CHAPTER VI. 



FIXATION. 



202. Fixation, a Chemical Change. When a fer- 

 tilizer is applied to a soil, chemical reaction takes 

 place between the soil and the fertilizer. There is 

 a general tendency for the soluble matter of fertilizers 

 to undergo chemical change and become insoluble. 

 This process is known as fixation. If a solution of 

 potassium chloride be percolated through a column of 

 clay, the filtrate will contain scarcely a trace of potas- 

 sium chloride, but instead calcium and other chlo- 

 rides. . The element potassium of the potassium chlo- 

 ride has been replaced by the element calcium present 

 in the soil. As a result of this change between the 

 two bases, an insoluble compound of potash is formed 

 in the soil. 



203. Fixation Due to Zeolites. It has been shown 

 by experiments, particularly by those of Way and 

 Voechler, 53 that fixation is due mainly to zeolitic sili- 

 cates (See section 62). Sandy soils containing but 

 little clay have only feeble power of fixation. Clay 

 soils when digested with hydrochloric acid to remove 

 the zeolitic silicates, lose their power of fixation. 

 The fixation of potassium chloride and the liberation 

 of calcium chloride may be illustrated by the follow- 

 ing reaction : 



Zeolite. Zeolite. 



H,O + CaCl 2 



etc 



