282 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 5 



of another cation, but there is little evidence to support the view that 

 the physiological effect of an anion sucli as chloride is any different 

 whether its salt-forming companion is sodium or calcium. 



Some of the constituents of salinity are essential to plant nutrition. 

 This is certainly true of such cations as calcium, magnesium, and 

 potassium, and of such anions as sulphate and nitrate, to enumerate 

 only a few. However, the concentration levels at which several of 

 these constituents commonly occur in saline irrigation water is much 

 above the optimum for plant nutrition. It is probably true that in 

 respect to many of the ions or elements that occur in solution in 

 natural waters or in the soil solution there are concentration levels 

 that are below the optimum for plant nutrition as well as higher con- 

 centration levels at which the same ions or elements become toxic. 

 This is not known to be true of all of these constituents. Chloride, 

 for example, is probably not essential, at least to many plants, and 

 there is some reason for believing that it may cause some depression 

 of growth when it occurs in concentrations that are much lower than 

 those that are associated with definite symptoms of malnutrition or 

 that kill the plant. 



There are very pronounced differences in the toxicity of the differ- 

 ent solution constituents. Boron, for example, may cause serious 

 injury when its concentration in irrigation water is less than 1 part 

 per million or when in the soil solution its proportion is not more 

 than 3 or 4 parts per million. With chloride, as another example, 

 concentrations up to 150 to 200 parts per million in irrigation water 

 may not cause obvious symptoms of injury even though the soil solu- 

 tion resulting from the use of such water may contain 3 or 4 times 

 that concentration. 



There are also very great differences among crop plants in respect 

 to their limits of tolerance for any one constituent of the soil solu- 

 tion or for all of those constituents taken together. This aspect of the 

 subject is discussed in some detail in a later paragraph. 



In the case of plants it is probably true that nearly every one of 

 the dissolved constituents of the soil solution has some effect, for good 

 or ill, on the plant. This is apparently not true of the relationship 

 between these solutes and the physical condition of the soil. Our 

 concern about the soil is due to the fact that its function as a suitable 

 reservoir for the soil solution is profoundly influenced by its physical 

 condition in relation to its permeability, and that this in turn is 

 influenced by the nature of the cations introduced into the soil solu- 

 tion by the irrigation water. These cations participate in reactions 

 of exchange between the solution and the soil. So far as we now 

 know, such reactions do not occur between the soil and the dissolved 

 anions. These anions, or some of them, may have some effect on the 



