2 BULLETIN 1122, U. S. DEPARTMENT OF AGEIGULTURE. 



had similar absorptive capacities. In 1912 Mitscherlich (16, p. 476) 

 made a general statement that the hygroscopicity of noncolloidal 

 soil particles was almost nothing, but that the colloids in different 

 soils must have different capacities for absorbing water vapor. Evi- 

 dence recently accumulated in this laboratory also seems to show 

 that practically all the absorptive capacity of the soil is located in 

 the colloidal material. For instance, samples of the colloidal mat- 

 ter, or ''ultra clays, "^ extracted from a large number of different 

 soils by use of a high-power centrifuge,* show absorptive capacities 

 for malachite green, ammonia gas, and water vapor which are two 

 to twenty times higher than those of the original soils. 



Moore, Fry, and Middleton, (17) of this bureau, have suggested 

 that the colloidal content of the soil may be measm-ed by comparing 

 the absorptive power of the soil for malachite green or ammonia gas 

 with the absorptive power of the mixed colloids, or ultra clay, ex- 

 tracted from the soil, if soil absorption is due almost entirely to the 

 colloids. While the various ultra clays absorb widely different 

 amounts of malachite green and of ammonia gas, they have a much 

 more constant absorptive capacity for water vapor. It was, there- 

 fore, suggested by Robinson (21) that the colloidal content of the 

 soil might be estimated by dividing its water-absorption value by 

 a factor ^hich is an average of the specific absorptions of a large 

 number of different ultra clays, if the noncolloidal particles do not 

 absorb appreciably. This latter method would obviate the neces- 

 sity of extracting a sample of colloid from the soil. 



Although there is considerable evidence that by far the greater 

 part of the soil's absorptive capacity is due to colloids, it is obviously 

 important to obtain quantitative data on the relative absorptions 

 of the colloidal and noncolloidal constituents, if absorptive methods 

 are to be used for determining the amount of colloids in soils. This 

 paper deals chiefly mth the absorptive capacities of the noncoUoidal 

 soil particles for malachite green, water, and ammonia. The absorption 

 of such particles is compared with the average absorptive capacities 

 of the ultra clays extracted from different soils. Various colloidal 

 mineral powders and synthetic gels were also tested for absorption 

 in order to throw light on the nature of the soil colloids. 



PREVIOUS WORK. 



Several investigators, including Konig and Hasenbaumer {14), Ogg 

 and Hendrick (18), and Atterberg (4) have fractionated soils into va- 

 rious sized particles and determined the absorptive power of the 

 different fractions for substances, such as water, dyes, gases, or 

 salts. The results on the whole show that the absorption by the 

 different sized particles increases with the fineness, although not 

 in a regular manner, and that the finer soil fractions above colloidal 

 size absorb appreciably but much less than the colloidal material. 

 These determinations of the noncolloidal absorption can not be ac- 

 cepted without question, however, since the various soil fractions 

 were probably not entirely free from colloidal material. 



•The tern) "ultra clay" has been suggested to denote the mixture of colloidal constituents present in 

 the soil. 

 * Details of the process are given in the paper by Moore, Fry, and Middleton (17). 



