CHAPTER XXXIII 



FIXATION OF PLANT FOOD BY SOILS 



WHEN soluble plant food is applied to the soil, it is as a very 

 general rule changed into insoluble forms by reaction with the soil. 

 Nitrogen in the form of nitrate is an exception to this rule, the 

 only method of changing nitrate nitrogen to the insoluble form 

 being by the growth of some plant which converts it into organic 

 nitrogen, as already explained. 1 



The fixation of bases includes not only the metals, but also the 

 ammonium group, the soluble base taking the place of some other 

 element in an insoluble polysilicate, as illustrated in the following 

 general equation: 



Al x Fe x Mg x Na x Ca (SiO 3 ) x (H 2 O) x + 2 KC1 



= Al x Fe x Mg x Na x K 2 (SiO 3 ) x (H 2 O) X + CaCl 2 . 



This equation typifies the reaction of soluble potassium chlorid 

 with a zeolitic compound, resulting in the fixation of potassium 

 and the liberation of calcium, which passes off in the drainage 

 waters in combination with the acid radicle which formerly carried 

 the potassium. 



Other mineral bases and even ammonium may be fixed in a simi- 

 lar manner, but the ammonium fixation is very temporary, because 

 under usual conditions nitrification proceeds rapidly and the 

 ammonia nitrogen passes into soluble nitrate nitrogen, a fact which 

 is well illustrated by the following data from Rothamsted. 



The ammonium salts consisted of equal parts of the sulfate and 

 chlorid. Warington makes the following comments : 



"At the first running of the drain pipe (after October 25) sufficient time had 

 not elapsed for the complete decomposition of the ammonium salt and the 

 fixation of the ammonia. Some undecomposed salt of ammonium is thus 



1 Even low forms of plant life, as fungi and bacteria, may aid in this process. 



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