THE COLLOIDAL MATTER OF THE SOIL 135 



As may naturally be inferred the influence of the colloidal 

 matter on soil conditions, especially as related to plants, is 

 extremely important. This influence is exerted in a number 

 of ways, modifying the physical and chemical as well as the 

 biological activities within the soil. 



One important attribute imparted to soil by colloid develop- 

 ment is high absorptive power. This power extends not only 

 to condensation of gases, but also to water and to materials 

 in solution. The water of condensation on dry soil particles 

 when exposed to a saturated atmosphere is largely determined 

 by the colloidal content. The absorptive capacity for mate- 

 rials in solution affects both bases and acid radicals, although 

 the former is usually more strongly influenced. This has a 

 very important bearing on the economic use of fertilizers and 

 on the loss of plant nutrients from the soil. Colloidal mate- 

 rial may also function as a catalyst^ in that it may force 

 certain reactions that otherwise might proceed but slowly. 



Since an adjustment is always taking place between the 

 soil colloidal material and the soil solution as far as soluble 

 constituents are concerned, it is readily seen that not only 

 the concentration but also the composition of the latter is at 

 least partially a function of the colloidal matter of the soil. 

 Colloidal matter, moreover, does not exert the same absorptive 

 power for all material but is capable of a certain amount of 

 selection. For example, if ammonium sulfate is added to a 

 soil, the ammonia is strongly taken up, which tends to release 

 the sulfate ion. The continuous use of such a fertilizer on a 

 soil low in active bases will ultimately result in an acid con- 

 dition. This is another example of the practical importance 

 of the soil colloidal matter. 



The movement of air and water in the soil is strongly in- 

 fluenced by colloidal materials. In a fine soil in which the 

 individual pore spaces are normally very minute the develop- 



^ A catalyst is a material capable of hastening or retarding a chemical 

 reaction, the catalytic agent itself not entering into the reaction. 



