BULLETIiq- 1122, U. S. DEPARTMENT OF AGRICULTUEE. 
PREPARATION OF SOIL FRACTIONS. 
Our aim was to secure only two fractions of the soil, colloid and 
noncolloid; but we found the separation was facilitated in some cases 
by dividing the noncolloidal fraction into two groups — a coarser 
fraction containing the gravel and sands and a finer fraction consist- 
ing of particles ranging approximately from 0.001 to 0.050 millimeter 
in diameter.^ The colloidal fraction, as previously noted, was made 
up of particles less than 0.001 millimeter in diameter. 
Fractions were obtained from four soils. Details of the procedure 
were as follows: 
Samples of 50 to 100 grams of soil were used, depending on the 
probable amount of colloidal matter present. The samples were 
first agitated with distilled water containing ammonia (about 1 
part of ammonium hydroxide to 3,000 parts HjO). The suspension 
was allowed to settle for several hours and the supernatant liquid 
containing the finer soil particles was then decanted and whirled for 
45 minutes in a bottle centrifuge having a diameter of 22 inches and 
a speed of 850 revolutions per minute. The material remaining in 
suspension was poured into the jar containing the colloid fraction. 
The residues from the centrifuge were combined with the residues 
left after decantation and were rubbed gently with a rubber pestle, 
having just enough water present to make a paste. About a liter 
of water was then added and the subsidence and centrifuging re- 
peated. After 40 to 60 such treatments the residues yielded only 
the merest trace of colloid and it was evident that no more colloid 
was to be extracted by these methods. The colloidal matter was 
concentrated to a thin paste by drawing off the water through Pas- 
teur-Chamberland filters, using suction. The paste of colloidal 
matter, as well as the coarse residues, was then air-dried. 
ABSORPTION BY THE FRACTIONS. 
The amounts of the different fractions separated in this manner 
from four soils of the humid region, together with the absorption 
per gram of these fractions for malachite green, water vapor, and 
ammonia gas are sho^^^l in Table 1. The quantities of the different 
fractions obtained are expressed as percentages of the whole soil 
Table 1. — Absorptive capacities of different soil fractions. 
Part of 
whole 
soil. 
Absorption per gram 
of material. 
Description of soil fractions. 
Mala- 
chite Water, 
green. \ 
Am- 
monia. 
Huntinsrton loam, soil: 
CoUoid . 
Perct. 
10.3 
21.9 
64.0 
13.3 
19.4 
63.3 
14.4 
20.6 
61.9 
42.4 
53.1 
Gram. 
0.0949 
.0288 
Gram. 
0. 2221 
.0476 
Gram. 
0.0270 
Fine fraction . . . 
.0098 
Coarse fraction . . . 
.0114 ! .0215 
.0062 
Huntington loam, subsoil: 
Colloid 
.0918 
.2996 
.0226 
Fine fraction 
.0143 
.0053 
.1398 
.0222 
.0053 
.3720 
.0665 
.0292 
.2732 
.0546 
.0041 
.3072 
.0072 
Coarse fraction 
.0031 
Sassafras silt loam, subsoU: 
CoUoid 
.0293 
.0079 
.0004 
Sharkey clay, soil: 
Colloid 
.0516 
.0657 ! .0772 1 .0172 
a Very little material above 0. 050 millimeter present. 
8 This group corresponds approximately to the combined silt and clay groups of the Bureau of Soils 
classification, except that in this classification no lower Umit is given for the clay {10). 
