42 BULLETIN 1152, U. S. DEPARTMENT OF AGRICULTURE 
CHARACTER OF SOIL COLLOIDS 
Data are now available on the properties and ultimate chemical 
composition of a considerable number of different colloidal soil mate- 
rials. It is thus possible to judge something of their general nature 
as compared with that of colloids which have been studied more 
extensively. All the extremes in composition and properties which 
might be encountered in colloidal soil materials probably are not 
represented in those studied in this and preceding publications, but 
the kinds of colloidal materials found in most soils are doubtless 
fairly well represented. 
Although wide variations have been encountered in chemical com- 
position and in all properties except specific gravity, the different 
soil colloids do not form sharply defined groups, since distinct gaps 
do not occur in chemical characteristics or properties when a suffi- 
ciently varied lot of colloids is examined. The colloids might, how- 
ever, be divided roughly into k * high." " medium," and " low " groups 
on the basis of chemical composition or on the basis of a number of 
properties. Rough grouping of soil colloids on many such bases 
coincide fairly well because of the correlations obtaining between dif- 
ferent properties and between properties and chemical composition. 
Colloids, for instance, in the high group would usually have a high 
ratio of silica to aluminum and iron, a high content of total and 
exchangeable monovalent and divalent bases, and a relatively low 
content of water held at 110° C. (47, p. 15) ; the reaction in distilled 
water would be alkaline or only slightly acid and the average size 
of particles small : while comparatively high values would be shown 
for swelling, imbibition, moisture equivalent, volume of floe, heat of 
wetting, adsorptive capacity for vapors and electrolytes, and the 
quantity of methylene blue required to render the colloids isoelectric. 
Soil colloidal materials as a class are evidently nearer the lyophobe 
than the lyophile type of colloid, judging by the degree they possess 
those properties which have been generally used for characterizing 
the two types. The soil colloids are much lower in viscosity, swell- 
ing, and imbibition, and are far less stable toward electrolytes than 
the typical lyophiles; but some of the colloids in the high group 
differ considerably from the typical lyophobes. The Fallon colloid, 
for instance, shows a volume increase in water of 150 per cent as 
compared with no increase for ignited silica gel and a 900 per cent 
increase for gelatin. 
Bentonite appears to be closely related to the soil colloids, al- 
though its properties approach those of the lyophile colloids more 
nearly than most colloidal soil materials. 
The characterization of the soil colloids as neither typically 
lyophile nor typically lyophobe accords with opinions of other 
investigators. Hardy (22) describes clay colloids as inelastic 
emulsoid gels with a reticulate structure. Bradfield (9) regards 
the finest colloidal soil material, that which is essentially submi- 
croscopic, as an emulsoid. and the coarser material, made up of 
particles ranging from 0A to 1:2 microns in diameter, as a suspensoid. 
This investigation has dealt only with the properties of the col- 
loidal soil material and not with the properties of the whole soil. 
However, many of the properties of a soil are largely affected by 
