26 



MISCELLANEOUS PUBLICATION NO. 1065, U.S. DEPARTMENT OF AGRICULTURE 



Inorganic Salts and Minerals 



Farmers in the Imperial Valley of California 

 know about salt. This very productive valley is 

 entirely dependent on Colorado River water for 

 survival as an economic entity. About 3 million 

 acre-feet of Colorado Eiver water is brought into 

 this valley each year through the All-American 

 Canal. Colorado River water at Imperial Dam 

 contains about 750 p.p.m. dissolved salts — a rather 

 moderate level. But this concentration means ap- 

 proximately 1.2 tons of salt per acre-foot of water. 

 An irrigator applying 5 feet of water over the 

 season, also applies 6 tons of salt per acre. 



Since evapotranspiration consumes water and 

 leaves the salt behind in the soil, extra water over 

 and above consumptive use must be added to leach 

 the salt residue below the root zone, to avoid ex- 

 cessive salt accumulation. The saltier the available 

 water supply, the more water will be required for 

 leaching. Since the valley receives over 3 million 

 tons of salt in the irrigation water each year, 3 

 million tons of salt must be removed annually from 

 the irrigated soils into the elaborate drainage 

 system in order to maintain a favorable salt bal- 

 ance. Fields that are improperly managed with re- 

 spect to leaching and drainage produce only one 

 crop — "Imperial Valley snow" — a white crust of 

 sodium sulfate. When this happens, agriculture is 

 not onl} T hurt, it is virtually eliminated. Adverse 

 effects on production take place at even modest 

 levels of salinization because of stunted crop 

 growth. 



Inorganic salts and minerals that impair the 

 qualit}- of soils and waters derive from natural 

 deposits, acid mine drainage, industrial proc- 

 esses, and return flow from irrigated areas. The 

 main losses to agriculture occur through salt ac- 

 cumulation on irrigated soils with its consequent 

 effect on crop production. 



There are about 30 million acres of irrigated 

 land in the 17 Western States. Salinity is a poten- 

 tial hazard on about half of this acreage, and crop 

 production has been affected adversely by salt in 

 the soil on about 8 million acres. 



Rainfall generally contains negligible quanti- 

 ties of salt, but as the water flows over and through 

 the land, its salt content increases. Salts are formed 

 in the soil by solubilization of soil minerals, or 

 they may be present in geologic deposits such as 



saline shales. In arid climates, soils may contain 

 appreciable quantities of salts because rainfall and 

 drainage are often inadequate to leach out the salt. 

 Thus, surface and ground waters in arid regions 

 may contain moderate, and even high, concentra- 

 tions of salt as a result of normal or natural proc- 

 esses. When such natural waters are used by man, 

 the salt content may be increased by the addition 

 of salts not originally present in the water. Thus, 

 metropolitan and industrial wastes can increase 

 the salt burden of the water system. Such salt 

 additions are usually minor in importance, al- 

 though in certain cases, such as in the contamina- 

 tion of surface and ground waters by brines from 

 oil well fields, the additions can be critical to the 

 further use of the water for agriculture. 



The salt concentration of the water system may 

 also be increased by evaporation of the water. In- 

 dustrial and municipal use of water contributes 

 relatively little to this effect, since only about 7 

 percent of water diverted for these uses is evapo- 

 rated. Agricultural use, on the other hand, results 

 in evaporation of 60 percent of the diverted water. 

 Thus, agricultural use tends to concentrate the 

 salt in the water, whereas industrial and municipal 

 use largely adds exogenous salts. 



In addition to total salt content, the salt com- 

 ponents may be altered by the use of the water. 

 For municipal and some industrial uses (boiler 

 water) , it is necessary to soften the incoming water 

 supply by replacing calcium and magnesium with 

 sodium. The effluent water will, therefore, be en- 

 riched in sodium at the expense of calcium and 

 magnesium, and its quality for agricultural use 

 thereby impaired. Reclamation of sodic soils effects 

 similar changes in the quality of the water used 

 in leaching, although total salt content as well as 

 the proportion of sodium may be greatly increased 

 in the effluent. 



Initial use of water for irrigation also results 

 in changes in composition, as well as concentration, 

 of dissolved salts. Some precipitation of lime and 

 calcium sulfate may occur as the volume of the 

 water is reduced by evaporation, whereas the more 

 soluble sodium salts do not precipitate. The effluent 

 water, therefore, is often relatively more en- 

 riched in sodium chloride than in the sulfates 

 or bicarbonates of calcium. Both increasing salt 

 concentration and altered ionic compositions of 



