IRRIGATION AND SEWAGE FARMS 133 



[since it chiefly depends on the surface for purification, and 

 I especially on the nitrifying organisms, which, as requiring air, 

 [do not work well in the depth, and disappear altogether at a 

 [certain distance below the surface. R. Warington tested for 

 ithem in the heavy soil at Rothamsted by their power of 

 nitrifying weak urine. Thirty-eight out of thirty-nine samples 

 down to 3 feet were active ; at 5 feet half were inert, and below 

 6 feet the organisms seemed to be absent. The action extended 

 ;for only 18 inches in clay, but to a greater depth in sand, and, 

 besides the scanty aeration, the deficiency of phosphates in the 

 lower layers adversely affected nitrification. Soil which rapidly 



(nitrified when in a moist, aerated condition became a vigorous 

 denitrifying medium when water-logged. 



Very near the surface aerobic organisms greatly pre- 

 dominate ; deeper down the anaerobes increase until we come 

 to a layer in which practically only anaerobic bacteria are 

 found, while deeper still there may be no organisms.^ 



For these reasons sewage is preferably made to pass obliquely, 

 by digging deep trenches at the lower end of the farm, and the 

 feeders are made to follow the contour of the ground. When, 

 owing to geological structure, the liquid can rise again as 

 springs, the absence of the first nitration may be concealed by 

 a second process occurring in the ascent to the surface. On 

 areas at a distance from habitations, with a porous soil (espe- 

 cially under rice cultivation, as in India), broad irrigation has 

 been successful, ditches and intercepting drains being provided, 

 and all wells on the sewage area, or within a radius likely to be 

 affected, being closed. 



The action of soil on sewage consists of (i) mechanical 

 straining; (2) mordant or '* adsorptive " effect, chiefly possessed 

 by the hydrous silicates of clay, which, if it be made sufficiently 

 porous, takes up soluble matters, but soon becomes saturated ; 

 (3) biological changes. While it was originally held that the 

 "cleansing power" of a soil was determined solely by its 

 physical condition, porosity, freedom from clogging and water 

 retention, it has since been proved that chemical composition 

 and bacterial efficiency have a high influence. Sir E. Frank- 

 land, in 1870, comparing the soil of the Barking sewage farm, 

 in which nitrification was absent or very slow, with a loam 

 from Dursley, in Gloucestershire, which showed his highest 

 efficiency (purifying sewage at the rate of nearly 100,000 gallons 



^ Sims Woodhead, Baltimore Sewerage Commission, 1899, p. 105. 



