but its composition is very 

 variable, and the dry matter may contain anything from J 

 to 2 per cent, of nitrogen. Not infrequently these sludges 

 are mixed with some phosphatic fertilizer to render them 

 more generally useful. Popular conceptions are apt to 

 exaggerate the fertilizing importance of town sewage. The 

 average produce of one man in one year is about ii lbs. 

 nitrogen, 2| lbs. phosphoric acid, and 2| lbs. potash. The sum 

 of all the population is, no doubt, large, but the problem of 

 this utilization presents very great difficulties, excepting on 

 a small scale. 



Miscellaneous Organic Mixed Fertilizers.— The drop- 

 pings of poiiltry form a very well-known and much-prized 

 manure for intensive purposes. Birds do not secrete waste 



