EDAPHIC OR SOIL FACTORS: CHEMICAL 1 G3 



lack of sensiliveness toward electrolytic effects, it gives an acid 

 reaction. 



Protective Colloids. — Adsorptively unsaturated, highly disperse 

 luiinus is, because of its lack of sensitiveness toward electrolytic effects, 

 an efficient protective colloid. Iron hydroxide, aluminum hydroxide, 

 silicic acid, etc., are much more sensitive toward coagulating electro- 

 lytes. When the particles are protected by the enveloping colloid 

 particles of the less sensitive protecting colloid of the highly dispersed 

 humus, they themselves are no longer sensitive toward electrolytic 

 effects, so that changes of dispersion are retarded and precipitation may 

 be stopped completely, under certain circumstances. Even very small 

 quantities of protective colloids may prevent changes in the dispersity 

 of the disperse phase. Unsaturated humus, for example, keeps various 

 soil colloids (Al(OH):;, re(0H)3, etc.) in soluble, highly dispersed 

 condition. It protects them from precipitation, and renders them 

 highly mobile, susceptible to leaching out. The dark waters of regions 

 of crystalline rocks and of moors owe their color to the jyrotective 

 colloid action of the acid humus. The color is due to leached humus 

 colloids. 



The formation of podsol also depends upon the ready displacement 

 of protected colloids (Fe(OH;,), A1(0H;,) sols), which are washed out 

 and disappear from the upper horizons of the soil, to be flocculated 

 below and to enrich the lovrer horizons where the electrolytic content is 

 greater. 



Ionic Exchange. — The phenomena of exchange in many soils, 

 especially significant in the study of fertilizers, are classed as "ionic 

 (or basic) exchange." The various soil colloids are the carriers of the 

 interchange, where the negative particles show basic exchange while 

 the electropositive particles show an exchange of anions. The 

 exchange of bases in agricultural soils has long been known but is 

 properly interpreted only through colloid chemistry. The gels of 

 aluminum hydroxide and silicic acid, important components of clay, 

 bind the ions of ammonium and potassium more strongly than tiiose of 

 sodium and even calcium at certain concentrations. The latter cdii for 

 this reason be displaced by the former and substituted adsorptively. 

 While ammonium and potassium remain insolubly combined in the soil, 

 the dissolved sodium and calcium ions flow away, and the soil is 

 decalcified by the addition of potassium. Similar phenomena of 

 exchange also occur in natural soils and are of the greatest importance 

 to the development of vegetation. Jenny (1926) explains the leaching 

 of the rendzina soils of the Alps and their transformation into podsol 

 and alpine humus soils, from the viewpoint of base exchange. The H 



