146 
Transactions of the Boyal Society of South Africa. 
The effect of irrigation, as practised at Van Wyks Ylei, 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 
Carnarvon Division — the presence of magnesium chloride in the soil was 
noticed. In the Eobertson Division, on the other hand, where rain is 
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 30 inches, while the sulphates have remained at the lower 
levels. 
Calculating from the figures in the last ta.ble, the percentage composition 
of the salt at the soil surface at T is as follows : — 
Sodium chloride 33*20 
Magnesium chloride 1"80 
Calcium chloride 60*30 
Calcium sulphate 4*15 
Calcium carbonate *55 
With this may be compared the following results, quoted by Hilgard 
(" Soils : their Formation, Properties, &c.," p. 442), of an analysis of 
alkali occurring in California (Imperial) : — 
Potassium chloride 1"15 
Sodium nitrate 8*21 
Sodium chloride 31*82 
Sodium carbonate *58 
Magnesium chloride 2*81 
Calcium chloride 58*42 
There is considerable resemblance in the respective percentages of 
sodium, calcium, and magnesium chlorides. 
X. 
