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



2. The chemical composition of the soluble 

 salts. 



3. The content of adsorbed sodium on the 

 soil. 



4. The nature and content of calcium min- 

 erals in the soil. 



5. The composition and adequacy of sup- 

 ply of available irrigation water. 



The Bankhead-Jones Act of 1935 authorized the 

 establishment of certain regional research labo- 

 ratories. The directors of the agricultural experi- 

 ment stations of the 11 Western States decided on 

 the establishment of a salinity laboratory. This 

 came into being at Riverside, Calif., in 1038. 



The U.S. Salinity Laboratory has carried on a 

 continuing cooperative research program with the 

 agricultural experiment stations of the 18 West- 

 ern States on salt-affected soils, salts in irrigation 

 and drainage waters, and the reactions of economic 

 plants to salty water and soils. 



Research findings have been made available 

 through publication in scientific journals and sum- 

 marized in agricultural bulletins and information 

 bulletins for use by farmers. Information has gone 

 directly to engineers and agronomists employed 

 by action agencies such as the Soil Conservation 

 Service, U.S. Bureau of Reclamation, water and 

 drainage districts, and pollution control boards. 

 USDA Handbook 60 {126), which summarizes 

 much of the findings up to 1051, has been trans- 

 lated into four languages and is widely used inter- 

 nationally as a text and reference book. 



irrigation Water Quality 



Studies have been underway for a number of 

 years, and are continuing more intensively, on 

 developing sound criteria for assessing irrigation 

 water quality. Experience has shown that a single- 

 valued criterion of water quality for irrigation 

 under average conditions requires a high degree 

 of subjective judgment in its use. Research data 

 must be accrued to enable quantitative assessment 

 of irrigation water quality with reference to soil 

 properties, climate, irrigation management, and 

 salt tolerance of plants. 



The research approach underway to meet this 

 objective is to determine "the potential leaching 

 fraction" for a given salt-affected soil, taking into 

 account soil infiltration rate, evapotranspiration 

 and drainage rates, and the critical water manage- 



ment practices, irrigation frequency, and duration. 

 On calculation of the potential leaching fraction, 

 the maximum permissible salt content of an irri- 

 gation water could be calculated by reference to 

 the maximum permissible limits of salts in the 

 soil solution for specific crops. 



From the maximum permissible salt content of 

 the soil solution for the specific crops to be grown, 

 one can determine the fraction of the total amount 

 of irrigation water of a given quality that must 

 be allowed to drain. 



The above statement ignores the effect of com- 

 position of irrigation water ; the tendency of high 

 sodium waters and, to some extent, of high bicar- 

 bonate waters to increase the exchangeable sodium 

 percentage in the soil is an additional complicat- 

 ing factor. 



Another aspect of water quality that needs fur- 

 ther study is its relationship with hydraulic con- 

 ductivity. The initial use of high salt water for 

 reclamation of sodic soils, with the salt concen- 

 tration of the water decreasing gradually with 

 time, tends to maintain a higher conductivity and 

 hence enhances the passage of leaching water. 

 This can result in drastically reduced time re- 

 quirements for reclaiming sodic soils. 



The research on water quality will not only pro- 

 vide more rational characterization of water qua- 

 lity, but will also permit more efficient use of 

 water having a quality which is partially impaired 

 by a salt burden. 



Soil Chemistry 



Much research has been underway over the 

 years, and is continuing, on cation exchange re- 

 actions in salt-affected soils. Knowledge of such 

 chemistry is essential for making sound recom- 

 mendations on the management of saline and sodic 

 soils. 



It is known that calcium ions are adsorbed on 

 clay particles with a much higher energy of reten- 

 tion than potassium ions are, and that potassium 

 ions are retained with a higher energy than sodium 

 ions. The physical properties of soil colloids are 

 in turn affected by the nature of adsorbed cations. 

 Calcium-saturated soil colloids flocculate readily, 

 whereas those colloids predominantly saturated 

 with sodium tend to be highly dispersed. Soil 

 with appreciable levels of adsorbed sodium tend 

 to be intractable and impervious to water. Soils 

 begin to take on the characteristic of "sodium 



