BIO GRANDE AND OTHER ALLUVIAL SOILS. 11 



areas are naturally well drained. The lower lying districts have 

 been injured from seepage and accumulation of alkali. These un- 

 fortunate conditions are of more widespread occurrence upon this 

 type than upon the adjacent San Luis sand. 



SAN LUIS LOAM. 



The third member of the San Luis series of soils, the San Luis loam, 

 covers but limited areas of the fan slopes. This type ranges in tex- 

 ture from a fine sandy loam to a light loam and represents a finer grade 

 of material, transported by mountain torrents or derived in part from 

 wash from slightly more elevated adjacent soils. It is similar in 

 appearance to the San Luis sandy loam, but is of heavier character 

 and becomes sticky and plastic when wet. It carries but little coarse 

 sand, but contains some fine gravel. It occupies lower levels and 

 depressions between adjacent low slopes of coalescing detrital fans 

 and in places is affected by accumulation of seepage water. The soil 

 material is inclined to puddle, and the surface frequently becomes 

 baked and hard. It is usually underlain at a depth of from 2 to 3 

 feet by sand or sandy loam, grading into the usual gravelly subsoil 

 of the San Luis series. The surface is such as to favor irrigation, 

 but the type is generally poorly drained and contains injurious 

 quantities of alkali salts. It produces good yields of native grasses 

 where flooded, and when drained becomes a valuable soil for the 

 production of grains, alfalfa, and other staple crops of the valley. 



BIO GRANDE AND OTHER ALLUVIAL SOILS. 



The recent soils of the river flood plains embraced within the limits 

 of the survey of 1903 in the Rio Grande fan district are classed as 

 the Rio Grande series, which, like the San Luis series, has a wide 

 distribution in the valley outside the limits of the survey. They 

 are dark brown to black in color, carry considerable quantities of 

 organic matter, frequently support some timber growth, and are 

 usually underlain by sandy and gravelly subsoils. 



Of the members of this series recognized and mapped in the earlier 

 survey the Rio Grande sandy loam is the more extensive, occupying 

 a belt from one to several miles wide along the Rio Grande River. 

 It is a friable soil of fine sandy loam texture and easily maintained 

 in good tilth. The surface soil is less gravelly than the soils of the 

 San Luis series, owing to superficial covering of fine alluvium, but it 

 is underlain, generally at about 2 feet, by a sandy and gravelly sub- 

 soil of loose, porous structure similar to the San Luis series, which 

 in this type is frequently exposed at the surface over small areas. 

 The type is of gently sloping surface, favoring irrigation, but cut 

 in places by remnants of former stream courses or bands of gravel 



