Studies in Soil Physics. 35 



ing much clay, there are observed phenomena quite analogous 

 to those of colloidal solutions or of the so-called "true" suspen- 

 sions of fine particles in water. * Chief among these is the floc- 

 culation or loosening of structure produced on clay soils by added 

 lime. These phenomena are very complex and are still but little 

 understood. It is probable that they are related to surface 

 forces residing on the solid-liquid surface J between the soil 

 particles and the soil water. They are observed only in 

 clay soils but it is probable that they are not confined thereto. 

 There is, indeed, every reason to believe that they are duplicated 

 in kind in all soils at the solid-liquid surface, although in most 

 cases they are entirely masked by the forces at the liquid-gas 

 surface. In these flocculation effects we have, then, a factor 

 affecting structure in all cases but varying in its importance with 

 variations in mechanical composition. In soils composed largely 

 of the coarser particles the interspaces are all relatively large, 

 water-films are set up in all of them and the water-film forces have 

 their maximum effect on structure. In the same soil the amount 

 of solid-liquid surface (in relation to the masses of soil particles 

 and of water) is relatively small and the flocculation effects de- 

 pendent upon forces there resident are correspondingly unim- 

 portant. As the soil becomes finer in mechanical composition 

 the interspaces become smaller and smaller (in individual size, 

 not total amount), in more and more cases the water-films on 

 opposite sides of the interspace come so close together that they 

 fuse entirely, until in the heaviest clay soils the larger proportion 

 of the interspaces are probably too small to contain active water- 

 films at all. The liquid-gas surfaces are nearly absent and their 

 surface tension forces are relatively unimportant. On the other 

 hand, the relative area of this solid-liquid surface has tremend- 

 ously increased, the forces resident on it are much greater in 

 total amount, and the flocculation which they cause is in control 



*Such systems show a whole series of characteristic properties and reactions of which the 

 most striking is the flocculation, or gathering of the particles into aggregates, which 

 follows the addition of small quantities of most electrolytes. On these matters see 

 Ostwald, — Grundriss der KoUoidchemie, Dresden (1909). For a short summary see 



Free,— Jour. Frank. Inst.. 169: 421-438; 170: 46-57 (1910). 



JAs distinguished from the liquid-gas (water-air) surface. The main items of evidence in 

 favor of the existence of important forces on solid-liquid stirfaces are furnished by the 

 phenomena of adsorption, or the concentration of liquids, gases or dissolved substances 

 on the surface of solid particles present in the system. These phenomena seem to have 

 a close relation to those of flocculation. The information concerning adsorption in 



soils is summarized by Patten and Waggaman, — Bull. 52. Bureau of Soils. IT. S. Dept. 

 of Agriculture (1908). On the relation of adsorption and flocculation see Patten, — 

 Trans. Amer. Electrochem. Soc, 9: 277-290 (1906): 10: 67-74 (1906);i.ll :J87-407 

 (1907); Freundlich,— Zeits. Phys. Chem.. 73: 385-423 (1910). 



