20 SOILS OF THE SAN LUIS VALLEY, COLORADO. 



much of the valley, will doubtless result in some loss of pressure and 

 diminished flow. It is to be regretted that conditions are not such 

 as to permit general storage of the water in artificial reservoirs and 

 the regulation of flow. A more abundant supply would thus be 

 assured, while prevention of further waste would improve many areas 

 now swamped with seepage or rendered worthless by the accumulation 

 of alkali. 



The alkali salts consist mainly of sulphate of sodium, or Glauber's 

 salts, and of chloride of sodium, or common salt. With these fre- 

 quently occur smaller quantities of sulphate of magnesium, or epsom 

 salts, bicarbonate of sodium, or common cooking soda, and chloride 

 and sulphate of potassium. The carbonate of sodium the so-called 

 " black alkali " is of only occasional occurrence. Calcium sulphate, 

 or gypsum, is found more frequently. This is a relatively insoluble 

 salt of beneficial rather than injurious effect, owing to its chemical 

 action in converting the black alkali into less harmful salts. 



These salts are derived by decomposition of the minerals forming 

 the soils a process begun before the mineral fragments were trans- 

 ported and deposited in the valley and still continuing. The salts 

 are nearly all readily soluble and are carried from place to place 

 in the drainage waters. When, owing to a high water table or to 

 outcropping of underlying impervious strata, the water approaches 

 the surface sufficiently to maintain a moist column of soil between the 

 water table and the surface, an upward capillary movement of the 

 solution takes place, the moisture evaporating from the surface being 

 continuously renewed from below. As the moisture evaporates the 

 salts which it contains crystallize and deposit in the soil at or near 

 the surface, forming, when the quantity is great enough, a crust or 

 a powdery efflorescence. As evaporation in the San Luis Valley is 

 particularly rapid and the supply of impregnated water practically 

 constant, the tendency under present conditions is toward an increas- 

 ing accumulation of the deleterious salts. With the lowering of the 

 water table the rate of evaporation is decreased or the movement 

 discontinued. The occurrence of rains or the application of irriga- 

 tion waters may reverse the process and cause leaching from the sur- 

 face by percolating waters of a large proportion of the salts, which 

 return to the subsoils or subsoil waters, and under favorable drain- 

 age conditions may be discharged into the country drainage and 

 permanently removed. 



The seepage and alkali problems are thus intimately related and 

 yield readily to the same treatment. The first step is the removal of 

 the seepage waters and the lowering of the water table. The posi- 

 tion at which the water table becomes a source of danger depem 

 upon textural and structural characteristics of the soil and subsoil 

 In the open porous soils of the San Luis series the upward movement 



