SOIL SURVEY OF THE MIDDLE GILA VALLEY, ARIZONA. 17 



land and are confined to the small rocky buttes and mountain 

 slopes. 



Old vaUey-flUng soils. — The soils of the old valley-filling province 

 occupy about 75 per cent of the area. They are derived from 

 weathered and otherwise modified, relatively old, water-laid, un- 

 consolidated deposits, which oricrinally had their source in a variety 

 of formations in which granites and related rocks seem to have 

 predominated. The color of the surface soils ranges from brown 

 to reddish brown or red. Oxidation is well advanced and the soils 

 are characteristically low in organic matter. An abundant supply 

 of lime is present. Heavier, more compact subsoils occur in places, 

 and frequently a greatly compacted, or in some cases cemented, layer 

 lies within a few inches of the surface. These soils cover practically 

 the whole of the area except the valley of the Gila River. The sur- 

 face is smooth and comparatively flat or gently sloping. The soils 

 have a moderate elevation, and in places are eroded. Drainage is 

 good over most of this area. Three series of soils are recognized — 

 the Pinal, Mohave, and McClellan. 



The soils of the Pinal series are pale red or grayish red to light 

 reddish brown in color, and predominantly gravelly or stony. Ce- 

 mented, calcareous, gravelly hardpan or "caliche" layers are prom- 

 inently developed. The surface is gently rolling and dissected and 

 drainage is good. The Pinal series is inextensive in this area and 

 unimportant agriculturally. Only one type, the Pinal gravelly 

 sandy loam, occurs in the survey. 



The soils of the Mohave series have a range in color from light 

 reddish brown to dull red or dull reddish brown. They are usually 

 underlain by grayish or pinkish-gray very compact subsoils, fre- 

 quently containing irregular, partially cemented seams or layers, 

 with an approach to a dense, cemented " caliche " or hardpan. The 

 soils are low in organic matter and high in lime. They generally 

 kave a smooth and gently sloping surface (see PI. I, fig. 1), which 

 is little affected by erosion. Surface drainage is usually well estab- 

 lished, but the subdrainage is restricted and some areas are affected 

 by alkali salts. Two types of this series are mapped, the Mohave 

 sandy loam and fine sandy loam. 



The McClellan soils are browner than those of the two series just 

 described. The surface soils are low in organic matter, high in lime, 

 and underlain by a very compact, grayish, calcareous subsoil, which 

 retards the movement of moisture and limits root development. The 

 clay-loam member of the series is usually very low, flat, and poorly 

 drained, while the loam has a gently sloping surface and much 

 better drainage. Part of the series is affected by the accumulation 

 of alkali salts. 



155289°— 20 3 



