ALKALI PROBLEM IN IRRIGATION SCOFIELD. 215 



The way in which the soluble salts are deposited from evaporating 

 water at the surface of the soil is shown in plate 1. In this case a 

 deep layer of soil that had been soaked with salty water dried out 

 by surface evaporation. The water moving upward carried the dis- 

 solved salts Avith it until it was vaporized at the surface, when the 

 salts were deposited in crystalline form. 



Another example of the surface deposition of salts in the dry 

 country may be seen in the flat valleys of rivers or creeks. Many 

 desert streams meander through such valleys and where the subsoil 

 on either bank is readily permeable to water there is an appreciable 

 lateral seepage from the stream bed, particularly in times of high 

 water. Some of the water from this underflow is drawn upward 

 through the layers of soil and evaporated, leaving its dissolved salts 

 behind. In the course of time, these surface accumulations of salt 

 reach such concentrations as to prevent the growth of plants and 

 thus render useless large areas of potentially fertile and easily irri- 

 gable land. 



From these examples it may be seen that the alkali salts of arid 

 lands owe their origin chiefly to the natural processes of soil weather- 

 ing. The marked irregularities of their distribution are due to sub- 

 sequent transportation in water and their deposition at the places 

 where the water evaporates. 



The processes of soil formation from the breaking up of rock 

 masses proceed very slowly, so that the release of soluble salts to 

 the soil water takes place gradually. The abundant deposits of 

 alkali salts that are now to be found in spots in the desert may repre- 

 sent the accumulations of many centuries of time and the drainage 

 from many square miles of surface. If these salts were evenly dis- 

 tributed throughout the region in which they were formed they 

 would seldom be harmful, but when the salts from many acres are 

 all deposited in a small spot they are likely to prove troublesome if 

 that spot is selected for crop production. An example of the ac- 

 cumulation of salts in a low spot in the desert is shown in plate 2, 

 figure 1. Such a low spot as this may serve for many years as an 

 evaporating basin for the flood waters that bring in the salts from 

 the surrounding land. After a time such a basin may be completely 

 filled and covered by drifting soil only to be discovered later when an 

 attempt is made to use the land for crop production. Often these 

 spots are small in area, covering only a few acres, but in other 

 cases such as the Great Salt Lake in Utah the evaporating basin 

 covers many square miles. 



ALKALI SALTS AND THE GROUND WATER. 



The alkali salts of the desert have little practical significance ex- 

 cept in relation to the ground water. It is only when they are dis- 



