ABSOEPTIOlSr BY SOIL CONSTITUENTS. 7 



It will be noted that the soil fractions containing particles from 

 0.001 to 0.050 millimeter in diameter had absorptive capacities averag- 

 ing about 25 per cent as great as the corresponding colloids. The 

 coarser soil fractions, made up of particles ranging mostly from about 

 0.050 to 2 millimeters, had absorptive values which were for the most 

 part much lower than those of the finer fractions. 



The absorptions per gram of the noncolloidal soil fractions are thus 

 low as compared with those of the colloids. Nevertheless, if we take 

 into account the relative amounts of the different fractions present 

 in the soil it would appear that from 20 to 70 per cent of the total 

 absorptive capacities of the four soils was due to the noncolloidal 

 particles in these soils. If these data could be taken as accurately 

 representing the absorption values of the noncolloidal part of the soil, 

 it would be evident that the colloidal content of the soil could not be 

 estimated with even approximate accuracy from the absorption of 

 the whole soil. 



Although the above data on the absorptive capacities of the non- 

 colloidal soil particles are probably as good as can be obtained by this 

 method of investigation, it is believed that they are not reliable and 

 that they must be considerably higher than the true values. It 

 should be borne in mind that a relatively small contamination with a 

 highly absorptive material, such as the colloidal matter, would appre- 

 ciably enhance the absorptive capacity of the noncolloidal fractions. 

 Furthermore, we have no positive proof that the noncolloidal frac- 

 tions were free from colloids. We merely know that all colloid 

 possible was extracted with the means we employed. It is very pos- 

 sible that by some further treatment still more colloid could have been 

 brought into suspension and separated, just as traces of ammonia or 

 gentle rubbing yielded more colloid when mere agitation with dis- 

 tilled water was ineffective." 



In the process of separating the colloidal and noncolloidal portions 

 of the soil the size of the particles in the two products was carefully 

 checked by microscopical observation. It was evident from this 

 control that the noncolloidal fraction was practically free from par- 

 ticles smaller than 1 micron in diameter and the colloidal matter 

 contained but very few particles larger than 1 micron. However, if 

 our methods of dellocculation were not perfect, aggregates of colloidal 

 matter, similar to clumps of dried ferric hydroxide, or aluminum 

 silicate gels, or partially decomposed organic matter might well have 

 gone into the noncolloidal fraction. Examination of the dry non- 

 colloidal fractions with the petrographic microscope indicated that 

 this actually occurred. The fractions from the different soils which 

 were made up of particles of 0.001 to 0.050 millimeter in diameter 

 contained from 60 to 75 per cent of particles which could readily be 

 identified as crystalline minerals; the rest of the material was 

 without crystalline properties and had the same general appearance 

 as dried inorganic gels or as the dried soil colloids, which we have 

 separated from the soil with a supercentrifuge. 



ESTIMATION OF NONCOLLOIDAL ABSORPTION FROM MINERAL 



PARTICLES. 



Since a complete separation of colloidal from noncolloidal soil 

 material is apparently impossible, owing to the difficulty of dis- 



6 Williams (26) relied largely on boiling to bring about dispersion. However, boiling a fine fraction of 

 Huntington subsoil for 18 hours failed to bring about a state of dispersion whereby all the colloid could 

 be removed. 



