nesium, with their purgative properties, cannot be present in quantity without seriously 

 affecting the direct use of the water by man and his domestic animals and that 'black 

 alkali' (sodium carbonate and bicarbonate), even in small amounts, is not acceptable 

 to plants at present grown as crops. 



Ideas on the standard of water acceptable to man for drinking have change con- 

 siderably in recent years. We may now take as a fact that water with a salinity of 

 3000 parts by weight of NaCl per million of water can be drunk regularly by human 

 beings in a desert climate, that a figure of 4000 unaccompanied by important quantities 

 of other salts is acceptable, and that for short periods even a figure of 5000 is endur- 

 able. Domestic animals are even more tolerant of dissolved constituents than man 

 though there is no close agreement on the worst limits of quality. Thus to take three 

 examples from Australia, we find Jewell^^^ in Victoria, stating that 3000 parts of total 

 dissolved salts (not simply sodium chloride) per million is safe for working horses, 

 dairy cattle and pigs, and setting a normal limit of 7000 and an emergency one of 

 10,000 for grazing cattle and sheep. Jack(2)^ in South Australia states that horses will 

 thrive on water with 1 ounce of sodium chloride per gallon (6260 parts per million) and 

 sets the upper limits for living as 7800 for horses, 9400 for cattle and 15,600 for sheep 



— unless magnesium sulphate is present, when the figures must be lowered. Edge- 

 worth David and Browne(3) giving figures expressed as total solids, set limits even 

 beyond those of Jack's — 8000 for horses in work, 13,500 for horses at grass, 14,000 

 for cattle and 19,000 for sheep. However we attempt to reconcile these somewhat dis- 

 crepant figures, it is apparent that man is rather less tolerant than are his herds, but 

 that both can drink water which, as will be seen later, is of a quality not infrequently 

 obtainable in deserts. 



To obtain water of a quality suitable for crop - irrigation is a far more difficult 

 matter. There are certain salts — the alkali carbonates and bicarbonates (black alkali) 



— which are only acceptable to plants in very small concentration. Figures of between 

 100 and 200 parts per million have been given as limits and such amounts are often 

 exceeded in desert waters considered to be of good drinking quality. Apart from these 

 special constituents, the total amount of dissolved solids in irrigation water must also 

 be much below the limits of drinking water. It is not that many plants are intolerant of 

 brackish water. I have myself seen date palms and tamarisk growing well in ground 

 water with 6000 parts of NaCl per million — a water which a man could not take; but 

 the process of irrigation in a desert climate is inevitably accompanied by evaporation, 

 with the gradual concentration of salts in the soil to a point where plant growth is in- 

 hibited. The practical limit that has been given for the total solids in irrigation water 

 is only 700 per million. No doubt some easing of the stringency of this figure is per- 

 missible in semi-deserts, where there is sufficient rainfall to leach out some of the 

 accumulating salts, but in this paper I am concerned primarily with true deserts. In 

 these, the high quality necessary for irrigation water is the controlling factor on pro- 



(1) Jewell, N.R., 1927, Water for Stock. }. Agric. Victoria. 



(2) Jack, P.L., 1914, Bull. Geol. Surv. S.Australia, No. 3- 



(3) Edgeworth David, Sir T.W. & Browne, W.R., 1950. The Geology of the Commonwealth of 

 Australia. 2, 514-593. 



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