22 BULLETIN 1452, U. S. DEPARTMENT OF AGRICULTURE 
determine particle size (page 4). Factors determining the volume 
of floe hav.e been studied so little that it is not known just what the 
comparative volumes of different colloids indicate concerning the na- 
ture of the materials. It seems evident from the data in Table 11, 
however, that this simple determination may be used to distinguish 
roughly different kinds of soil colloids. 
ADSORPTION OF ELECTROLYTES 
The adsorption of vapors by soil colloids has been discussed on 
page 10. According to certain theories, this type of adsorption is 
conditioned primarily by the pores of the material. The adsorption 
of ions from solution, although not unaffected by size of pores or by 
surface exposed, is coming to be considered more as a chemical re- 
action taking place at the surface than as a distinctly physical action. 
A recent article by Michaelis discusses this view of adsorption (38). 
Some data are available concerning the adsorptive capacity of the 
isolated colloidal soil material for electrolytes, but there is no in- 
formation on the adsorption of nonelectrolytes from solution. A 
previous publication shows that some soil colloids adsorb six times 
as much malachite green as others; (20) and Kelley and Brown (29) 
have shown that colloids extracted from three different soils adsorb 
different quantities of NH 4 from an NH 4 C1 solution." 
There is much information in the literature on the capacity of the 
whole soil for absorbing inorganic salts and dyes from water solu- 
tion, although very little is known about the absorption of nonelec- 
trolytes. Since it has been shown that practically all of the adsorp- 
tion of the soil is due to the colloidal material (1, 29), many of the 
facts established for the whole soil may be taken as applying to the 
colloid. 
The following facts concerning the adsorption of eletrolytes by the 
colloidal soil material may be taken as established by investigations 
conducted on the whole soil: (1) Anions, such as CI, N0 3 , S0 4 , etc., 
are adsorbed little, if at all, from salt solutions; (2) the quantity 
of cation adsorbed increases with the concentration of the solution, 
approximately as expressed by Freundlich's well-known formula; 
(3) when a cation is adsorbed an approximately equivalent quantity 
of other cations is released to the solution. 
It is thus apparent from the work on soils that the colloidal soil 
material has a considerable capacity for the polar type of adsorption, 
or, as it is commonly called, for "base exchange." However, as the 
earlier work was conducted on soils which contained undetermined 
quantities of colloidal materials of unknown compositions, it is not 
evident from the earlier work just how much the different colloidal 
soil materials vary in their capacities for exchange adsorption with 
inorganic salts nor just what part of the bases in the colloidal mate- 
rial are capable of exchange. This information is obviously needed 
to characterize the different soil colloids and to throw light on the 
condition of the basic constituents of the material. 
Six soil colloids were therefore tested to determine their compara- 
tive adsorptive capacity for the same inorganic salt solution, and to 
determine what part of each base in the colloid exchanges with the 
base of the salt solution. In base-exchange investigations of soils, 
ammonium chloride solutions have probably been used more than 
