SOIL CHANGES DUE TO IRRIGAT 



be sandy; if the soils are loamy or clayey, the sediments 

 will be correspondingly more rich in clayey materials. 

 Usually, however, only the silty or finer particles reach 

 the lower portions of the river where the irrigation canals 

 are taken out. The coarser or more sandy particles are 

 deposited in the first quiet places of the river and do not, 

 ordinarily, reach the lower lands, except, perhaps, in 

 times of high water, when even the sand deposits of earlier 

 years may be torn up and whirled down to the irrigated 

 districts. 



The top or surface soil is always most vigorously 

 affected by sunshine, air, water and biological agencies; 

 therefore the top soil is the most fertile part of the soil. 

 It is this fertile soil layer that is washed into the rivers, 

 finally perhaps to be deposited on the farmers' fields. 

 Eventually, then, the farmer covers his own land with the 

 fertile surface soil of the mountain slopes and upland 

 valleys. 



72. Composition of river sediments. River sediments 

 have been analysed in the United States, in Europe and 

 in Egypt. The results show that river muds are somewhat 

 richer in the essential plant-foods than the ordinary 

 fertile soils which the water serves. It has been estimated 

 by Forbes that the market value of the fertilizing con- 

 stituents in three samples of Salt River mud, to the acre- 

 foot of water, varied from $7.98 to $25.51. These figures 

 should be given respectful consideration by the farmer 

 who does not content himself with using one acre-foot 

 of water. When the fertilizing value of these sediments 

 is considered in connection with the fertilizing value of 

 the dissolved materials, one of the great advantages of 

 irrigation is made evident. Under many of the rivers of 

 the irrigated section, proper methods of irrigation should 



