36 FIELD OPERATIOjSTS OF THE BUREAU OF SOILS, 1917. 



The climate is warm and arid. The mean annual precipitation at 

 Phoenix is 6.8 inches and at Maricopa 6.24 inches. Two wet seasons 

 occur, one in the sprino^ and one in the late summer. The mean an- 

 nual temperature at Maricopa is 69.6°, Maximum temperatures as 

 recorded at this station range between 110° and 126°. The lowest 

 temperatures are usually above 25°, but a minimum of 8° has been 

 recorded. The average date of the last killing frost in the spring is 

 March 7, and that of the earliest in the fall, November 27. 



Irrigated farming was carried on in the Gila Valley by prehistoric 

 races, and by Indians for centuries preceding settlemept by the 

 whites. Grain was the chief crop grown by the Indians, and is still 

 very important. 



The principal agricultural products are cereals, chiefly wheat, hay, 

 mainly alfalfa, and live stock. Corn is grown to some extent. 

 Egyptian corn and milo are becoming important crops. Long-staple 

 Egyptian cotton has been introduced and promises to become a very 

 im]:>ortant money crop with both white farmers and Indians. 



The methods in use by the white farmers are good, but the Indian 

 is slow to adopt modern methods and machinery. 



The i^ossibility of irrigation and presence of alkali have had more 

 influence on the distribution of crops than has soil character. 



The Pima Indians and Mexicans supply most of the farm labor. 

 Over 95 per cent of the farms were operated by the owners. 



The soils of the Middle Gila Valley fall into three main groups — 

 residual, old valley filling, and recent alluvial. The old valley-filling 

 group is most extensive, and includes the desert or upland soils. The 

 recent-alluvial group includes the soils at present of greatest agri- 

 cultural value. The residual group is unimportant, including the one 

 nonagricultural type, Eough stony land. 



Five series, including eight soil types, are recognized and mapped 

 in this area, in addition to Eough stony land and Riverwasli. The old 

 . valley-filling soils cover about 75 per cent of the area. They are 

 derived from weathered and altered old-alluvial deposits, and are 

 characterized by a limy, compacted or cemented subsoil or hardpan. 

 The surface is smooth or undulating. Three soil series are recognized 

 under this group, the Pinal, Mohave, and McClellan. 



The Pinal gravell}^ sandy loam has a cemented, gravelly hardpan. 

 The Mohave series consists of light-textured, red or reddish-brown 

 soils, with a compact subsoil approaching a hardpan. Two types, 

 in which some alkali occurs, are included in this series. The McClel- 

 lan series is darker and browner in color than the Mohave. Two 

 types are included, the heavier of which has a level surface and con- 

 tains alkali. 



The recent-alluvial soils, confined to the bottoms and low terraces 

 of the Gila River, are classified with the Gila and Pima series. These 



