270 NATURE AND PROPERTIES OF SOILS 



if a residue is left, it is unchanged except in concentration. 

 Such would be the case in the absorption of certain dyes, of 

 gases and of hydroxides of various kinds, where the molecule 

 is fixed intact. This first form of absorption is by no means 

 as important as the selective absorption of ions. 



Certain compounds, called electrolytes, 1 tend when in solu- 

 tion to ionize or split up into ions. Thus potassium nitrate, 

 a neutral salt, breaks up into K + and NO" 3 ions, the degree 

 of ionization depending on the concentration of the solution. 

 When such a solution is brought into contact with soil, the 

 latter usually, but not always, exerts a greater affinity for the 

 basic ion, leaving an excess of the acid radical in solution. 

 The water present furnishes small amounts of H + and OH' 

 ions, thereby encouraging the formation of KOH, which is 

 absorbed intact, together with the K + and OH" ions. This 

 action, therefore, leaves the H + and NO~ 3 ions preponderant in 

 the solution, which is of necessity acid in reaction due to the 

 hydrogen ion concentration. This selective absorption may be 

 demonstrated with any neutral salt and any neutral absorbent, 

 the resultant extract always being acid due to the selective 

 absorption of the basic ions. 



142. Substitution of bases. 2 — Associated with the selec- 

 tive absorption of bases from solution there is a liberation of 



1 According to the theory of the electrolytic-dissociation or ioniza- 

 tion, many compounds under certain conditions break up into electrically 

 charged portions called ions. Ions may be single atoms or a group of 

 atoms. Many inorganic substances are almost completely ionized. A 

 few organic compounds exhibit marked dissociation but many are not 

 appreciably affected. 



Water dissociates into H+ and OH- ions to the extent of about .00001 

 of a per cent, or 1 part in 10,000,000. An acid yields hydrogen ions 

 and other ions carrying the remainder of the molecules. Alkalies give 

 hydroxyl ions and other ions consisting of the remaining portion of the 

 molecules. The acidity or alkalinity of a solution is determined by its 

 hydrogen-ion concentration. 



2 Van Bemmelen, J. M., Das Absorptionsvermogen der Ackererde; 

 Landw. Vers. Stat., Band 21, Seite 135-191, 1877. 



Sullivan, E. C, The Interaction between Minerals and Water Solu- 

 tions; U. S. Geol. Survey, Bui. 312, 1907. 



Wiegner, G., Zum Basenaustausch in der Ackererde; Jour. Landw., 

 Band 60, Seite 111-150, 197-222. 1912. 



