KEARNEY." PLANT LIFE ON SALINE SOILS 111 



ally not so much the chemical composition of the soil solution as 

 its concentration and the resulting osmotic pressure which affect 

 vegetation."^ 



CONCENTRATION OF THE SOIL SOLUTION 



Saline soils are extremely diverse in respect to texture, water- 

 holding capacity, humus content, and fertility. The one thing 

 that they have in common is a high concentration of the soil 

 solution. This factor, also, is extremely variable, since not 

 only the absolute quantity of salts may differ enormously within 

 very short distances, but the concentration of the solution fluc- 

 tuates continually, being diluted by rainfall or by inundation 

 and concentrated by evaporation. 



The fluctuations are greatest, of course, in deserts, where rain- 

 falls of brief duration alternate with long periods of extreme 

 drought. A salt content amounting to 3 per cent of the dry 

 weight of the soil, to the depth penetrated by the plant roots, 

 is not uncommon in arid regions. With this salt content, and 

 with a water-holding capacity of 50 per cent, the soil, even when 

 saturated with water, would have a solution concentration of 6 

 per cent, or twice that of sea-water. If the soil dried out to the 

 wilting point for plants,^ the concentration would reach 30 per 

 cent, which is bej^ond the point of saturation for sodium chloride. 



Even in humid climates the periodical fluctuations are by no 

 means negligible. Hi'F reported that in a salt marsh in Brit- 

 tany, heavy rains lasting two days reduced the concentration 

 of the soil solution to one-sixth of what was observed immedi- 

 ately before the rain began. If the original concentration had 

 been that of sea- water, this would correspond to a fall in osmotic 

 pressure of from about 22 to about 3.5 atmospheres.^ 



* This does not hold in the case of sodium carbonate ("-black alkali") which, 

 because of its strong alkalinity, is much more toxic than other salts of sodium. 



* L. J. Briggs and H. L. Shantz. The wilting coefficient for different plants 

 and its indirect determination. U. S. Dept. Agr. Bur. Plant Ind. Bull. 230. 1912. 



« T. G. Hill. The Bouche d'Erquy in 1908. New Phytol. 8: 97. 1909. 



' Careful computations of the osmotic pressure of sea water have recently 

 been published by R. H. True {Osmotic experiments with marine algae. Bot. 

 Gaz. 65:71. 1918). 



