give a usable parameter which represents a soil 

 salinity value which can be correlated with plant 

 growth. 



Salinity is most readily measured by determining 

 the electrical conductivity (EC) of a solution. This 

 method relates to the ability of salts in solution to 

 conduct electricity and results are expressed as 

 miUimhos (mhosx 10'^) per cm at 25 C. Salinity 

 of irrigation water is expressed in terms of EC, 

 and soil salinity is indicated by the electrical con- 

 ductivity of the saturation extract (ECe). 



Temperature and wind effects are especially im- 

 portant as they directly affect evapotranspiration. 

 Periods of high temperature or other factors, such 

 as dry winds, which increase evapotranspiration 

 rates, not only tend to increase soil salinity but 

 also create a greater water stress in the plant. The 

 effect of climatic conditions on plant response to 

 salinity was demonstrated by Magistad, et al. (99). 

 Some of these effects can be alleviated by more 

 frequent irrigations to maintain safer levels of 

 soil salinity. Particular problems occur where high 

 rates of evapotranspiration occur on soils with 

 low infiltration rates so that it may be sometimes 

 virtually impossible to replace the soil moisture 

 rapidly enough during the crop growing season to 

 prevent stress. 



Plants vary in their tolerance to soil salinity and 

 there are many ways in which salt tolerance can 

 be appraised. Hay ward and Bernstein (65) point 



out three: (1) the ability of a plant to survive 

 on saline soils — salt tolerance based primarily on 

 this criterion of survival has limited application in 

 irrigation agriculture, but is a method of appraisal 

 which has been used widely by ecologists; (2) 

 the absolute yield of a plant on a saline soil — 

 this criterion has the greatest agronomic signifi- 

 cance; (3) relative yield on saline soil compared 

 to nonsaline soil — this criterion is useful for com- 

 paring dissimilar crops whose absolute yields 

 cannot be compared directly. The U.S. Salinity 

 Laboratory (181) has used the third criterion in 

 establishing the list of salt tolerance of various 

 crops shown in table IV-3 (p. 117). These salt 

 tolerance values are based upon the conductivity 

 of the saturation extract (ECe) expressed in 

 mmhos/cm at which a 50-percent decrement in 

 yield may be expected when compared to yields 

 of that plant grown on a nonsaline soil under 

 comparable growing conditions. 



Work has been done by many investigators, 

 based upon both field and greenhouse research, to 

 evaluate salt tolerance of a broad variety of plants. 

 In general, where comparable criteria were used 

 to assess salt tolerance, results obtained agree quite 

 well with those shown in table IV- 14. 



Early investigations considered the question of 

 how increasing salinity levels in the substrate 

 affect plant growth i.e., is there a threshold con- 

 centration at which damage to the crop will occur 



FIGURE IV-1. Salt tolerance of vegetable crops* 



2 4 6 



14 



ECe IN MILLIMHOS 

 PER CM. AT 25 C 



Beets 



Spinach . . 

 Tomato . . 

 Broccoli . . 

 Cabbage . . 

 Potato ... 



Corn 



Sweetpotato 

 Lettuce ... 

 Bell pepper 

 Onion .... 

 Carrot ... 

 Beans ... 





X. 



::^a 











*The indicated salt tolerances apply to 

 the period of rapid plant growth and 

 maturation, from the late seedling stage 

 onward. Crops in each category are 

 ranked in order of decreasing salt tol- 

 erance. Width of the bar next to each 

 crop indicates the effect of increasing 

 salinity on yield. Crosslines are placed 

 at 10 , 25 , and 5Q-percent yield reduc- 

 tions. 



YIELD REDUCTION 



148 



