IRRIGATION AND DEY-FARMING 711 



with this, a mulch of dry soil from four to eight inches 

 (deep should be maintained. This is a protection against 

 too high a temperature in moist soil unprotected by shade, 

 as well as against loss of moisture. The surface of the 

 soil should be kept as nearly level as possible. 



Crops that are not planted in rows, such as grain, may 

 be cultivated with a fine-tooth harrow until they reach a 

 height of from several inches to a foot, at which stage 

 evaporation from the soil is largely prevented by the 

 shading of vegetation. If it is to be successful this culti- 

 vation must begin as soon as the seedlings appear above 

 the surface, in order that the roots may be forced deep 

 into the soil. Then the top may be much twisted with 

 but little injury to the plant, and that injury appears to 

 be more than counterbalanced by the tillering of the plant. 

 By prompt and thorough tillage following irrigation, very 

 much may be done not only to conserve soil moisture but 

 also to prevent the accumulation of alkali at the surface 

 by evaporation. 



609. Sewage irrigation. — A phase of the general prac- 

 tice of irrigation is the application of sewage l to the land 

 for purposes of crop production. This supplies plant-food 

 as well as water. The food content, however, is relatively 

 small, being about two parts in one thousand, of which 

 one-half is organic and one-half is inorganic material. 

 In European countries sewage irrigation is extensively 

 employed near cities, but in the United States the practice 

 has not been largely followed. The city of Boston has 

 carried out extensive experiments, and the city of Los 



1 Rafter, G. W., and Baker, M. N. Sewage Disposal in the 

 United States. New York. 1904. Also, Rafter, G. W. Sew- 

 age Irrigation. U. S. Geol. Survey, Water-Supply and Irrigation 

 Papers, Nos. 3 and 22. 1897 and 1899. 



