68 THE REACTIONS OCCURRING IN SOILS. IV. 



which the process occurs may be slightly alkaline or 

 neutral, but acidity or much alkalinity prevents nitri- 

 fication. Calcium carbonate acts very efficiently as 

 a basic material, the carbon dioxide being easily 

 expelled or perhaps used for the assimilation of car- 

 bon by the organisms. Sodium bicarbonate NaHCO H 

 is also suitable; but sodium carbonate Na. 2 CO 8 hin- 

 ders or entirely prevents nitrification. 



(c) Suitable temperature. Nitrification probably ceases 



about the freezing point of water and it is stopped 

 at a temperature of about 50 or 55" C. It is most 

 active about 36 C. 



(d) Sufficient moisture. The action is suspended if a soil 



be air-dried. 



(e) Absence of strong light. Bright light, e.g., sunlight, 



suspends the action of and eventually destroys the 

 organisms. 



(f) Presence of sufficient oxygen. Since the process is one 



of oxidation it is obvious that free supplies of oxygen 

 should be provided. Hence it cannot proceed in a 

 waterlogged soil and is greatly increased by stirring 

 the soil. :;: 



(g) While the work of the nitrous organism proceeds ob- 



viously best in the presence of considerable amounts 

 of ammonium salts, the nitric organism is rendered 

 inactive by more than small amounts of such com- 

 pounds and only produces nitrates when the ammo- 

 nium compounds have, to a great extent, been 

 destroyed by the nitrous organism. 



(h) The action of the nitric organism is hindered by the 

 presence of alkalis (sodium carbonate) or by calcium 

 chloride, but is favoured by bicarbonates and sulphates. 



(i) Nitrification can be stopped by the use of certain anti- 

 septics, e.g., chloroform or carbon disulphide vapour. 

 It has even been suggested to employ the latter in 

 autumn to lessen loss of nitrates by drainage from a 

 soil. I 



* Deherain. Compt. Rend. 11(5, 1091. 

 t Deherain, Ann. Agron, 1895, 501 ; J.C.S. 189C, ahst. ii., 329. 



