28 



FIELD OPERATIONS OF THE BUREAU OF SOILS, 1917. 



as $5 or $10 an acre to $100 an acre where <;ood stands of grain can 

 be obtained. 



The table below gives the resnlts of mechanical analyses of samples 

 of the soil and subsoil of the Gila silty clay loam : 



MccliiHik-dl an(iJii!<ci< of fUla .s(7/// chn/ loam. 



PIMA CLAY. 



The soil of the Pima clay consists of a dark-brown or dark gi-ayish 

 brown clay, in many places relatively high in silt, 18 to 24 inches 

 deep. It is sticky when wet and cracks when dry, but it contains 

 much organic matter and retains moisture well. The type through- 

 out contains a large amount of lime. The subsoil is a light grayish 

 brown silty clay loam to clay, extending to a depth of more than C 

 feet. It differs from the surface soil in its lighter color, and in being 

 compact, but it has not been materially changed by leaching, cementa- 

 tion, or weathering in place. Small areas here and there are under- 

 lain by gravel or sand at a depth of several feet, which alloAVS the 

 water to penetrate more rapidly, and a gravelly substratum underlies 

 the type at various depths. 



The Pima clay is not extensive, but it is important in the agricul- 

 ture of the area. The type is confined to the flood plain of the Gila 

 Kiver, where it occurs in irregular-shaped areas or in long, narrow 

 strips, generally roughly paralleling the stream. It is found in all 

 parts of the valley from the eastern to the western boundary of the 

 area. 



The surface is level or slightly depressed. Flood water stands on 

 the surface until removed by percolation or evaporation. This is a 

 slow process, and more time must elapse between overflows and culti- 

 vation than in case of some of the other bottom soils. The type main- 

 tains a better moisture supply during droughts than the river-bottom 

 soils of lighter texture. Parts of the type contain alkali, but in- 

 jurious accumulations are less common than in the Gila silty clay 

 loam. The type is not directly overflowed by the river, the water 

 coming from other sources. Water is easily applied in irrigation, 

 and much of the type is under gravity canals. 



In most places the vegetation consists mainly of a heavy growth of 

 mescpiite, but Avhere alkali is present such plants as greasewood, salt- 

 bush, and salt grass grow. 



