FIXATION 163 



water-soluble salts, particularly when the reaction is 

 one of union rather than replacement. Then the plant 

 is free to render soluble its own food in quantities 

 and at times desired. 



Farm manures and commercial fertilizers alike un- 

 dergo the process of fixation and, in studying ferti- 

 lizers, their Action upon the soil and the products of 

 fixation are matters of prime importance. 



Soil water obtained by leaching soils is an exceed- 

 ingly dilute solution of various mineral salts and organ- 

 ic compounds. Through rock disintegration, mineral 

 matter is rendered soluble, but the process of fix- 

 ation prevents accumulation in the soil solution of 

 compounds of such elements as potassium and phos- 

 phorus. As a result of disintegration and fixation, 

 numerous chemical changes take place iri the soil, and 

 the soil solution is an important factor in bringing about 

 these reactions. Many of the phenomena which have 

 been studied in connection with solutions in physical 

 chemistry, take place in the soil. Diffusion, absorption, 

 osmotic pressure and ionization, 88 disassociation of the 

 molecule in solution, all occur in soils and are due 

 largely to the physical and chemical action of the soil 

 solution. The soil solution from different soils varies 

 with the composition and disintegration of the soil ; 

 in the same soil at different times variations in the 

 composition of the soil solution are noticeable. The soil 

 solution is more important as an agent in bringing 

 about chemical and physical changes in the soil than 

 as a storehouse of plant food. 



