hundred parts of soluble matter maybe taken as the extreme limit of endurance 

 for plants, while 250 or 300 parts mark the danger point at which the results of 



the use of the water are very uncertain. 



******* 



The limit of endurance for most cultivated plants in a water solution is about 

 1 per cent or 1,000 parts of the readily soluble salts in 100,000 parts of water, 

 but it must be remembered that in field culture the water is applied to soils 

 already containing more or less of these salts, and also that evaporation and 

 consequent concentration immediately set in after the application of water. It 

 was found at Carlsbad that about 300 parts of soluble matter per 100,000 parts 



of water marked the extreme limit of safetv of the use of water at that place. 

 * * * * * * * 



One thing should be said in connection with the Carlsbad area, which seems 

 rather anomalous in view of the statements of other investigators, namely, that 

 with a water supply so near the limit of crop endurance as this becomes at times 

 and in those areas in which there is already a large accumulation of salts, 

 economy in the use of irrigation water, which is generally recommended in alkali 

 regions, is one of the worst methods which can be practiced. Where the soil 

 contains a relatively large amount of salt and but little water containing much 

 salt is frequently applied, the ordinary evaporation will increase the salt content 

 of the soil to such an extent that crops can no longer survive, whereas if ade- 

 quate drainage is provided, and a large amount of water is used, the excess of 

 salt resulting from the evaporation of previous applications of water, may be 

 removed, and the soil moisture be maintained at nearly the same concentration 

 as the water supply. 



The fact that the Arabs in Algerian oases are actually growing sensi- 

 tive plants by the aid of irrigation waters containing from 400 to 800 

 parts of soluble salts, in some instances 50 per cent sodium chloride, 

 shows that the Bureau has been on the conservative side in its esti- 

 mates, and should encourage a more hopeful feeling among the people 

 occupying areas where only alkali water is available for irrigation. 



The prerequisite to the use of water of high salt content in irrigation 

 is the knowledge that the methods employed are opposed to the teach- 

 ings of most American writers on the subject. Those who place the 

 low limit of safety for alkaline irrigation waters have taught that where 

 water was badly alkaline irrigation should be sparing. They have not 

 insisted on thorough drainage, and they have warned irrigators against 

 too frequent irrigation. With such practices the limit of concentration 

 which they set is probably high enough, and even then all except the 

 most sandy soils or those with exceptionally good natural drainage 

 would ultimately be damaged. 



The methods in the oases are quite different. The Arab gardens are 

 divided into small plots, about 20 feet square, between which run drain- 

 age ditches dug to a depth of about 3 feet. The soils being very light 

 and sandy, this ditching at short intervals insures the most rapid and 

 thorough drainage. Irrigation is by the check method, and application 

 is made at least once a week, though often two wettings a week are 

 deemed necessary. A large quantity of water is used at each irrigation. 

 Thus a continuous movement of the water downward is maintained, 

 there is little opportunity for the soil water to become more concentrated 

 than the water as applied, and the interval between irrigations being so 

 short but little accumulation of salt from evaporation at the surface 

 takes place. What concentration or accumulation does occur is quickly 

 corrected by the succeeding irrigation. 



