82 THE EEACTIONS OCCURRING IN SOILS. IV. 



(4) Similar compounds produced by the micro-organisms (or 



by the enzymes resulting from them) frequenting the 

 tubercles or nodules on the rootlets of leguminous 

 plants. 



(5) The products of the nitrification of organic compounds 



already present in the soil. This is also the work of 

 micro-organisms. 



(6) The small quantity of nitrogenous matter (in solution or 



suspension) brought in occasionally by flood water, and 

 its subsequent nitrification, if necessary. 



APPENDIX TO CHAPTER IV. 



The chemical changes occurring in a soil take place between 

 substances in highly dilute solution, and it is probable that 

 considerable help might be afforded by the application of 

 Arrhenius's electrolytic dissociation theory of solution to such 

 problems. 



It would be out of place to attempt complete exposition of 

 this theory here, but it may perhaps be advisable to very 

 briefly mention the chief conceptions contained in it. 



When a compound capable of undergoing electrolysis in 

 aqueous solution is dissolved in water, the mere act of solution 

 is accompanied by its partial or complete dissociation into ions, 

 as the metal and acid radical are called. Thus in a solution of 

 common salt there are a number of molecules of NaCl, but if 

 the solution be dilute the greater portion of the salt exists as 

 free ions of sodium and chlorine, the ions consisting of atoms 

 (in this case) each carrying a charge of electricity, positive in 

 the case of the metal, negative in the case of the halogen. 

 The degree of dissociation, i.e., the proportion of the total 

 quantity of salt which thus undergoes " ionisation," increases 

 with the dilution. 



The chemical changes which may ensue w r hen two solutions 

 are mixed are dependent upon the action of the ions upon each 



