66 CHEMICAL PROPERTIES OF SOIL 



42. Effect of Acidity. 



If the above experiment be repeated with the 

 omission of the calcium carbonate and the addition 

 instead of just sufficient dilute sulphuric acid to render 

 the solution faintly acid, the effect of acidity on nitrifica- 

 tion will be plainly seen. 



43. General Occurrence of Nitrifying Bacteria. 



{a) In Soils. — Several samples of soils should be 

 tested for nitrates according to the method previously 

 described (40 (^), p. 63). The presence of nitrates 

 indicates that the soil is capable of nitrifying. 



{U) In Dust. — Laboratory dust will generally respond 

 to tests for nitrites and nitrates, owing to the general 

 distribution of the nitrifying micro-organisms. If the 

 test is negative or equivocal, the existence of the 

 bacteria should be confirmed by seeding some of the 

 culture solution with a small quantity of the dust, under 

 the conditions detailed above. 



Denitrification. 



In the presence of a large excess of organic matter, nitrates 

 and nitrites are reduced by bacterial action, with the formation of 

 gaseous nitrogen. This is termed denitrification^ although the 

 term is also applied to the formation of nitrogen from ammonia 

 and nitrogenous organic matter. The bacterial activity may be 

 shown as follows : — 



44. Measurement of Denitrification. 



A litre flask is nearly filled with fresh horse dung or 

 manure, together with i or 2 grams of sodium or 

 potassium nitrate. The vessel is then completely 

 filled with water and closed with a rubber stopper 

 fitted with a delivery tube which passes under the 

 end of a long graduated tube filled with caustic-soda 



