Irrigated Agricueture in the San Luis Vaeeey 
7 
considerable amount of finer material, thus becoming sticky when 
wet. Stickiness is probably increased by the presence of alkali 
salts and by the absence of coarse sand or fine gravel in the surface 
layers. It is usually underlaid at a depth of two to three feet with 
sand or sandy loam which grades into the usual gravel underlying 
most of this region. This soil is very rich. When well drained it 
will produce heavy crops of grasses, alfalfa and grains, but much 
of it IS not now available for cropping because of seepage and alkali. 
The so-called Rio Grande series occurs in narrow belts along 
the Rio Grande. The series includes Sandy Loams, Loams and 
Clays, the first-named being the most extensive type. All of 
these soils are dark brown to black in color and usually have con¬ 
siderable organic matter in their native condition. The surface 
soils contain very little gravel, but generally at a depth of two feet 
or slightly over, they are underlaid with the prevailing gravel layer 
or layers peculiar to this region. 
The surface slopes of the Rio Grande types are gentle, favoring 
irrigation, though they are frequently cut up by old stream courses 
or banks of gravel. Much of their area is subject to overflow or 
is flooded to increase the growth of wild hay. If protected from 
overflow and excessive irrigation, Rio Grande Sandy Loam is one 
of the best adapted soils for potatoes, alfalfa and truck crops. 
The heavier Rio Grande types are also subject to overflow. 
They are very sticky and puddle easily, and these adverse physical 
conditions frequently are made worse by considerable quantities of 
alkali. When worked in just the right way and at the right time 
they give good crops of grass and wheat. However, their manage¬ 
ment is difficult as they require artificial drainage, very careful 
irrigation and plowing and other cultivation at just the right mois¬ 
ture point—conditions difficult to obtain. 
The San Luis and Rio Grande Series occupy the western and 
southern parts of the valley and include the more important agri¬ 
cultural types. The soils of the northern part of the Valley are 
somewhat heavier in texture—that is, they contain much more 
fine material. They were probably deposited by slower moving 
streams as the present grades of the streams are much lower than 
those in the western and southern parts of the Valley. The lime 
content ol the soils is also less. 
The soils in this section are chocolate brown to light brown in 
color, although occasionally they will be found almost gray in con¬ 
siderable patches. Close to the mountains and on the Valley slopes 
the soils are gravelly to sandy gravelly loams, but on the flatter 
Valley floor they run to loams and clays. Rather heavy percentages 
