tions in relation to salinity was demonstrated by 

 Magistad, et al. (99). In general, criteria regard- 

 ing salinity for supplemental irrigation in humid 

 areas can be more flexible than for arid areas. 



Soil characteristics represent another significant 

 difference between arid and humid regions. Soils 

 in arid regions generally tend to be neutral or 

 alkaline, whereas those in humid regions tend to 

 be acidic. Mineralogical composition will also vary. 

 The composition of soil water available for ab- 

 sorption by plant roots represents the results of an 

 interaction between the constituents of the irriga- 

 tion water and the soil complex. The final result 

 may be that a given quality deterrent present in the 

 water could be rendered harmless by the soil, re- 

 main readily available, or that the dissolved consti- 

 tuents of a water may render soluble toxic concen- 

 trations of an element which was not present in the 

 irrigation water. An example of this would be the 

 addition of a saline water to an acid soil resulting 

 in a decrease in pH and a possible increase in solu- 

 bility of elements such as iron, aluminum, and 

 manganese (51). 



Another significant characteristic of soils is their 

 adsorption, or ion exchange, properties. Not only 

 is the composition of the soil solution altered by 

 dissolved constituents in irrigation water, but the 

 physical properties of the soil may also be altered 

 by changes in ions adsorbed by the soil. 



General relationships previously derived for 

 SAR and adsorbed sodium in neutral or alkaline 

 soils of arid areas do not apply equally as well to 

 acid soils found in humid regions (92). Further- 

 more, the effect of a given level of adsorbed sodium 

 (exchangeable sodium percentage) on plant 

 growth will be determined to some degree by the 

 associated adsorbed cations. The amount of ad- 

 sorbed calcium and magnesium relative to ad- 

 sorbed sodium is of considerable consequence, 

 especially when comparing acidic soils to ones 

 which are neutral or alkaline. Another example 

 would be the presence of a trace element in the 

 irrigation water which might be rendered insoluble 

 when applied to a neutral or alkaline soil, but 

 retained in a soluble, available form in acid soils. 

 For these reasons, soil characteristics, which differ 

 greatly between arid and humid areas, must be 

 taken into consideration. 



Certain economic factors also influence water 

 quality criteria for supplemental irrigation. Al- 

 though the ultimate objective of irrigation is to 

 insure efficient and economic crop production, 

 there may be instances where an adequate supply 

 of good quality water is unavailable to achieve 

 this. A farmer may be faced with the need to use 

 irrigation water of inferior quality to get some 



economic return and prevent a complete crop 

 failure. This can occur in humid areas during 

 periods of prolonged drought. Water quality cri- 

 teria are generally designed for optimum produc- 

 tion, but consideration must be given also to sup- 

 plying guidelines for use of water of inferior quality 

 to avert a crop failure. 



Specific quality criteria for 

 supplemental irrigation 



A previous discussion of potential quality de- 

 terrents contained a long list of factors indicating 

 the current state of our knowledge as to how they 

 might relate to plant growth. Criteria can be es- 

 tablished in two ways: (a) by determining a con- 

 centration of a given deterrent which when ad- 

 sorbed on, or absorbed by, a leaf during sprinkler 

 irrigation results in adverse plant growth, and (b) 

 by evaluating the direct and/or indirect effects that 

 a given concentration of a quality deterrent will 

 have on the plant root environment as irrigation 

 water enters the soil. Neither evaluation is simple, 

 but the latter is most complex since so many vari- 

 ables are involved. Since sprinkler application is 

 most common in humid areas for supplemental 

 irrigation, both types of evaluation have consider- 

 able significance. The following discussion relates 

 only to those quality criteria that are specifically 

 applicable to supplemental irrigation. 



Salinity 



General concepts regarding soil salinity as pre- 

 viously discussed are applicable. Actual levels of 

 salinity which can be tolerated for supplemental 

 irrigation must take into consideration the leaching 

 effect of rainfall and the fact that soils are usually 

 nonsaline at spring planting. The amount of irriga- 

 tion water having a given level of salinity that can 

 be applied to the crop will depend upon the num- 

 ber of irrigations between leaching rains, the salt 

 tolerance of the crop, and the salt content of the 

 soil prior to irrigation. Since it is not reaUstic to set 

 a single salinity value, or even a range, that would 

 take these variables into consideration, a guide was 

 developed to aid farmers in safely using saline, or 

 brackish, waters (93). The following eC[uation was 

 used as a basis for this guide : 



ECe(() = ECe(i)-|- 



n(EC,„) 



where ECe(f) = electrical conductivity of the satu- 



173 



