187 



fcidered necessary to make analyses, but the soils taken from the higher 

 levels were examined for alkaline salts, with the results tabulated below : 



The effect of irrigation, as practised at Van Wyks Vlei, on this type 

 of soil is seen, in the case of soil T, by the accumulation of salts at the 

 surface. In its natural condition the vertical distribution of the saline 

 material is such as to leave the surface soil comparatively free, so that 

 shallow-rooted crops may be cultivated without difficulty. In the irrigated 

 soil the present failure to grow crops is easily explained by the existence 

 of the chlorides of Calcium and Magnesium in the upper soil layers. The 

 occurrence of these very soluble salts in " brack " is somewhat unusual, 

 and can only take place in arid regions like Carnarvon. At Thebus too, 

 where the rainfall is not abundant although less scanty than in the Car- 

 narvon Division the presence of Magnesium chloride in the soil was 

 noticed (see page 183). In the Robertson Division, on the other hand, 

 where rain is more frequent, and the atmosphere more humid, a surface 

 efflorescence containing these salts is an impossibility. 



It will be noticed, especially from the appended diagram, that, in the 

 irrigated soil, the chlorides are, for the most part, accumulated within the 

 upper thirty inches, while the sulphates have remained at the lower levels. 



Calculating from the figures in the last table, the percentage composi- 

 tion of the salt at the soil surface at T is as follows : 



Sodium chloride 33'20 



Calcium chloride 60'30 



Magnesium chloride 1'80 



Calcium sulphate 4' 15 



Calcium carbonate "55 



With this may be compared the following results, quoted by Hilgard 

 (" Soils: their formation, properties, etc.," p. 442), of an analysis of 

 alkali occurring in California (Imperial) : 



Potassium chloride 1'15 



Sodium nitrate 8' 21 



Sodium carbonate *58 



Sodium chloride 31'82 



Calcium chloride 58'42 



Magnesium chloride 2'81 



There is considerable resemblance in the respective percentages of 

 Sodium, Calcium, and Magnesium chlorides. 



