36 THE SOIL SOLUTION 



alkali silicates, the hydrolysis is practically complete and the 

 silica is nearly all present as colloidal silica and not as silicic 

 acid. Nevertheless at higher concentrations silicates are formed, 

 and there is abundant evidence in nature that the alumino- or 

 ferro-silicates are reacting with bases to form salts, for example 

 such as the micas. 1 If the hydrolysis were quite complete, it 

 would appear to follow that the reaction between water and the 

 silicate is irreversible. In that case it is difficult to see how any 

 silicate mineral could persist in the soil for any length of time, 

 and all soils should soon become sterile wastes composed essen- 

 tially of quartz, kaolin and ferruginous oxides. It has been 

 suggested that the original mineral particles are protected from 

 decomposition by the formation of a coating "gel." That is, 

 that silica, alumina, ferruginous or other materials result from 

 the decomposition of the minerals in a jelly-like form on the sur- 

 face of the soil grains, protecting them from further action of 

 the soil solution. 2 If diffusion can take place through the gel, 

 solution and hydrolysis of the mineral would proceed, although 

 the presence of the gel would probably retard the rate of the 

 reaction. If it be postulated, however, that diffusion through 

 the gel does not take place, the minerals of the soil can have no 

 influence on the composition of the soil solution, which is an 

 unthinkable alternative. The presence of such gels in the soil 

 has frequently been assumed, but satisfactory proof is generally 

 wanting. 



In general, the same kind of considerations developed for 

 orthoclase hold for the other soil minerals. If minerals of this 

 character be pulverized or ground reasonably fine and then be 



1 Van Hise, loc. cit., p. 693. 



2 A gel is a jelly-like substance, apparently continuous, which forms 

 either by the settling from suspension in a liquid of very fine particles 

 which then become aggregated ; or, is formed by the evaporation of a 

 liquid containing fine particles in suspension until the quantity of liquid 

 remaining is just sufficient to serve as a cementation medium holding the 

 suspended particles together in a semi-rigid mass. For an experimental 

 demonstration of the formation of such a gel, see, The effect of water 

 on rock powders, by Allerton S. Cushman, Bull. No. 92, Bureau of 

 Chemistry, U. S. Dept. Agriculture, 1905. 



