VIII.] MANURES DERI VED FROM HUMAN EXCRETA 243 



Manures derived from Human Excreta. 



Since the process of digestion in man does not 

 essentially differ from that of animals, the greater part, 

 and in the case of adults the whole, of the nitrogen* 

 phosphoric acid, and potash, contained in human food 

 is excreted in the urine and fseces. We have already 

 seen that when plants are grown to feed animals, the 

 nutrient constituents drawn from the soil are for the 

 most part returned to the land ; the only fertilising 



Table LXXV.— Composition of Human Excreta. 



constituents which leave the farm permanently are the 

 corn, the wool, and the fat stock for the use of man. 

 Even of these the husk of the grain, the wool, the bones 

 and hair find their way back to the land eventually, but 

 under modern conditions the permanently valuable con- 

 stituents of human food which pass into the excreta 

 are then wasted agriculturally by being washed away 

 into the rivers and sea. In the gross the waste is 

 enormous ; the only difficulty of preventing the loss lies 

 in the fact that most of the methods for rendering 

 serviceable the wasted material cost more than an equal 

 amount of fertiliser from some other extraneous source. 

 Wolff and Lehmann have estimated (Table LXXV.) 



