22 
BULLETIN 1193, U. S. DEPARTMENT OF AGRICULTURE. 
In the case of all soils, except the Sharkey (separation by super- 
centrifuge), the quantities of colloid isolated were all that could be 
obtained with the methods employed. The coarse and fine fractions 
of all soils except this one, therefore, contain only what we may term 
" unextractable " colloid. 11 The fine and coarse fractions of the 
Sharkey soil (separation by supercentrifuge) on the other hand con- 
tain a small amount of colloid capable of isolation in addition to the 
unextractable. 
The quantity of colloidal material present in these fine and coarse 
fractions was estimated with the petrographic microscope. The 
details and accuracy of the method have been described by Fry 
in another publication (13). 
Microscopical determinations of the colloidal material present in 
the fine and coarse fractions of Table 4 are reported in Table 5. 
Table 5 also shows the quantities of colloidal material in the fine and 
coarse fractions which the dye, water, and ammonia adsorption ratios 
adsorption per gram of fraction 
adsorption per gram of extracted colloid 
indicate. The adsorption ratios were calculated from data given in 
Table 4. 
Table 5. — Percentages of unextracted colloid in fine and coarse soil fractions. 
Percentage of colloidal material in the 
sample indicated by — 
Description of samples. 
Micro- 
scopical 
count. 
Dye 
adsorp- 
tion 
ratio. 
Water 
adsorp- 
tion 
ratio. 
Ammonia 
adsorp- 
tion 
ratio. 
Cecil clay loam, soil: 
Fine fraction 
Per cent. 
74 
4 
55 
4 
48 
8 
25 
10 
38 
2 
42 
52 
25 
97 
14 
Per cent. 
18.3 
.0 
!o 
30.3 
12.0 
Per cent. 
29.3 
1.1 
18.0 
1.5 
21.4 
9.7 
Per cent. 
46.1 
6.1 
Cecil clay loam, soil (duplicate determination): 
Huntington loam, soil: 
36.3 
23.0 
Huntington loam, subsoil: 
15. 6 22. 2 
31.9 
5.8 
15.9 
3.8 
17.3 
29.9 
23.0 
92.8 
17.7 
9.7 
20.0 
1.5 
20.1 
42.1 
27.5 
78.4 
8.7 
13.7 
Sassafras silt loam, subsoil: 
26.9 
1.4 
Sharkey clay, soil: 
33.1 
Sharkey clay, soil (separation by supercentrifuge): 
47.5 
32.7 
Vega Baja clay loam, soil (separation by supercentrifuge): 
104.3 
23.4 
Both microscopical determinations and adsorption ratios indicate 
that a considerable quantity of colloidal material is left in the frac- 
tions of finer soil residues in spite of the repeated washing and rubbing 
to which this material was subjected. In the coarser soil residues, 
however, very little colloidal material is present as a rule. One 
» It may well be that there is no real distinction between the colloid which was extracted and that which 
was not. 
