930 
Journal of Agricultural Research 
Vol. XXVIII, No. & 
heat of wetting data closely parallel the results obtained in a more extensive 
study of soil minerals with respect to adsorption ( l) y indicating that the heat of 
wetting of a soil is almost entirely a function of the colloidal material. 
HEAT OF WETTING AS A BASIS FOR ESTIMATING THE COLLOIDAL 
MATERIAL OF SOILS 
While the heat of wetting of a soil is doubtless due almost entirely to the col¬ 
loidal matter present, it is evident from the data given in Table I that the heats 
of wetting of different soils do not indicate the relative contents of colloidal 
material. This simple relationship does not obtain because of the wide varia¬ 
tion in the heats shown by different colloids. It is possible, however, that the 
approximate colloidal content of a soil could be estimated from the heat of wetting 
of the soil if the heat of wetting of the particular kind of colloid present in the 
soil were known. If a sample of the colloidal material of a soil were extracted 
and the heat of wetting determined, then the formula 
heat of wetting per gram of soil 
heat of wetting per gram of colloid^ * 
should indicate approximately the percentage of colloidal material in the soiL 
This general method of estimating the colloidal contents of soils from the stand¬ 
point of adsorptive capacities has been investigated in this bureau (4, 8, 13). 
Table II shows the percentages of colloidal material present in eight soils, as 
indicated by the ratio of heat of wetting of soil to that of colloid. The data 
for this calculation are given in Table I. Table II also gives, for comparison, 
the percentages of colloid in the same soils indicated by adsorption ratios. 
Table II .—Percentages of colloidal material in soils calculated by heat of wetting! 
ratio and by dye , water , and ammonia adsorption ratios 
Colloidal 
material 
in soil 
Colloidal material in soil in¬ 
dicated by adsorption ratios 
indicated 
by heat 
of wet¬ 
ting ratio 
Dye 
HjO 
NH 3 
Cecil clay loam subsoil.._... 
Per cent 
35.6 
Per cent 
25.4 
Per cent 
31.5 
Per cent 
26. O' 
Chester loam soil..... 
15.3 
11.0 
8.5 
16.7 
Marshall silt loam soil....... 
30.3 
26.3 
27.3 
33.6 
Miami silty clay loam subsoil_ ___ 
54.2 
54.8 
50.1 
54. £ 
Norfolk fine sandy loam subsoil.. .. 
25.0 
23.7 
20.5 
18.6 
Sassafras silt loam subsoil.... 
22.5 
13.2 
18.9 
21.8 
Sharkey clay soiL._..... 
58.8 
56.6 
61.2 
58.8 
Wabash silt loam soil.. 
29.5 
29.4 
29.8 
34.6 
The colloidal contents of the various soils indicated by the heat of wetting 
ratio agree well with the contents shown by one or more of the adsorption ratios. 
The heat of wetting ratio of soil and colloid obviously affords a more accurate 
estimation of the colloid content of a soil than the heat of wetting of the soil 
alone. It is probable, however, that even when heats of both soil and colloid 
are obtained the determination of the quantity of colloid in a soil is in most 
cases not strictly accurate, inasmuch as the heat of wetting ratios may be sub¬ 
ject to the same inaccuracies as were found to affect adsorption ratios. A pre¬ 
vious investigation U) indicated that in estimating the colloidal content of a soil 
from the adsorption ratio of soil and colloid there might be two sources of in¬ 
accuracy, the possibility of the sample of colloid extracted not being representa- 
