PROPERTIES OF THE COLLOIDAL SOIL MATERIAL 41 
arid soils are inclined to have high rr- ^ i, n ratios. However. 
ALUg-f-x 1 e 2 U 3 
a close correlation between high silica ratio and proportion of ex- 
changeable sodium would not be expected, since sodium-saturated 
colloids do not appear to be the rule even in arid regions. On the 
other hand, exchangeable hydrogen or aluminum and iron should 
usually form a relatively high proportion of the exchangeable cations 
in colloids with low silicia ratios. It is generally recognized that 
exchangeable acidity is largely confined to weathered soils of the 
humid regions, and that the colloids of such soils tend to have low 
silica ratios. Evidently these general relations between the proportions 
of the different exchangeable bases and the proportions of silica to 
aluminum and iron in the colloid are not due to a direct influence 
of the silica ratio or the kind of exchangeable base, but are due to 
both these chemical characteristics being governed by similar or 
related soil-forming conditions. 
The correlation between properties of the colloid and the silica 
ratio probably does not mean that all properties of the colloid are 
chiefly governed by the silica ratio directly. Structure of the col- 
loid — that is, size and surface of the particles and arrangement or 
localization of the chemical constituents — must also influence proper- 
ties of the soil colloidal material, just as structure influences the 
properties of other colloids. Furthermore, it should be borne in 
mind that the ratio ofr- ,^ , -a, — ~- takes into account only the chief 
constituents of the colloid and only the total quantity of these con- 
stituents. Other constituents of the colloid, such as organic matter, 
Ca, Mg, K, etc., would presumably have some effect on properties, 
and the condition of the elements in the colloid — that is, the form 
of combination in which they are present and their distribution in 
the colloid structure — should have an influence in determining prop- 
erties. As a matter of fact, properties which are dependent upon 
the adsorption of cations, such as adsorption of malachite green and 
certain aspects of electrical behavior, are evidently governed more 
directly by the exchangeable Ca, Mg, etc., than by the silica ratio. 
However, as previously mentioned, the silica ratio may determine 
the exchangeable cations within certain limits. 
It thus seems that the correlation between chemical constituents 
may be due in some cases to a direct and partial dependence of one 
constituent on another, and in other cases to both characteristics 
being dependent on similar or related conditions. 
Further investigation is needed to learn in what measure properties 
are directly or indirectly dependent on the ratio of 
SiQ 2 ' 
Al 2 3 +Fe 2 3 ' 
It seems, however, that the connection between these major constitu- 
ents of the colloid and properties may be indirect in many cases. 
Structure and the Ca, Mg, K, and Na present in an exchangeable 
condition may govern certain properties more directly than the 
silica ratio, but these may in turn be governed by the silica ratio to 
some extent. 
