40 
BULLETIN 1452, IT. S. DEPARTMENT OF AGRICULTURE 
of the exchangeable monovalent and divalent bases. Robinson and 
Holmes (47) have suggested that the a i q ■ p V) ratio and the 
n q , -p q ratio may both be determined to a large extent by the 
rainfall to which the material has been subjected, the silica, like the 
lime, being more easily removed from the colloid than iron and 
alumina. It is also possible that the relative quantities of silica. 
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2.0 2.5 
3.0 3.5 
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Si0 2 
Fig. 4. — Relation between malachite green adsorption and the \i o. + F e ,o„ 
aluminium, and iron in the colloid determine to some extent the 
total content of exchangeable bases. - 
Some very general relations between silica ratio and kind of bases 
might be expected that are not brought out hy the data given in this 
bulletin. For instance, a high proportion of exchangeable sodium 
would be expected only in colloids having a high silica ratio ; since 
colloids high in exchangeable sodium are probably confined largely 
to the alkali or salt soils of arid regions, and it appears from the 
analyses reported by Robinson and Holmes (4,7) that the colloids of 
