CHAPTER VI 



FIXATION 



214. 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 nitrate will contain scarcely a trace of potas- 

 sium chloride, but instead calcium and other chlorides. 

 The element potassium of the potassium chloride has 

 been replaced by the element calcium present in the 

 soil, and as a result of this exchange of base elements 

 an insoluble compound of potash is formed. Indepen- 

 dent of chemical action, a small amount of soluble salts 

 are absorbed physically by soils and retained by molec- 

 ular force. Absorption is a physical property of soils, 

 while fixation is due to a chemical change. 



215. Fixation due to Zeolites. It has been shown by 

 experiments, particularly those of Way and Voechler, 53 

 that fixation is due mainly to zeolitic silicates. Sandy 

 soils containing but little clay have only feeble power 

 of fixation. Clay soils when digested with hydrochloric 



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