882 
Journal of Agricultural Research voi. xxiv, No ic 
observed under these conditions; and consequently the distinction 
between lumps of aggregated colloids and soil minerals would reduce 
itself purely to a question of personal judgment based on appearance. 
Colloidal aggregates might bear very close resemblances to corroded 
mineral particles and ferruginous or rutilated quartz, for example. 
Even with a completely equipped petrographic microscope careful 
cognizance must be taken of the pseudo-similarities of the optics as 
well as the resemblances of some crystalline and amorphous materials,, 
such as the similarity between colloids subjected to strain and the 
matted structure of such minerals as chlorite. 
Table II .—Microscopic estimation of unexiracted colloids in fine and coarse residues of 
soils 
Sample. 
Quantity 
of fractions 
separated 
mechani¬ 
cally, as 
part of 
whole soil. 
Quantity 
of unex- 
tracted col¬ 
loid in resi¬ 
dues deter¬ 
mined mi¬ 
croscopi¬ 
cally, as 
part of 
residue. 
Unex¬ 
tracted 
colloid in 
combined 
fine and 
coarse resi¬ 
dues, as 
part of 
whole soil. 
Unex¬ 
tracted 
colloid, as- 
part of 
total CO- 
loidal nuK 
terialin 
soil. 
Cecil clay loam, soil: 
Colloid extracted. 
Per cent. 
9.4 
7.6 
83.8 
9.4 
10. 4 
78-3 
10.3 
21. 9 
64. 0 
13-3 
19.4 
63-3 
14.4 
20. 6 
61. 9 
42.4 
53 - I 
31 - 5 
38.9 
31.8 
30. 5 
38.6 
31.8 
Per cent. 
Per cent. 
Per cent. 
Residue finer particles. 
74 
4 
Residue coarser particles. 
9.0 
48.9 
Cecil clay loam, soil: 
Colloid extracted. 
Residue finer particles. 
55 
4 
Residue coarse particles. 
8.9 
48.6 
Huntington loam, soil: 
Colloid extracted. 
Residue finer particles. 
00 00 
Residue coarse particles. 
IS-6 
60. 2 
Huntington loam, subsoil: 
Colloid extracted. 
Residue finer particles. 
25 
10 
Residue coarse particles. 
II. 2 
45 - r 
Sassafras silt loam, subsoil: 
Colloid extracted. 
Residue finer particles. 
38 
2 
Residue coarse particles. 
9. I 
38.7 
Sharkey clay, soil: 
Colloid extracted. 
Residue fine and coarse particles com¬ 
bined . 
42 
22.3 
34 - 5 
Sharkey clay, soil: 
Colloid extracted. 
Residue finer particles. 
52 
25 
Residue coarse particles. 
00 
47. 2 
Vega Baja clay loam, soil: 
Colloid extracted. 
Residue fine particles. 
97 
14 
Residue coarse particles. 
41.9 
57-9 
The calculations given in column 4 of Table II show that from 8.9 
to 41.9 per cent of the whole soil was composed of col oidal aggregates 
not extractable by the methods used. It should be mentioned, however, 
that the Vega Baja soil which showed a total colloidal content of 72.4 
