ABSORPTION BY SOIL CONSTITUENTS. 13 



Although the mineral content assumed in this case is about the 

 average of a large number of soils, most soils do not contain this 

 amount of muscovite. The mica average of 5 per cent is due to the 

 very high mica content of a few important soil types. Usually the 

 muscovite content is below 2 per cent. If the hypothetical soil just 

 considered had contained 2 per cent of muscovite, only approximately 

 1 per cent of the total absorption would have been due to the crystal- 

 line minerals. If, however, the muscovite content had been as high 

 as 37 per cent, which has been reported for one of our most highly mi- 

 caceous soils, the mineral absorption values might have been as high 

 as 7 to 20 per cent of the total soil absorption, the dye absorption 

 being most affected by the mica. 



In the case of a peat soil, the total mineral matter is small in 

 amount, so the absorption of the minerals would be negligible. The 

 organic matter, jas indicated before, may be considered essentially 

 colloidal. 



EVIDENCE CONCERNING THE NATURE OF SOIL COLLOIDS AFFORDED 

 BY ABSORPTION DATA. 



It is difficult to understand why so small a part of the absorptive 

 capacity of the whole soil should reside in the noncoUoidal constitu- 

 ents unless the coUoidal constituents differ from the noncoUoidal in 

 other respects than mere degree of subdivision. The organic matter 

 of soils, which is classified as colloidal, is obviously of a different 

 nature from the mineral particles; but in many soils organic matter 

 forms only a small part of the total colloidal content. Other investi- 

 gations in this bureau indicate that the larger part of the inorganic 

 colloidal matter in soils is different from the noncoUoidal mineral par- 

 ticles in being similar to gels. This view that the soil colloids are 

 mostly of a gel-like nature is also held by Van Bemmelen {5) , Rohland 

 (22), Konig (13), Ehrenberg (9), Wiegner (25), and others. _ 



The fact that the mineral powders just above the colloidal size 

 had such low absorptions as compared with the ultra clays is in 

 harmony with the view that the two classes of material are of a 

 different nature. However, since most of the colloidal particles 

 are so small that their size can not be accurately determined micro- 

 scopically, the possibilities would still remain, so far as microscopical 

 observation is concerned, that the colloidal and noncoUoidal parts 

 of the soil are of the same nature and that the differences in absorp- 

 tian are merely a function of the size of the particles, the colloidal 

 particles being exceedingly fine as compared with the noncoUoidal 

 particles. 



It therefore became desirable to determine the absorptions of 

 mineral powders ground as finely as possible and to compare the 

 absorptions of these powders with those of the coarser powders and 

 ultra clays. 



ABSORPTIVE CAPACITIES OF MINERAL PARTICLES BELOW 1 MICRON IN DIAMETER. 



Six minerals which showed but little tendency to flocculate when 

 suspended in water were selected for grinding to colloidal dimensions. 

 The minerals were ground dry in a steel ball mill — muscovite for 10 

 days and the other minerals for about 3 days. The powders from the 

 bail mill were then shaken with distilled water and the greater part 



