OVER-IRRIGATION AND ALKALI 385 



for plant-production. By over-irrigation, water is lost, 

 plant-food is lost, the upper lands are impoverished and 

 the lower lands made useless. 



The whole process is well illustrated on a large scale 

 by the inland salt lakes, found in abundance over the 

 arid region, such as the Great Salt Lake. This lake is fed 

 by the seepage from the neighboring territory, and 

 by rivers flowing directly into it. It has no outlet. 

 Water is lost from the lake chiefly by evaporation, 

 and the soluble substances carried by the water entering 

 the lake are accumulated in the lake water. As a result, 

 the water has become so saturated that crystallization is 

 going on. On a smaller scale, in every valley bottom with 

 poor drainage, the process is being repeated. Large 

 areas have thus been and are being made alkaline. 



231. Upward leaching. Another phase of the alkali 

 question does not concern itself with seepage. Arid 

 soils are rich in soluble matters, which, when evenly 

 distributed throughout the soil, are advantageous hi 

 plant-growth. If by any chance the soluble substances of 

 the upper 6 to 10 feet of the soil are partly concentrated 

 near the surface, plant injury is almost sure to follow. 

 Such concentration frequently occurs under irrigation. 

 Water, added to the soil in moderation moves downward 

 only a few feet, but in its descent dissolves some of the 

 water-soluble soil constituents. By transpiration and 

 evaporation the water thus added moves upward and 

 carries with it the substances dissolved in its descent. At 

 the soil surface, the water evaporates and the salts are left 

 behind. As this is continued, the soluble soil constituents 

 tend to accumulate at or near the surface. This process 

 has been named upward leaching. It is a condition that 

 need not cause permanent injury, for it may be controlled. 



