128 Bacteria in Relation to Country Life 



standpoint, as indicated by the facts just noted, more 

 may be said in its favor from the sanitary standpoint. 

 The soil readily retains a large portion of the materials 

 suspended or dissolved in the sewage, and, under favor- 

 able conditions, purifies it to such an extent as to make 

 the effluent resemble good drinking-water. The sub- 

 stances retained in the soil are rapidly decomposed by 

 the bacteria and rendered harmless, while the plant-food 

 contained in them is made available. The purifying 

 power of the soil was well known, of course, in ancient 

 times, and utilized to a large extent in protecting the 

 health of man. With the growth of cities in the last 

 century, and the establishment of sewage systems, the 

 large streams became the receptacles of much sewage 

 until the resulting serious pollution called forth protests 

 and led to remedial legislation. Many communities 

 were thus compelled by law to purify their sewage 

 before discharging it into surface waters. They turned 

 to land treatment as a convenient method for their 

 purpose. 



Kinds of irrigation. — The application of sewage to 

 the land is designated as broad irrigation when the 

 liquid is distributed over a large area in order to promotq 

 the growth of some cultivated crop. It is designated 

 as intermittent irrigation when much larger quantities 

 are applied, at frequent intervals, to open, well-under- 

 drained soil. In this case, the land may be seeded and 

 cultivated, or left uncropped. When the volume of 

 sewage is large, and the area of soil available for broad 

 irrigation limited, the two methods may be combined, 

 giving rise to the mixed system of irrigation. 



