Selectocl Bil)liogiap]iy 1S9 



the soil 1h'c(miu\s more alkaline. A similar inercase in alkalinity is 

 obserxed when green manures are applied to flooded soils. In high- 

 moor peat soils, the addition of lime leads to acti\'e mtrifieation; 

 when the nitrates are redueed by denitrifying baeteria, the nitrogen 

 in the soils is rapidly depleted. 



Great losses of nitrogen may take plaee in a humid, hot climate; 

 the rate of loss is increased by liming; bare fallows in rainy season 

 were found to be especially wasteful because of the leaching of 

 nitrates in drainage waters. There is little danger from denitrifica- 

 tion in normal soils. The partial reduction of nitrates to nitrites and 

 ammonia, which is more extensive and carried out by larger num- 

 bers of microorganisms, does not involve any actual losses of nitro- 

 gen. The nitrates may completely disappear from the medium with- 

 out loss of nitrogen. The products formed from the nitrates (nitrites 

 and ammonia) can be further acted upon by nitrifying bacteria; the 

 part of the nitrate assimilated by microorganisms is merely stored 

 away in the soil in an organic form. 



It is often observed that addition of large quantities of undecom- 

 posed organic matter to a soil particularly rich in carbohydrates and 

 poor in nitrogen injures crop growth. This is not due to denitrifica- 

 tion, to which it has often been ascribed, but to the fact that, in the 

 presence of an excess of available organic matter, the fungi, actino- 

 mycetes, and \'arious heterotrophic bacteria synthesize an extensive 

 protoplasm. For this purpose, they assimilate the nitrates and am- 

 monium compounds present in the soil and thus compete with higher 

 plants. 



The conclusion ma)' be reached that the phenomenon of denitri- 

 fication is of no economic significance in well-aerated, not too moist 

 soils, in the presence of moderate amounts of organic matter or 

 nitrate. In soils kept under water for some time, as rice soils, how- 

 ever, addition of nitrates may prove injurious because of the forma- 

 tion of toxic nitrite. It may be added here that there is also no dis- 

 tinct parallelism between plant communities, the geological substrate, 

 and the presence and activities of denitrifying bacteria. 



Selected Bibliography 



1. Barritt, X. W., The liberation of elementary nitrogen by bacteria, Biochem. 

 J., 23:196.5-1972, 1931. 



