174 THE SOIL: INORGANIC MATTER 



is a very different process (Section 130). Absorption, as 

 barely indicated above, is of two kinds, chemical and physical. 

 138. Chemical Absorption. — Some elements are retained 

 in the soil by a chemical reaction which changes the element 

 from a soluble compound into an insoluble compound. This 

 may take place by double decomposition and subsequent 

 precipitation, or by simple precipitation. When, for example, 

 potassium sulphate or potassium carbonate in solution in the 

 soil moisture comes in contact with an insoluble compound 

 containing calcium, such as a silicate or a humate, there is 

 an interchange of bases, the potassium remaining as the 

 silicate and the calcium leaching away as the sulphate 

 or carbonate (bicarbonate would be the soluble form), thus: 



CaAl2(Si 3 8 )2 + K2CO3 + H2O +CO2 = K 2 Al2(Si 3 8 )2 + CaH 2 (C0 3 )2. 



Or take a compound like monocalcium phosphate. It is 

 precipitated by calcium bicarbonate or iron hydroxide as an 

 insoluble phosphate, thus: 



CaH 4 (P0 4 ) 2 + 2CaH 2 (C0 3 )2 = Ca 3 (P0 4 ) 2 + 4H 2 + 4C0 2 



and 



2Fe(OH) 3 + CaH 4 (P0 4 )2+ 2CO2 = 2FeP0 4 + CaH 2 (C0 3 )2+ 4H 2 0. 



In some cases the reaction occurs directly between a solid 

 and a compound in solution. In other cases two substances 

 in solution react and an insoluble precipitate results. In 

 one case it is an exchange of bases, and in this connection 

 it must be noted that other bases than potassium and calcium 

 exchange places in this way. Sodium, magnesium, and 

 ammonium exchange with one another and with potassium 

 and calcium. In the other case it is the formation of an 

 insoluble salt of an acid. 



There is still another case where the base of a salt is ab- 

 sorbed and the acid radicle left behind as an acid. There is 

 no exchange of bases. For instance a hydrated silicate 

 like kaolinite in the presence of a salt like potassium sulphate 

 and water will absorb the potassium, probably to form a 

 potassium silicate, and leave sulphuric acid. This applies 

 to salts like the chlorides and sulphates more particularly. 



