Investigations Begarding Brack" {Alkali) in Cape Colony Soils. 131 
soil from the surface — a method which has sometimes been employed with 
somewhat misleading results. 
It is to the presence of alkaline salts, comprising the chloride, sulphate, 
and carbonate of sodium, that the brackness of a soil is due. Of these 
salts, sodium sulphate is the least and sodium carbonate the most injurious 
on account of the corrosive effect of the latter on the bark, and its puddling 
of the soil and so interfering with tillage ; the last-named salt is the cause 
of what is commonly termed black brack, the dark colour being due to the 
organic matter dissolved by the alkaline carbonate, while the chloride and 
sulphate, which do not dissolve organic matter, form what is called white 
brack. In small quantities these alkaline salts are contained in nearly all 
soils, being derived from the insoluble sodium silicates, which slowly 
decompose under the influence of air, water, and the solar heat into the 
soluble salts above mentioned. Hence there is ever going on a constant 
addition to the soil of soluble sodium salts, and, unless means exist for a 
constant, and as rapid a removal of these salts, they would in time so 
accumulate in the soil as to render it incapable of cultivation. 
As a rule natural drainage is sufficient to remove the noxious salts 
from the soil, and they are continually carried down to the sea by the 
rain-water in which they are dissolved. Exceptions to this rule, however, 
exist : thus, when the surface soil is porous but has an impermeable layer 
underneath, the alkaline salts accumulate, as it were, in a basin from 
which there is no outlet ; or the soil may itself be so compact or other- 
wise constituted as to prevent proper drainage ; lastly, a high-water line 
in the subsoil may prevent the exit of the injurious compounds ; f in all 
these cases, unless a remedy be provided, there can be but one result, an 
accumulation in the soil of the salts which render it brack. 
When, therefore, drainage of a soil becomes impossible, the question to 
be considered is whether the alkaline salts may collect at any particular 
level, or whether, on the other hand, they will be more or less evenly 
distributed throughout a mass of soil several feet in thickness. In such 
localities where rain is frequent and the temperature not excessive the 
probability is that the salts will be fairly evenly distributed throughout 
the soil, so that comparatively little, if indeed any, injurious effect on the 
vegetation ensues. If, however, the rainfall is scanty and the climate 
warm, or if there even be a brief period of heavy rainfall succeeded 
by a dry season, time and opportunity are afforded for the harmful salts 
* Such a possibility demonstrates the importance of properly ascertaining, by 
mechanical analysis or otherwise, the physical condition of the soil all the way from 
the surface, down to a depth of from 4 to 6 feet, wherever an irrigation project is under 
consideration. 
f This again shows that if a soil is likely to prove brack under irrigation, no investiga- 
tion can be considered complete unless a series of determinations of the height of the water 
table in the subsoil has been made. 
