14 
BULLETIN 618, U. S. DEPARTMENT OF AGRICULTURE. 
the intervening States are included except Oklahoma, New Mexico, 
Arizona, and Washington. 
The adaptation and value of durum wheat vary with the environ- 
ment. There is naturally a wide variation in soil and climate within 
this great expanse of territory. The eastern portion is distinctly 
humid, and the soil consists of glacial drift and alluvium. Farther 
west, in the Great Plains area, the precipitation decreases, the altitude 
increases, and the growing season becomes shorter. The soils are 
mostly heavy clay loams, varying to heavy clays or gumbo on the 
one hand and to sandy loams and sands on the other. 
Fig. 7. — Heads of four different groups of durum wheat. A, Kubanka; B, Pelissier; C, Velvet Don; 
D, Kahla. (About half natural size. ) 
West of the Rocky Mountains new sets of conditions present them- 
selves. The precipitation usually is low, and sometimes very low. 
The altitude in the great interior basins is high, except in the Columbia 
Basin, where it seldom exceeds 2,000 feet. In the California valleys 
the altitude is low. The soils throughout are mostly light in texture. 
For convenience in presenting and interpreting results the terri- 
tory under discussion is separated into three different divisions, 
according to the environmental conditions. These three divisions 
are called (1) the Prairie States, or subhumid area; (2) the Great 
Plains, or semiarid area; (3) the basin and coast or arid area, includ- 
ing the Salt Lake Basin, Snake River Basin, Humboldt Basin, 
Harney Valley, Columbia Basin, and the California valleys. 
