18 FIELD OPERATIONS OF THE BUREAU OF SOILS, 1917. 



Recent -alluvial soils. — The recent-alluvial soils of this area, while 

 not the most extensive, are the most important agriculturally. They 

 have been made to produce crops since prehistoric times. They are 

 confined to the Gila River bottoms and consist of recent sedimentary 

 materials laid down by this stream. As distinguished from the old 

 valley-filling soils, they are naturally friable and lack a compacted 

 and cemented or altered subsoil. They are much higher in organic 

 matter than the old valley-filling types. The soil profile represents 

 the various coarser or finer strata laid down by the stream under 

 different conditions of velocity, volume, and load of suspended ma- 

 terial. The surface is generally level and smooth, and the soils are 

 in places poorly drained and affected with alkali salts. They are 

 also subject to overflows. Two series are included under this soil 

 province — the Gila and the Pima. 



The Gila series consists of brown soils, ranging from rather light 

 grayish brown to rather dark brown in color, frequently with a 

 slight reddish or chocolate tint. The subsoils are generally similar 

 in color and texture, but they may consist of variably textured, alter- 

 nating strata. They are free from any consistent compact or hard- 

 panlike layers. There is a rather high percentage of organic mat- 

 ter in the surface soil, and both surface soil and subsoil are calcareous. 

 In many places the content of mica is large. The surface of the 

 Gila soils is usually smooth and level or v^ry gently sloping, and 

 drainage is only fairly good. Alkali salts are sometimes present in 

 excessive quantities. The soils are subject to periodic overflow and 

 have a high water table. 



The Pima soils are dark brown or dark grayish brown, with, in 

 many places, a slight reddish or rich chocolate brown tint. The sub- 

 soils are of similar or of lighter and more grayish color, particularly 

 where highly calcareous. Both surface soil and subsoil are cal- 

 careous, effervescing freely with dilute acid. The subsoil is similar 

 in texture to the surface soil or consists of variably textured strata 

 without cementation or alteration in place through weathering. 

 The surface is generally smooth and level. The soils are subject to 

 overflow, and at times are poorly drained. In some places the Pima 

 soils contain alkali salts, but the affected areas are easily reclaimed 

 by drainage and flooding. 



In the following pages of this report the various soils of the Mid- 

 dle Gila Valley area are described in detail, and their relation to 

 agriculture discussed. The distribution of the soils is shown on the 

 map accompanying this report, and the table below gives the actual 

 and relative extent of each : 



