ration extract after irrigation is 

 completed; 

 ECe(i) = electrical conductivity of the soil 

 saturation extract before irriga- 

 tion; 

 ECiw = electrical conductivity of the irriga- 

 tion water; and 

 n = number of irrigations. 



To utilize this guide, one must first consider the 

 salt tolerance of the crop to be grown and the soil 

 salinity level [ECe(f)] which will result in a 15 or 

 50-percent yield decrement for that crop. Then, 

 after evaluating the level of soil salinity prior to 

 irrigation [ECed,] and the salinity of the irrigation 

 water, the maximum number of permissible irri- 

 gations can be calculated. These numbers are 

 based on the assumption that no intervening rain- 

 fall occurs in quantities large enough to leach salts 

 from the root zone. Should leaching rainfall occur, 

 the situation could be reevaluated using a new 

 value forECc(i). 



Using values based on a 50-percent yield decre- 

 ment (table IV-14, p. 150), and categorizing the 

 salt tolerance of crops as highly salt tolerant, 

 moderately salt tolerant and slightly salt tolerant, 

 the guide shown in table IV-20 was prepared to 

 indicate the number of permissible irrigations 

 using water of varying salt concentrations. This 

 guide is based on two assumptions: 



(1) That no leaching rainfall occurs between 

 irrigations, and 



(2) That there is no salt accumulation in the 

 soil at the start of the irrigation period. If 

 leaching rains occur between irrigations, 

 the effect of the added salt will be mini- 



TABLE IV-20. Permissible Number of Irriga- 

 tions in Humid Areas With Saline Water 

 Between Leaching Rains for Crops of 

 Different Salt Tolerance ' (97) 



Number of irrigations for 

 Irrigation water crops having 



Electrical 



Total conductivity Low Moderate High 



salts mmhos/cm. salt salt salt 



mg/l at 25 C tolerance tolerance tolerance 



640 1 7 15 



1,280 2 4 7 11 



1,920 3 2 4-5 7 



2,560 4 2 3 5 



3,200 5 12-3 4 



3,840 6 12 3 



4,480 7 — 1-2 2-3 



5,120 8 -. 1 2 



1 Based on a 50-percent yield decrement. 



174 



mized. If there is an accumulation of salt 

 in the soil initially, such as might occur 

 when irrigating a fall crop on land to which 

 saline water had been applied during a 

 spring crop, the soil should be tested for 

 salt content and the irrigation recommen- 

 dations modified accordingly. 



SAR Values and Exchangeable Sodium 



The principles relating to this parameter and 

 the degree to which sodium is adsorbed from water 

 by soils are generally applicable in both arid and 

 humid regions. Some evidence is available (92), 

 however, to indicate that, for a given water quality, 

 less sodium is adsorbed by an acid soil than by a 

 base-saturated soil. For a given level of exchange- 

 able sodium, preliminary evidence indicates more 

 detrimental effects on acid soils than on base- 

 saturated soils (94). Since experimental evidence 

 is not conclusive at this point, detrimental limits 

 for SAR values previously listed will also apply to 

 supplemental irrigation. 



Acidity and Alkalinity 



The effect of the pH of irrigation water on crops 

 results primarily from the resultant effect on the 

 soil to which it is applied. The only consideration 

 not previously discussed relates to soil acidity 

 which is more prevalent in humid regions where 

 supplemental irrigation is practiced. Any factor 

 which will drop the pH below 4.8 may render 

 soluble toxic concentrations of iron, aluminum, 

 and manganese. This might result from application 

 of a highly acidic water, or from a saline solution 

 applied to an acidic soil. Since the nature of the 

 soil is the major determining factor, it is not feasi- 

 ble to set limits on the pH of the water. Specific 

 consideration must be given to each individual set 

 of conditions. 



Trace Elements 



Criteria and related factors previously listed are 

 equally applicable to supplemental irrigation. Cer- 

 tain related qualifications must be kept in mind, 

 however. First, foliar absorption of trace elements 

 in toxic amounts is direcfly related to sprinkler 

 irrigation. Critical levels established for soil or cul- 

 ture solutions would not apply to direct foliar 

 injury. Regarding trace element concentrations in 

 the soil resulting from irrigation water application, 



