8 The Phili'p'pine Journal of Science 1914 



soils and that microorganisms have the exclusive pov^er of 

 forming ammonia from nitrogenous organic matter, and Remy^* 

 has shown that the ammonification pouter of SQJI in a general 

 way measures the amounts of nitrogen available for the growing 

 crop. 



Nitrifying bacteria. — Nitrifying bacteria, under proper con- 

 ditions, convert ammonia into nitrites and nitrates. 



Denitrifying bacteria. — Denitrifying bacteria consist of those 

 which merely reduce nitrates to nitrites and progressively to 

 ammonia, which does not necessarily involve loss of nitrogen, 

 and those that convert the nitrates and nitrites into gaseous 

 nitrogen which is thereby lost from the soil.^^ The organisms 

 causing the liberation of soil nitrate as gaseous nitrogen have 

 been found to be present in fresh horse dung and also on the 

 surface of old straw.^" When the horse dung has become rotted, 

 denitrifying bacteria seem to have disappeared, giving way to 

 nitrifying organisms. 



Nitrogen fixation. — The processes of ammonification, nitri- 

 fication, and denitrification have simply to do with the existing 

 supply of nitrogen in the soil with reference to its retention 

 or liberation from the soil, while the fixation of atmospheric 

 nitrogen by soil microorganisms is one of vital importance in 

 increasing the nitrogen content of the soil.^^ For centuries it 

 has been recognized that long-cultivated soils when left to 

 themselves for a number of years partially recover their lost 

 fertility. It was later recognized that such fertility accumula- 

 tion is largely due to increase in the content of nitrogen. Later 

 it was explained by Boussingault'* as being mostly the work 

 of microorganisms. The discovery by Hellriegel and Wilfrath 

 of the fixation of nitrogen in the root tubercles of legumes, 

 together with the studies of Berthelot and other investigators, 

 definitely established that increase in nitrogen in the soil is 

 due to fixation by bacteria that may be living within the soil 

 itself (nonsymbiotic) or those living within the root tubercles 

 of legumes (symbiotic). 



"Centralbl. f. Bakt., etc., 2 Abt. (1902), 8, 667; Fiinfte Internat. Kong. 

 Angeiv. Chem. (Behn) (1903), 793. 



" Loc. cit., Hilgard, Soils, 146 ; Vorhees and Lipman, A Review of Inves- 

 tigations in Soil Bacteriology. Bull. U. S. Dept. Agr., Off. Exp. Sta. 

 (1907), 194, 71. 



"Loc. cit, Hilgard, Soils. 148; Bull. Bur. Agr. Intell. (1913), 4, 1528-9. 



"Bull. U. S. Dept. Agr., Off. Exp. Sta. (1907), 194, 76. 



"76id., 76. 



