130 Bacteria in Relation to Country Life 



for the character of the land and of the sewage, as well 

 as the climatic conditions, necessarily play here a pre- 

 dominating part. 



The effectiveness of broad irrigation in sewage- 

 disposal is further affected- by the constant supply of 

 sewage, irrespective of the season or the needs of the 

 crops. It should be remembered that in the summer 

 there is a rapid evaporation of moisture, not only from 

 the soil, but also a transpiration of water from the 

 foliage of growing plants. The pumping action of the 

 ■latter is extremely important in quickly disposing of 

 excessive moisture, since it is estimated that about 

 three hundred tons of water must be transpired through 

 the foHage in the production of one ton of dry matter. 

 Moreover, the summer temperatures stimulate the ac- 

 tivities of the soil bacteria and make possible thereby 

 a rapid decomposition of the organic matter. In the 

 winter months, on the other hand, the evaporation of 

 water directly from the soil is greatly reduced, the 

 removal of moisture by transpiration entirely dis- 

 continued, and the decomposition of the organic matter 

 markedly retarded. It frequently happens, thus, that 

 offensive conditions are created in the vicinity of sewage- 

 irrigated farms in the fall and winter months, even when 

 no cause for complaint exists in the summer. 



Intermittent and mixed irrigation. — In intermittent 

 irrigation, the business of crop-production becomes of 

 secondary moment. The purpose sought here is to secure 

 the greatest efficiency for any given soil area in the 

 purification of sewage. Porosity, aeration, and a vigorous 

 bacterial flora are the main desiderata here. The land 



