IRRIGATION AND SEWAGE FARMS 137 



B. coli and its allies are not discoverable, or are present in 

 imall numbers only, in virgin soils, such as those from unculti- 

 vated uplands.^ Dr. W. G. Savage obtained similar results 



[from a number of mountain samples, and concluded that if 

 )resent in ordinary hillside soils the coli has been derived from 

 Lnimal excreta.^ 

 Dr. Sidney Martin has proved that B. typhosus in virgin soils 



[attenuates and after a short time disappears, but in those v^hich 

 contain large quantities of organic matter, particularly from 

 jewage, it will multiply even through extremes of heat and cold 



[(from 37° to 3° C.), and under conditions of dryness, and will 

 ;urvive for at least 450 days alone, or 50 days in presence of 



[other bacteria. 



Although no injury to health has been directly attributed to 

 sewage farms, the possibility of the survival, or even the multi- 



fplication, of pathogenic organisms on such farms must be taken 



into account when the drainage waters pass into or near 



idrinking-water supplies. (See further p. 193 et seq.) 



Suitable Crops. 



Unless proper cultivation is adopted on sewage farms, the 



[soil is given up to a profuse growth of weeds, which have to be 



[removed and the soil broken up to keep it porous, without any 



return for the labour. The conditions are different from those 



>f ordinary agriculture, inasmuch as, although the liquid un- 



loubtedly contains the elements of plant food, they are supplied 



'00 continuously and in too great dilution with water, while 



the volume is usually greatest at a season when it is absolutely 



injurious to crops. Therefore for successful cultivation the 



[plants must only receive the sewage as they want it, the 



remainder being treated by other methods. It is also neces- 



iry that the plants should be of such a character as can be 



[grown on a ridge, so as to prevent the liquid at any time 



flooding their growing tops. The difficulties are obviously 



lessened under the "separate system" of sewerage, in which 



^.surface and storm water are excluded (see Fourth Report of 



Royal Commission on Sewage, 1904, part iv., p. 5). 



Deherain^ has determined the quantity of water exhaled in 

 |one hour by certain growing leaves exposed to the sun : 



^ Report of the Medical Officer of the Local Government Board, 1899-1900, 

 p. 510. 



'^ Journal of Hygiene, July, 1902, p. 331. '■^ Chimie Agricole, p. 281. 



