ESTIMATION OF COLLOIDAL MATERIAL IN SOILS. 37 
The water adsorption ratios indicate that the 32 soils reported in 
Table 2 contain from 6 to 61 per cent of colloidal material, the 6 
different loam soils averaging 20 per cent of colloid. In the case of 
many soils the quantity of colloidal material present exceeds the per- 
centage of "clay" shown by the old system of mechanical analysis, 
as practiced in this bureau. 20 Thirty of the soils contain an average 
of 19 per cent clay by mechanical analysis and 23 per cent of colloid 
by the water adsorption ratio. The widest difference occurs in the 
case of the Sharkey soil, with 36 per cent of clay as compared with 
61 per cent of colloidal material. The clay fraction is supposed to 
contain all particles below 5 microns in diameter, but it has been 
pointed out by Davis (7) and it is evident from the results reported 
in this paper that aggregates of colloidal material above clay size are 
fairly abundant in the soil. 
The data in this paper show that, on the whole, soils contain only 
a slightly larger amount of the finest material than has heretofore 
been reported as "clay" (particles below 0.005 millimeter in diam- 
eter). The old clay fraction of a soil then, except in the case of 
certain very heavy soils, gave, as a rule, a rough idea of the quantity 
of finest material in the soil. It is apparent, however, from the data 
reported here that the clay fraction did not contain all the finest 
material and that most of the material which was included in this 
fraction is much finer than had been thought and is moreover pre- 
ponderantly of a quite different character from the mineral grains 
present in the fractions of larger particles. 
The old view that the soil consists chiefly of mineral grains of vary- 
ing sizes coated possibly with a thin film of colloidal material ob- 
viously needs to be modified. If 6 to 70 per cent of the soil consists 
of colloidal material of a gel-like nature, this material evidently is 
not present as a film only. 
It seems evident that the large quantity of colloidal material in 
the soil must be as much a determining factor of the chemical be- 
havior as of the physical character of a soil. And since the colloidal 
material consists chiefly of the more insoluble products formed from 
the decomposition of the soil minerals, it should constitute one of the 
more important criteria for the classification of soils. 
A knowledge of the total amount of colloidal material present in 
soils is only a preliminary step toward studying the colloids and 
investigating further the chemical and physical processes going on in 
the soil. Information concerning the different kinds and conditions 
of the colloidal materials present in a given soil is also needed. Evi- 
dence of this need is given in the preceding pages. The mixed colloids 
extracted from one soil, for instance, were seven times as adsorptive 
for malachite green and three times as adsorptive for ammonia as the 
colloids extracted from another soil. Furthermore, the mixed collo- 
idal material in a given soil appears to be present in more than one 
physical condition, judging from the varying ease with which parts of 
it are brought into suspension. 
*> Mechanical analyses of 30 of the soils were kindly made by A. A. Riley and J. B. Spencer according to 
the methods described in Bulletin 8 of the Bureau of Soils (Fletcher, 9). 
