36 
BULLETIN 1193, U. S. DEPARTMENT OF AGRICULTURE. 
moreover, were arrived at by two independent methods of determi- 
nation. 
Some idea of the quantities of colloids that are present in different 
kinds of soils may be gained from the data given in Table 2. The 
quantities indicated by the water-adsorption ratios in this table are 
probably the most accurate; since data given in Table 10 show that 
when no correction is applied to the ratio 
adsorption per gram soil 
adsorption per gram colloid 
for alteration in the adsorptive capacity of the colloid produced by 
extraction, the results obtained by water adsorption are likely to be 
the most accurate. The values given in Table 2 are calculated on 
the basis of a sample of the colloidal material which was graded by 
the supercentrifuge; that is, the dispersed particles appeared to be 
below 0.3 micron in diameter. Uncorrected water adsorption ratios 
calculated on the basis of a sample of colloidal material graded in 
this way are little different as a rule from uncorrected ratios calcu- 
lated on a sample of colloidal material graded with one micron as 
the largest diameter of the colloidal particles. This is apparent 
from the data given in Table 11. Hence the water adsorption ratios 
given in Table 2 probably indicate fairly closely as a rule the true 
percentages of colloidal material as denned in the first part of this 
paper. 
Table 11. — Percentages of colloidal material in soils shown by adsorption ratios based 
on colloidal material below 0.3 micron and below 1 micron. 1 
Adsorption ratio X 100 based 
on 0.3 micron colloidal ma- 
terial, substance adsorbed 
being- 
Adsorption ratio X 100 based 
on 1 micron eolloidal ma- 
terial, substance adsorbed 
being- 
Per- 
centage 
colloidal 
material 
less than 
Soil. 
Dye. 
H z O. 
NH 3 . 
Dye. 
H 2 0. 
NH,. 
1 micron 
present 
by ex- 
traction 
and 
micro- 
scopical 
determi- 
nation. 
10 
14 
12 
13 
59 
10 
23 
20 
19 
65 
15 
33 
30 
22 
70 
9 17 
26 24 
20 26 
20 21 
53 
21 
39 
37 
25 
66 
18 
26 
Huntington loam, subsoil 
Sassafras silt loam, subsoil 
Sharkey clay, soil 
25 
24 
65 
1 Values taken from Tables 2 and 10. 
It will be seen that the uncorrected water and ammonia adsorption 
ratios based on the two samples of colloidal material agree quite well 
in the case of all soils except the Cecil. 19 
19 In other words, the adsorptive capacities for water and ammonia of the extracted, supercentrifuge and 
1-micron graded colloids were practically the same. (Compare also data on adsorptive capacities of colloids 
in Tables 1 and 3 with similar data in Table 4). The 1-micron colloid as a rule is a little lower in adsorptive 
capacity for water and ammonia and markedly lower in adsorptive capacity for dye than the supercen- 
trifuge colloid. This may well be due to the inclusion in the 1-micron colloid of a certain amount of com- 
mon soil mineral particles ranging between 0.3 micron and 1 micron in diameter which would probably have 
a distinctly smaller adsorptive capacity than the gel-like colloidal material, especially for malachite green. 
A previous investigation showed that prepared common soil mineral particles below 1 micron in size were 
low in adsorptive capacity, particularly for dye, as compared with the average colloidal material graded 
by the supercentrifuge. (Anderson, 1.) It is possible, however, that the difference in adsorptive capaci- 
ties of the two samples of colloidal material is due simply to different degrees of alteration in adsorptive 
capacity produced by slightly different methods of preparing the samples. Unfortunately it was not 
possible to obtain data on the possible extent to which the colloids reported in Table 1 may have been 
altered by extraction. 
