SOIL SURVEY OF THE MIDDLE GILA VALLEY, ARIZON^A. 33 



cent. A very small proportion of sodium bicarbonate is also present. 

 Of the principal bases which combine to form the alkali salts, sodium 

 is the only one of importance represented in the surface crust. 

 Potassium, magnesium, and calcium exist only in very small 

 amounts, or traces. 



The alkali salts are all highly soluble and their presence in the 

 surface soil is due principally to the evaporation of soil waters. 

 They have for the most part been transported to the localities in 

 wdiich they occur by subsurface and surface drainage waters carrying 

 them in solution. 



The conditions of drainage in the bottom soils differ from those in 

 the upland or desert soils. The soils of the Gila River bottoms, as 

 has been pointed out, are affected by seepage and have a high water 

 table ; this, with their texture and structure, which favor capillarity, 

 has caused the large accumulations in the valley. The heavy alkali 

 crusts formed during the dry season are partly removed in surface 

 drainage waters or are carried downward during the rainy period. 



The desert or upland soils generally have impervious subsoil lay- 

 ers, and where they are affected by alkali, little free movement of the 

 salts in subsurface waters is possible. In many places the crusts of 

 surface salts are very thin even during the driest season, although 

 the areas are barren of vegetation and the total average alkali con- 

 tent high. Many of the barren alkali flats are interrupted or bordered 

 by slightly higher lying soils of lighter texture, which are frequently 

 free from surface accumulations of alkali. The flats receive run-off 

 from the adjacent soils, and the water remains until removed by 

 evaporation or percolation. Both the suspended sediments and the 

 salts in solution are deposited, and upon drying form a smooth, hard, 

 surface which greatly retards the movement of the salts. 



Owing to these different conditions the distribution of alkali salts 

 in the 6-foot profile varies greatly with the different soils and situa- 

 tions. Alkali in general is more uniformly distributed through the 

 soil profile in the recent-alluvial soils than in the old valley-filling or 

 desert types with compacted subsoils. Where the distribution is un- 

 equal, the zone of maximum concentration usually lies between the 

 first and third feet in the recent-alluvial soils, while in the desert 

 types the zone of highest salt content is usually deeper. 



The distribution and degree of concentration of the alkali areas are 

 shown on the accompanying alkali map. The mapping is based upon 

 field determinations with the electrolytic bridge method. The aver- 

 age salt content of the 6-foot profile, based upon the content of each 

 foot section, is determined. The results are expressed in terms of 

 parts per 100,000. Five grades of alkali land are shown. Grade A 

 comprises soils in which the average concentration of salt is under 



