SALINITY OF IRRIGATION WATER — SCOFIELD 283 



physical condition of the soil, particularly when they occur in high 

 concentrations in the soil solution, but that effect is probably not 

 exerted through exchange reactions. 



REACTIONS OF BASE EXCHANGE 



It has long been known that one of the important phases of the 

 so-called '' alkali problem " in irrigated lands had to do with what 

 was described as the puddling effect on the soil of " black alkali " 

 or sodium carbonate. A puddled or impermeable soil is one that 

 when wetted expands or becomes gelatinous so that water does not 

 move through it readily. This condition occurs in irrigated soil 

 on which soft water is used because saline soft waters contain 

 sodium. It is only within the last decade or two that substantial 

 progress has been made in understanding the nature and extent of 

 the reactions by which such effects are produced. These are known 

 as reactions of base exchange because they take place between the 

 basic ions or cations of the solution and ions of the same group 

 that are attached to some of the solid particles of the soil. The fact 

 that exchange reactions take place between the soil and the salts 

 of its solution was reported by an English chemist, J. Thomas Way, 

 as early as 1852. But it was not until some 35 years later when 

 the phenomenon of ionization was discovered that it became possible 

 to understand how these reactions occur. The finiits of these dis- 

 coveries have also been extensively utilized in industry in the process 

 of artificial water softening by which the calcium and magnesium 

 are removed from hard water intended for domestic use or for 

 laundries. 



The aspects of the subject of base exchange that are dominant 

 in irrigated lands have to do with exchange reactions involving 

 sodium and calcium. It is now known that deflocculated or im- 

 permeable soils contain appreciable quantities of sodium combined 

 with the exchange complex. This sodium has been taken up by 

 the soil from the soil solution in which sodium occurred, and such 

 reactions take place regardless of the nature of the anion that was 

 associated with sodium in the solution. In other words, sodium 

 may be absorbed by the soil from solutions of sodium chloride or 

 sodium sulphate as well as from solutions of sodium carbonate or 

 " black alkali." It is implicit in the concept of the reactions of 

 cationic exchange that for each ion absorbed from the solution by 

 the soil another ion must pass into the solution from the soil ; also 

 that these reactions occur in the direction of attaining ionic equi- 

 librium of concentration between the soil and its solution. 



These facts concerning the reactions of base exchange afford a 

 way of understanding the effects of the salinity of irrigation water 



