PROPERTIES OF THE COLLOIDAL SOIL MATERIAL 
25 
But it has been shown that the quantity removed by normal NH 4 C1 
in the procedure followed in this work is fairly definite and near 
the maximum removable by neutral salt solutions from the whole 
soil (16, 29). It seems apparent, therefore, that at least the magne- 
sium, potassium, and sodium present in the colloidal soil material 
are present in two conditions. Until more is known about the con- 
stitution of the colloid the two kinds or conditions of magnesium, 
etc., may be defined simply by the facts as exchangeable and non- 
exchangeable. 
Although the absolute quantity of any exchangeable base varies 
widely in different colloids, the proportion of one exchangeable base 
to another is fairly uniform. The proportionality between equiva- 
lents of exchangeable bases is shown in Table 14. 
Table 14. — Equivalents of Ca, Mg, K, and Na in 100 equivalents of monovalent 
and divalent oases exchanged 
Kind of colloid 
Ca 
equivalent 
Mg 
equivalent 
K 
equivalent 
Na 
equivalent 
Ca+Mg 
K+Na 
Ca+Mg 
Na 
Fallon -- 
70.4 
69.0 
67.6 
67.0 
61.2 
52.9 
• 
15.5 
26.1 
27.3 
33.0 
28.2 
34.5 
6.7 
3.0 
5.1 
Trace. 
3.5 
4.6 
7.4 
1.9 
6.1 
13.4 
11.6 
50.1 
Norfolk 
Trace. 
7.1 
8.0 
8.5 
6.9 
12.6 
10.9 
In these six colloids, Ca constituted 53 to 70 per cent of the total 
equivalents of bases exchanged, while the absolute quantity of Ca 
exchangeable in the different colloids (Table 13) varied between 0.13 
per cent and 2.13 per cent CaO. Why there should be a tendency 
to uniformity in the proportion of the various exchangeable bases has 
not been explained, although Hissink (2Jf), and Kelley and Brown 
(29) have obtained similar results in work on the whole soil, and 
have suggested that the relative proportions of the bases determine 
the properties of the colloid. According to Hissink, the properties 
of the colloid are influenced by the proportions between exchange- 
able divalent and monovalent bases. Kelley and Brown suggest the 
ratio — ^r — - as a determining factor in properties. These two 
ratios are shown for the different colloids in Table 14. 
In the case of these six colloids the adsorptive properties, heat of 
wetting, swelling, etc., do not correlate well with the ratios proposed 
by Hissink or by Kelley. Apparently, in the case of these widely 
different colloids, the properties are dependent more on other chem- 
ical characteristics, which will be discussed later, than on the pro- 
portions of the exchangeable bases. Considering the low accuracy 
with which the quantity of sodium can be determined in a normal am- 
monium chloride extract of a soil, this ratio obviously may not be 
very accurate in the case of soils containing small quantities of ex- 
changeable sodium. It is, of course, probable that within general 
types of soil the properties of the different colloids may correlate 
well with the proportions of the exchangeable bases, as found by 
Hissink and by Kelley. 
