18 BULLETIN 1122, U. S. DEPARTMENT OF AGRIOULTUEE. 
noncolloidal absorption might reach 10 to 20 per cent of the whole 
soil absorption. 
Absorption by noncolloidal constituents should, therefore, not 
seriously affect absorptive methods for determining the amount of 
colloids in soils. 
Evidence concerning the nature of the soil colloids is afforded by 
the absorptive capacities of various substances. Minerals ground 
to a state of subdivision probably equal to that of the mixed soil 
colloids, or ''ultra clays," absorbed less than the average ultra clay. 
The average absorption of peat and synthetic inorganic gels, on the 
other hand, was more nearly like that of the ultra clays. 
Since soil colloids appear to be of quite a different nature from 
the noncolloidal particles, and not merely finely comminuted min- 
erals, it is understandable why such a large part of the absorption 
of a soil should be localized in the colloidal matter. 
