the maximum permissible concentrations of stron- 

 tium-90 and radium-226 would be permissible for 

 at least nine and 40 years, respectively, before det- 

 rimental effects would be noted. 



Impairment of soil quality 



Sodium Hazard 



Sodium in irrigation water may become a prob- 

 lem in the soil solution as a component of total 

 salinity increasing the osmotic concentration, and 

 as a specific source of injury to fruits. It is mainly 

 a problem, however, due to its effect on soil struc- 

 ture, infiltration, and permeability rates. Since good 

 drainage is essential for management of salinity in 

 irrigation, and for reclamation of saUne lands, good 

 soil structure and permeability must be main- 

 tained. A high percentage of exchangeable sodium 

 in a soil containing swelling-type clays results in a 

 dispersed condition unfavorable for water move- 

 ment and plant growth. 



The organic and clay fractions of the soil possess 

 ion exchange properties. These fractions carry pre- 

 dominantly negative charges and, therefore, ab- 

 sorb positive ions (cations); predominantly cal- 

 cium, magnesium, potassium, sodium, hydrogen, 

 and aluminum. The distribution of adsorbed ca- 

 tions in the soil is in equilibrium with the soil solu- 

 tion. Anything that alters the composition of the 

 soil solution, such as irrigation or fertilization, dis- 

 turbs the equilibrium and alters the distribution of 

 adsorbed ions in the soil. When calcium is the pre- 

 dominant cation adsorbed on this exchange com- 

 plex, the soil tends to have a granular structure 

 which is easily worked and readily permeable. 

 When the amount of adsorbed sodium exceeds 10 

 to 15 percent of the total cations on the exchange 

 complex, the clay becomes dispersed and slowly 

 permeable unless a flocculated condition is main- 

 tained due to a high concentration of total salts. 

 Where soils have a high exchangeable sodium con- 

 tent and are flocculated due to the presence of free 

 salts in solution, subsequent removal of salts by 

 leaching will cause sodium dispersal to occur un- 

 less leaching is accomplished using additions of 

 calcium or calcium-producing amendments. 



Adsorption of sodium from a given irrigation 

 water is a function of the proportion of sodium to 

 divalent cations (calcium and magnesium) in that 

 water. To estimate the degree to which sodium will 

 be adsorbed by a soil from a given water when 

 brought into equilibrium with it, the U.S. Salinity 

 Laboratory (181) proposed the sodium adsorption 



ratio (SAR); see footnote, page 155. As soils tend 

 to dry, the SAR value of the soil solution increases 

 even though the relative concentrations of the 

 cations remain the same. This is apparent from 

 the above equation where the denominator is a 

 square-root function. This is a significant factor in 

 estimating sodium effects on soils. 



The SAR value can be related to the amount of 

 exchangeable sodium in the soil expressed as a 

 percentage of the total exchangeable cation con- 

 tent. This latter value is called the exchangeable 

 sodium percentage (ESP). From empirical deter- 

 minations, the U.S. Salinity Laboratory obtained 

 an equation for predicting a soil ESP value based 

 on the SAR value of a water in equilibrium with it. 

 This is expressed as follows : 



100[a+b(SAR)] 

 l + [a+b(SAR)] 



The constants "a" (intercept representing experi- 

 mental error) and "b" (slope of the regression 

 line) were determined statistically by various in- 

 vestigators who found "a" to be in the order of 

 — 0.06 to 0.01 and "b" to be within the range of 

 0.014 to 0.016. Thus, ESP as calculated from SAR 

 value will normally have a value slightly higher 

 than the SAR. This relationship is shown in the 

 nomogram [fig. IV-4 developed by the U.S. Salin- 

 ity Laboratory (181)]. For sensitive fruits, the 

 tolerance limit for SAR of irrigation water is about 

 4. For general crops, a limit of 8 to 1 8 is generally 

 considered within a usable range although this 

 depends to some degree on the type of clay min- 

 eral, electrolyte concentration in the water, and 

 other variables. 



The ESP value that significantly affects soil 

 properties varies according to the proportion of 

 swelling and nonswelling clay minerals. An ESP of 

 10 to 15 percent is considered excessive if a high 

 percentage of swelling clay minerals such as mont- 

 morillonite is present. Fair crop growth of alfalfa, 

 cotton, and even olives, have been observed in 

 soils of the San Joaquin Valley with ESP values 

 ranging from 60 to 70 percent {148). This condi- 

 tion is being studied further and is apparently the 

 result of a high percentage of nonswelling clay 

 minerals. 



Prediction of the equilibrium ESP from SAR 

 values of irrigation waters is complicated by the 

 fact that the salt content of irrigation water be- 

 comes more concentrated in the soil solution. Ac- 

 cording to the Salinity Laboratory {181), shallow 

 ground waters 10 times as saline as the irrigation 

 waters may be found at depths of 10 feet and a salt 

 concentration two to three times that of irrigation 

 water may be reasonably expected in the first-foot 



164 



