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ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 35 



the several constituents of salinity and that the group of species 

 comprised in one system of agriculture may be wholly dilTerent from 

 that of another system. These are examples of the variables that 

 must be considered in determining the relationship between the qual- 

 ity of irrigation water on one side and of a profitable and enduring 

 system of irrigation agriculture on the other. The demand for that 

 comparison is frequent and insistent. It is not much less difficult 

 to answer when its scope is limited to one constituent and one crop 

 species. Not the least of the difficulty lies in our lack of precise 

 knowledge in respect to the reaction of any given crop species to 

 any given concentration of a solution constituent as influenced by 

 climatic conditions, by stage of growth, or by the associated 

 constituents. 



Despite the recognized existence of all of these variables in the 

 equation, it is possible to establish certain criteria by which to esti- 

 mate the potential effect of the salinity of an irrigation supply on 

 a given soil with a given climate and a given group of crop species. 

 These criteria are of necessity based on field observation and on data 

 of the quality of irrigation water. As an example of the sort of 

 criteria that may be used, the following table is given. It repre- 

 sents the conclusions of a number of men who are well informed on 

 conditions in one irrigated region and is used in connection with the 

 appraisal of irrigated farms. In this table the boron constituent is 

 emphasized because it is regarded locally as of critical importance. 



Table 1. — An example of the permissible limits adopted for a definite region of 

 classes of irrigation ivater with respect to certain of its characteristics 



' The concentration of dissolved salts in water may be measured by either of two methods, that of elec- 

 trical conductance and that of evaporating the water and weighing the residue. 



' This percentage represents the proportion of sodium to the total cations and is computed from the 

 data of analysis, reported as milligram equivalents, by dividing the sum of the values for sodium and 

 potassium by the sum of the values for all the cations 



In the application of the class limits given in the table considera- 

 tion is given to (1) the crop group, (2) soil type, (3) climatic con- 

 ditions, (4) relative quantity of irrigation water to rainfall. As 

 applied specifically to the boron conditions the crop groupings are: 



