FIXATION 193 



the iron and aluminum compounds of soils, forming in- 

 soluble phosphates. Experiments show that in a loam 

 soil from 2000 to 8000 pounds per acre of phosphoric 

 acid may undergo fixation. Drainage waters contain 

 only traces of phosphates. At the Rothamsted Experi- 

 ment Station the plots receiving an annual dressing of 

 phosphates for fifty years contained 83 per cent of the 

 surplus fertilizer, half of which was in available forms 

 soluble in one per cent citric acid. 95 



219. Fixation of Potash. The potash compounds of 

 fertilizers readily undergo fixation, the sodium and cal- 

 cium of the soil being replaced by the potassium of the 

 fertilizer. Drainage waters contain larger amounts of 

 sodium than of potassium compounds, due to greater in- 

 solubility of the potash of the soil. Fixation of potash 

 occurs mainly in the surface soil, where it is held in 

 forms insoluble in water, a portion being soluble in 

 dilute acids. 



220. Nitrates cannot undergo Fixation. - - Nitrogen in 

 the form of nitrates or nitrites cannot undergo fixation. 

 This is because all of the ordinary forms of nitrates are 

 soluble. If potassium nitrate be added to a soil, calcium 

 or sodium nitrate will be obtained as the soluble com- 

 pound. The potassium undergoes fixation, but the ni- 

 trate radical does not. Chlorides also are incapable of 

 undergoing fixation, because all of the chlorides found 

 in soils are soluble. 



