THE CHEMICAL COMPOSITION OF SOIL COLLOIDS 
29 
iron and aluminum. The source of this soluble matter is in the 
decomposition of the minerals and the leaching of colloidal matter. 
The composition of soil solutions may give some indications of 
differential solubility of the constituents of the colloidal matter. 
The material in the soil solution, however, comes from several 
sources — that already in solution in the soil, that resulting from the 
decomposition of soil minerals, and that from the colloidal matter 
alone. Hence it is not certain what proportion of the total soluble 
matter found- comes from the colloidal matter. The analyses of the 
solutions from which seven soil colloids were separated showed that 
the sums of the lime, soda, magnesia, and potash exceeded the sums 
of the silica, iron, and alumina, and that the silica content was about 
three to four times the sum of the iron and alumina. The solutions 
analyzed were in contact with the colloidal matter for a much longer 
time than they were in contact with the larger mineral particles of 
the soil, and it is probable that these solutions may more nearly 
represent the relative quantities of the various constituents that 
would be found in an extract of the colloid alone than the ordinary 
soil solution would. If the composition of the solution from which 
the colloidal matter was extracted bears a quantitative relation to 
the composition of a solution of the pure colloid, then there are 
indications that there is a differential solubility of the silica and the 
sum of the iron and alumina. 
It seems from the above that there would be some change in the 
composition of the colloidal matter when subjected to the continuous 
action of water, as, for instance, in dialysis. Accordingly, dialysis 
was tried on one soil colloid. For this purpose a quantity of air- 
dried material equivalent to 4.8 grams of moisture-free Chester loam 
colloid (sample No. 6) was subjected to a dialysis covering 41 days. 
The sample was suspended in 1,500 cubic centimeters of water in the 
inner compartment of a dialyzer. The outer compartment held 
about 1,800 cubic centimeters. Heavy parchment paper was used for 
the membrane. The water in the outer compartment was replaced 
by fresh distilled water eight times at intervals of from 4 to 7 days, 
and the combined diffusate, totaling 12,630 cubic centimeters was 
evaporated for analysis. The analytical results are given in 
Table 7. 
Table 7. — Soluble salts separated by dialysis from Chester loam colloid 
Constituents 
Quantity 
obtained 
from 4.8 
grams 
colloid 
Part of 
total 
amount 
present 
in colloid 
Constituents 
Quantity 
obtained 
from 4.8 
grams 
colloid 
Part of 
total 
amount 
present 
in colloid 
Si0 2 -.-. 
Grams 
0. 0065 
Trace. 
.0044 
.0044 
.0073 
Per cent 
0. 0039 
MgO 
Grams 
0. 0128 
.0103 
.0051 
.0025 
Per cent 
9.34 
Ti02— . 
K2O-... 
25.2 
AI2O3 
.0033 
.0058 
89.0 
Na20 
100.0 
Fe203 
SO3- -. 
74.4 
CaO .... 
The total soluble material passing through the membrane was 
0.0532 grams or 1.1 per cent of the 4.807 grams of moisture-free 
colloid acted upon. The average concentration of the diffusate, ex- 
clusive of organic matter, nitrates, chlorides, and phosphates, was 
4.21 parts per million. 
