142 MOVEMENT OF SOLUBLE SALTS 



roots in deep arid soils, and since the first flush of water 

 passing through a soil usually carries considerably more 

 salts than the subsequent water, the usual movement of 

 alkali under natural conditions is toward the lower point 

 of rain penetration. In sandy soils or in regions where 

 the rainfall is greater, the penetration of the water is 

 greater than on the more impervious soils or where the 

 rainfall is light, and the accumulation of the salts at dif- 

 ferent depths varies accordingly. It was found in Cali- 

 fornia (16) that on a sandy loam soil with a rainfall of 8 

 inches the greatest accumulation of salts was at a depth of 

 3 to 4 feet, whereas in a coarse sandy soil in the same place 

 the depth of greatest salts was below 4 feet. Where the 

 rainfall was only 3 inches the maximum salt was at about 

 18 inches in a sandy loam soil, whereas with 15 inches the 

 bulk of the salts was at 5 feet. 



Salt Movement with Water. When these arid lands 

 are brought under irrigation, however, this balanced con- 

 dition is frequently upset. The soil is kept so much 

 more moist that capillary action is much easier, and not 

 infrequently seepage and over-irrigation raise the water- 

 table so high that upward movement is possible from the 

 free water in the soil. Under such conditions, the alkali 

 accumulations of the lower depths are moved to the upper 

 zone of soil where they become of greatest injury to plants. 

 It is in this manner that many of the formerly productive 

 irrigated lands have been rendered useless. 



Diffusion of the salts in the soil plays a local part in the 

 movement of alkali, but, according to the laboratory 

 work of McCool and Millar (23) and others, diffusion 

 causes changes for only a few inches about concentrated 

 salt solutions, and the field observations of Mackie (24), 

 Headden (14), Hansen (7), and others show that because 



