104 IRRIGATION PRACTICE 



by the accumulation of the annual silt deposits, which 

 effectually shut out air from the soil and make it almost 

 impossible for water to penetrate into the lower soil 

 layers. If this one danger be avoided, the suspended 

 matter in irrigation waters may be made a source of wealth 

 to the irrigation farmer. 



75. Effect of sediments on crop yields. Forbes has 

 made some interesting experiments on the effects of the 

 river silt on the production of crops in Arizona. Similar, 

 but not so carefully made, observations have been made 

 in other sections of the world. The general conclusion 

 seems to be that wherever water, carrying sediments, is 

 applied without attention being given the silt deposits, 

 the crop-yields tend to decrease. Whenever, however, 

 the physical disadvantages discussed above are offset by 

 proper tillage, great financial advantages result from the 

 fertile matter carried by the irrigation waters. In fact, 

 the fertile suspended matters, carried at the irrigation 

 season, should increase materially the value of water- 

 rights from such sources. The tremendous value of the 

 overflow of the Nile, heavy with suspended matter brought 

 from the African highlands, is a familiar historical fact. 

 In India, South Africa, Europe and the United States, 

 there are districts in which the lands have higher values 

 because of the quantities of sediment carried by the 

 irrigation streams. 



The irrigation farmer deals with a much more compli- 

 cated problem than does his brother who depends simply 

 upon the natural precipitation for the moisture supply. 

 To the irrigation farmer the soil is one factor, the rainfall 

 another, and the water that he uses may be almost as 

 important a factor as the soil itself. 



76. Water and soil life. Soil moisture also exerts a 



