66 



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



Plant Tolerance 



Irrigation agriculture must provide for im- 

 proved growth of crop plants or it has no justifica- 

 tion for existence. Thus, when soils or irrigation 

 water have an undue burden of soluble salts, it is 

 essential to know how much salt various crops 

 will tolerate at different stages of growth. Re- 

 search has shown that certain barley varieties are 

 quite tolerant to irrigation water containing 20,000 

 p.p.m. of dissolved salt. Sugar beets are very sen- 

 sitive to salt at the seedling stage but highly toler- 

 ant after the young plants have become well 

 established. Beans are very sensitive to salty soils. 

 Most horticultural fruits are sensitive to salty 

 soils, but there are exceptions. 



A study undertaken to ascertain the effects of 

 various rootstocks in permitting grapes to be 

 grown on salty soils showed that the rootstock 

 tremendously affected the amount of chloride that 

 accumulated in grape leaves. It was also f ound that 

 if the chloride ion accumulated to appreciable 

 levels in a grape leaf, it could cause necrosis, or 

 death, of the leaf. When grapes were grown on a 

 rootstock known as "Salt Creek,"' only one- 

 fifteenth as much chloride accumulated in the 

 leaves as when grown on ''Cardinal'' rootstock. 

 Consequently, grapevines growing in soils having 

 appreciable chloride salts would show serious in- 

 jury if grown on Cardinal roots, whereas the same 

 varieties would show virtually no injury when 

 grown on Salt Creek roots. 



Research is underway in the selection of genetic 

 lines of Pima cotton that are more tolerant to 

 saline conditions of the Southwest. Pima S-3, re- 

 cently released for commercial production, appears 

 to be considerably more tolerant to saline condi- 

 tions than the current commercial variety. 



Limited research has shown the feasibility of 

 developing forage grasses that are superior with 

 respect to establishment, persistence, and yield on 

 saline soils. Grass varieties differ markedly in salt 

 tolerance. Evidence indicates that there are good 

 possibilities in selecting within species for im- 

 proved salt tolerance. 



Research on the nature of salt tolerance and 

 selection or development of varieties superior in 

 salt tolerance, will provide valuable information 

 and planting material in the future. As economic 

 pressures on water supplies force agriculture to use 

 more and more water of impaired quality with 



respect to salt loading, salt tolerant crops will in- 

 crease in importance. 



Decreasing Salt in Drainage Water 



Plants preferentially absorb water and leave the 

 dissolved salts in the soil. This preference by 

 plants for water over salt increases with increasing 

 salinity of the water. Since this discrimination in 

 plant absorption is a basic plant characteristic, it 

 would appear that little can be done to alter the 

 fact that drainage waters from irrigated areas 

 will be higher in salt concentration than the orig- 

 inal irrigation water, and approximately in in- 

 verse proportion to the change in water volume 

 during the passage of the water through the plant 

 root zone. Since virtually all of the salt in the 

 drainage water was endogenous to the irrigation 

 water, no substantial decrease in total salt burden 

 of the water is possible without adversely affecting 

 the agriculture of the area. In fact, increasing the 

 salt tolerance of plants — thereby enabling them to 

 concentrate the water still further — will only 

 result in a still greater increase in the salinity of 

 the drainage water. However, the reduction in 

 volume of the drainage effluent may be decidedly 

 beneficial if alternative methods for disposal of 

 drainage effluent other than by return to streams 

 or reusable water reservoirs are developed. 



Xot all e^apotranspiration is beneficial or un- 

 avoidable. Excessive irrigation, by keeping the 

 soil surface moist for longer periods, promotes 

 surface evaporation. In only rare cases is the re- 

 sultant lowering of soil temperature of definite 

 benefit to the crop. Therefore, improved irrigation 

 efficiency will not only conserve water but will also 

 decrease the volume of drainage water and the 

 quantity of salt that must in some way be removed 

 from the crop fields. Research on planting tech- 

 niques for furrow-irrigated crops has produced 

 planting methods that do not require preplanting 

 irrigations for thorough leaching of salt out of the 

 plow layer. This, again, conserves water and 

 reduces nonbeneficial evaporation of water from 

 the soil. The elimination of weeds in fields, and of 

 phreatophytes along canals and field borders, will 

 further conserve water and decrease the magnitude 

 of the saline water disposal problem. 



The removal of drainage waters from irrigated 

 lands, which prevents rising water tables and a 

 buildup of salt in the soil, is essential for the main- 

 tenance of the project. The collected drainage 



