34 FIELD OPEKATIONS OF THE BUREAU OF SOILS, 1917. 



200 parts per 100,000 of air-dry soil; jrrade B represents soils con- 

 tainiii<r between 200 and 400 parts; j^rade C betAveen 400 and GOO 

 parts; grade D between 600 and 1,000 parts; and grade E those con- 

 taining over 1,000 parts. 



Alkali conditions in the field are indicated largely by surface ap- 

 pearance, vegetation, and topography, and the differentation of the 

 alkali soils into the above grades is aided largely by field observa- 

 tions of these factors. On soils free from alkali or possessing only 

 small amounts in the root-feeding zone, such native plants as desert 

 sage, creosote bush, rabbit brush, and several \arieties of cactus thrive. 

 On the more heavily impregnated soils squaw-bush, salt-blight, 

 greasewo"od, and pickleweed grow. Of course, where alkali is con- 

 centrated in the sul)soil the vegetation is not a safe guide, but enough 

 checks are made with the bridge to guard against mistake of this 

 kind. 



The areas of alkali soil increase in size and in degree of salt 

 concentration as the lower levels to the west are approached. Many 

 of the flat, shallow valleys that traverse the area in a northwesterlj'^ 

 direction have only slight indications of alkali at their higher levels, 

 but include areas of high alkali concentration in their lower reaches. 

 WTiile much of the soil of the Gila River bottoms below Blackwater 

 contains alkali, alkali-free areas occur in places. Above Blackwater. 

 the alkali areas are smaller and of less importance. The immediate 

 overflow plain of the Gila River is little affected by alkali, the low 

 terrace between the overflow plain and the uplands containing the 

 areas of highest concentration. The largest alkali-free areas lie in 

 the southern part of the survey, between McClellan Wash and Casa 

 Grande, and east and northeast of McClellan Wash. An extensive 

 alkali-free area roughly parallels the Southern Pacific Railroad. 

 The slopes about the mountains and foothills are generally free from 

 injurious amounts of alkali, except in the vicinity of seepage 

 channels. Plate A shows tlie general distribution of alkali lands in 

 the area. 



All of the soil types except the Pinal gravelly sandy loam arc 

 affected in places by alkali. The Gila fine sand and silty clay loam 

 frequently contain injurious amounts, but the Pima clay, one of the 

 important agricultural soils, generally contains only small amounts. 

 The McClellan loam, silty phase, is free from alkali, while the shal- 

 low phase of the same type generally contains varying amounts. 

 The Mohave sandy loam and fine sandy loam are generally well 

 drained and contain alkali only locally or in the marginal zones of 

 the areas. 



Prioi- to the coming of the white man the Indian reclaimed small 

 patches of land in the Gila River bottoms, using the floodwater of 

 the (Jila River for irrigation. In manv instances lands high in 



