12 SOILS OF THE SAN LUIS VALLEY, COLORADO. 



where such channels have been filled. Much of the type is subject to 

 overflow or is purposely flooded to promote the growth of wild hay. 

 Surface flooding, coupled with the porous subsoil, favoring rapid 

 percolation, has tended to keep the soil relatively free from alkali 

 salts, although these occur in certain areas. With protection from 

 overflow and efficient irrigation, cultivation, and drainage, it is 

 admirably adapted to the production of alfalfa, sugar beets, and 

 potatoes, and constitutes the most promising trucking soil in the 

 district. 



Other less extensive members of the Rio Grande series occur as 

 narrow to moderately wide belts parallel to the course of the Rio 

 Grande or less important streams. They range from fine sandy 

 loams and loams to heavy silt and clay loams, often carrying some 

 gravel. Usually within a few feet of the surface they are under- 

 lain by sandy and gravelly subsoils. Like the Rio Grande sandy 

 loam, they are often subject to overflow. The heavier members 

 become sticky when wet, puddle readily, and are maintained in good 

 tilth with difficulty, except when in a favorable condition as regards 

 moisture. When artificially drained, effectively irrigated, and thor- 

 oughly cultivated they are better adapted to the production of grains 

 and grasses than the lighter soil types. Alfalfa, sugar beets, and 

 hardy vegetables can be successfully grown upon these soils. Owing 

 to the compact structure of the soil and consequent slow rate of mois- 

 ture movement by percolation or capillarity, these heavier types are 

 'not well adapted to the method of subirrigation usually practiced in 

 the valley. 



SOILS OF THE NORTHERN PART OF THE VALLEY. 



In the more northern portions of the valley the soils are prevail- 

 ingly heavier in character, the alluvial fan-forming streams being of 

 less volume or lower gradient, causing less extensive distribution of 

 the coarser material. The quantity of lime in the soils is also less. 

 The sandy and gravelly subsoils often disappear or occur at greater 

 depth, except upon the more pronounced slopes in the immediate 

 vicinity of the mountains. 



Upon the more elevated portions of the fans and foot slopes the 

 surface soils are usually of chocolate-brown to light-brown color and 

 generally gravelly sandy loams. They are underlain by porous sub- 

 soils of coarse texture and are related to the San Luis series. The per- 

 centage of gravel, particularly in the subsoil, is excessive. The soils 

 support a sparser growth of native desert vegetation than do the 

 lower lying soils of the valley. They usually occur above the sources 

 of water available for irrigation and are utilized only for grazing. 

 Near the valley margins the soils merge into a number of intenne- 



