38 THE SOIL SOLUTION 



disturbing factors. 1 Disturbing factors, however, enter into all 

 cases under field conditions, such for instance as the presence 

 of some uncommon or unusual mineral in appreciable amounts, 

 differences in temperature, surface effects, or extraneous sub- 

 stances. These will be considered later, but another disturbing 

 factor requires immediate consideration. 



In every soil, varying proportions of the soluble mineral con- 

 stituents are present otherwise than as definite mineral species; 

 that is, they are present as solid solutions, or absorbed on the 

 soil grains or perhaps absorbed in some other manner. The con- 

 centration of the liquid solution in contact with a solid solution 

 or complex of absorbent and absorbed material is dependent 

 upon the relative masses of solution and solid. Thus, the con- 

 centration of a solution with respect to phosphoric acid, when 

 brought into contact with so-called basic phosphates of lime or 

 iron, is dependent in a marked way upon the proportion of solu- 

 tion to solid. 2 Consequently it is to be expected that an aqueous 

 extract of a soil will vary in concentration with the proportion 

 of water used; and that with the same proportion of water, 

 different soils or different samples of the same soil will yield 

 different concentrations. 



How far absorbed mineral constituents affect the solubility 

 of the definite minerals in the soil or influence the concentration 

 of the soil solution, it is not possible to predict with any approach 

 to certainty. Those soils which hold the most moisture are 

 generally the best absorbers. Moreover, the soluble mineral 

 constituents of the soil, for instance potassium or phosphoric 

 acid, are absorbed to a very high degree from dilute solutions. 



1 Feldspars certainly, and phosphorites possibly, are mineral compo- 

 nents of the soil; and these substances when ground sufficiently fine have 

 been added to soils with sometimes an increased production of crop. Other 

 minerals, such as leucite, have given similar results. But also apparently 

 pure quartz sand sometimes accomplishes the same results, as for example, 

 in the experiments of Hilgard cited above. It has not been shown, 

 however, that the addition of any of these substances produces an appre- 

 ciable change in the concentration of the soil solution. 



2 The action of water and aqueous solutions upon soil phosphates, by 

 Frank K. Cameron and James M. Bell, Bull. No. 41, Bureau of Soils, 

 U. S. Dept. of Agriculture, 1907. 



