ABSORPTION BY SOIL CONSTITUENTS. 17 



determining the colloidal content of a soil obviously would not be 

 affected appreciably by the noncolloidal constituents. One of the 

 fundamental objections against absorptive methods of estimating 

 the colloids is thus removed. It is not intended to imply, however, 

 that even relative colloidal contents of different soils are indicated by 

 the mere absorptive capacities of the soils for such substances as 

 malachite green or ammonia; since the colloids from different soils 

 absorb widely varying amounts of these substances. 



The almost complete localization of absorption in the colloidal 

 matter probably is due prunarily to the colloidal material being of a 

 quite different nature from the larger soil particles. While the non- 

 colloidal part of the soil is made up of unaltered soil minerals, the 

 colloidal material is doubtless made up chiefly of inorganic gels, the 

 insoluble decomposition product of soil minerals, together with 

 amorphous organic matter. 



The results on the noncolloidal absorption of soils are evidently at 

 variance with some of the older, but still prevalent, concepts regard- 

 ing soil absorption. It has been generally recognized that the organic 

 matter constitutes an especially absorptive part of the soil, and the old 

 view was that the rest of the absorption was due to mineral particles 

 of all degrees of fineness, the finer the particles the more absorptive 

 the soil. Absorption was thus regarded as proportional to the sur- 

 face exposed, and the surface, except for the organic matter, was con- 

 sidered as being almost entirely that of mineral particles. The more 

 highly absorptive soils were supposed to contain some especially ab- 

 sorptive minerals called zeolites. It is now well recognized that 

 zeolites, if present at all in soils, can be there only in very small 

 amounts. 



It is now coming to be realized that the highly absorptive inorganic 

 material in soils is not the definite crystalline zeolites, but amor- 

 phous material such as ferric hydroxide, aluminum hydroxide, silicic 

 acid, and aluminum silicate gels. The data here presented tend to 

 confirm the more modern view of soil absorption, and gives for the 

 first time a quantitative estimate of the amount of absorption due 

 to the colloidal material and the amount due to the noncolloidal 

 particles. 



SUMMARY. 



It was impossible to determine directly the amount of absorption 

 in soils due to colloids and noncolloids from a fractionation of the 

 soil into these two classes of material, since we could not separate 

 all the colloidal matter from the finer mineral particles. 



Since the noncolloidal part of soils is made up of minerals of 

 different degrees of fineness, it should be possible to calculate the 

 magnitude of the noncolloidal absorption in the soil from the ab- 

 sorptive capacities of minerals powdered to definite sizes. 



Each mineral has a characteristic absorptive capacity for different 

 substances. Several of the most abundant minerals in soils, such 

 as quartz and the feldspars, absorb practically no malachite green, 

 water or ammonia. 



It was calculated from the absorption of mineral powders and the 

 mineralogical compositions of soils that as a rule less than 5 per 

 cent of the total absorption of the soil is due to the noncolloidal 

 part. In the case of the most highly micaceous soils, however, the 



