208 BACTERIOLOGICAL EXAMINATION OF SOILS 



(iii) Find out if the soil possesses denitrifying power. 



Prepare the following solution : 



Grape sugar * v . . , 1.5 gr. 



Sodium nitrate . ,," . . .3 gr. 



Di-potassium phosphate . * . . i gr. 



Tap water . . . ..,.' 100 c.c. 



Add 10 c.c. to test-tube, and sterilize. Then add 5 c.c. of the 

 soil and water mixture, and incubate at 25 C. 



Test for nitrate every day with diphenylamine and sulphuric 

 acid as in Ex. 74 until the reaction disappears. 



(iv) Test the nitrifying power of the soil. 



Prepare the following solution : 



Ammonium sulphate . . .>. .4 gr. 

 Di-potassium phosphate (K 2 HPO 4 ) .2 gr. 

 Tap water 100 c.c. 



Take 25 c.c. and add i gr. basic magnesium carbonate. 

 To this add 20 c.c. of the soil and water mixture, and incubate 

 at 20 C. 



After six weeks estimate the nitrate present 



(a) by Schloesing's method, or 



(b) by the colorimetric method of Grandval and Lajoux. 

 The latter depends on the principle that picric acid is formed 



when nitrates are treated with an excess of phenol-sulphuric acid, 

 and this compound gives a strong yellow-coloured solution of 

 ammonium picrate when dissolved in weak ammonia. 



An empirical colorimetric scale is first prepared as follows : 

 Dissolve 7.22 gr. of potassium nitrate in i litre of distilled 

 water. Take 50 c.c. of this, and add it to i litre of water; from 

 the latter measure out 100 c.c., and add it to a third litre of 

 distilled water, i c.c. of the last dilution contains .005 gr. of 

 nitrogen. 



