10 SOILS OF THE SAN LUIS VALLEY, COLORADO. 



greater part of the district lying between Mosca, Hooper, and Ala- 

 mosa on the east and the mountains on the west. In this district, 

 which was covered by a soil survey conducted by the Bureau of Soils 

 in 1903, 1 the soils are of porous and gravelly character and under- 

 lain by gravelly subsoils. The prevailing types are of reddish-brown 

 color, and are classed as the soils of the San Luis series, of which 

 three members, viz, the San Luis sand, sandy loam, and loam, were 

 recognized and mapped. They occur over extensive areas outside 

 the limits of this earlier survey and represent the commonest soil 

 features of the southern and western parts of the valley. 



SAN LUIS SAND. 



The San Luis sand occurs as a broad belt extending from the. base 

 of the mountains to the vicinity of Center and Monte Vista, or as 

 narrow and irregular areas extending into the valley to the vicinity 

 of the valley trougli. It is of porous, incoherent, leachy character 

 and consists mainly of fragments of the volcanic rocks of the adjacent 

 mountains. It is somewhat subject to drifting by winds, forming 

 occasionally slight ridges or dimes. It is underlain by a still coarser 

 and more porous gravelly subsoil at a depth of from 2 to 4 feet. It 

 never puddles or becomes sticky when wet and is readily permeated 

 by water. These characteristics^ favor rapid drainage, but some 

 of the lower and more gentle slopes have become swampy or water- 

 logged areas and more or less affected by alkali as the result of 

 seepage of irrigation water from the more elevated slopes. Origi- 

 nally over much of the area covered by this soil type a compact and 

 partially cemented layer occurred at the depth of penetration by 

 rains. Under irrigation this has disappeared, except upon the higher 

 ridges, owing to softening and dissolution of the cementing materials, 

 rendering the soil still more leachy and porous and requiring copious 

 irrigation to supply growing crops. 



SAN LTTIS SANDY LOAM. 



The San Luis sandy loam ranks second in extent of area covered. 

 It is a coarse, reddish-brown soil, carrying a large quantity of gravel 

 and underlain by a coarse, porous, gravelly subsoil. The surface 

 material extends to the depth of 18 to 36 inches and is of slightly 

 heavier character than the San Luis sand. It is noticeably sticky 

 when wet, and is marked by narrow strips occupied by heavier phases 

 of the soil. The surface is subject to minor irregularities, but 

 readily leveled and prepared for irrigation, not, however, withoi 

 some danger of exposing the coarse, gravelly subsoil. It is readi 

 penetrated by drainage or irrigation waters and the more elevat 



i Soil Survey of the San Luis Valley, Colorado. Field Operations, Bureau of Soils, 1903. 



