PROPERTIES OF THE COLLOIDAL SOIL MATERIAL 6 
Description of the Sharkey, Marshall, Huntington. Sassafras. 
Susqehanna, Norfolk, and Fallon 3 soil types are given in a previous 
publication of this bureau (35). The sample of Aragon soil was 
selected because of its unusual chemical composition. 
The colloids were isolated essentially as described in a previous 
publication (20, p. 16). The soils were treated with large quan- 
tities of water and the turbid extracts passed through a super- 
centrifuge to grade out noncolloidal particles. In the preparation 
of these" samples no ammonia was used, except in the case of the 
Aragon soil, which without ammonia, settled clear very quickly in 
pure water. Dispersion was facilitated by rubbing the plastic mass 
of the fine soil material under water. The products obtained were. 
in each case, submicroscopic and were comparable to the prepara- 
tions previously studied. 
PROPERTIES OF COLLOIDAL SOIL MATERIALS 
SIZE OF PARTICLES 
Size of particles has generally been considered a fundamental 
distinguishing characteristic of materials in a colloidal condition. 
In fact, in many instances differences in colloidal properties have 
been assumed to be proportional to particle size. It should be borne 
in mind, however, that such a proportionality between particle size 
and properties holds only when one deals with a particular kind of 
colloidal material For example, the properties of different prepa- 
rations of colloidal sulphur parallel to a certain extent the size of 
the particles (47). but it is not to be expected that a sulphur sol 
and a gold sol having particles of the same size would have the 
same properties. The chemical composition of colloidal materials 
extracted from different soils has been shown to vary widely. It is 
thus probable that differences in behavior of the various colloidal 
soil materials may be in part dependent upon chemical composition, 
and not entirely a function of size of particles. 
The colloidal material exists in the soil chiefly as a gel; that is, 
as an intimate aggregation of ultimate particles. \Vhen the mate- 
rial is dispersed in water the dilute sols are stable, but the suspended 
matter may consist of both ultimate particles and aggregates. 
Determinations of size by most methods would not distinguish be- 
tween ultimate particles and aggregates; nevertheless such determi- 
nations should characterize the different colloids more or less, even 
if the particles are chiefly aggregates, provided the materials are 
dispersed by a uniform treatment. If the different materials are 
dispersed in the same manner, variation should occur in the size of 
aggregates owing to variation in the nature of the materials. 
The upper limit of size of particles in the different* soil colloids 
was of course determined by the method of preparation, the maxi- 
mum diameter of the particles being about 0.3 micron. It is evident, 
however, that the upper limit of size offers no indication of average 
size, inasmuch as the samples used contained all the colloid passing 
3 The sample of Fallon soil used contained about 0.5 per cent soluble salts. It was 
selected for tbis work particularly on account of its bigbly impervious character. The 
soluble salts were, of course, removed by the procedure followed in isolating colloidal 
material. 
