144 ^OIL BACTEEIOLOGY 



and in order to clarify add about 2 grams of calcium oxid. 

 To secure a fair sample, mix by rubbing in a mortar or by 

 shaking in a wide-mouthed bottle. Filter through folded 

 filter-paper until clear. Take a convenient volume, for 

 example, 25 c.c, and determine the nitrate present. This 

 is equal to 5 grams of soil. Use the colorimeter to compare 

 the standard solution with the unknown. 

 Formula for calculating results: 



100 S _ K 



X = — . -- , d . -r- . M 

 W A U 



Where X = Number of milligrams of N as NO3 per 100 grams dry soil. 

 W = Weight of dry soil. 

 S == Cubic centimeters of water added to W. 

 A == Aliquot taken for evaporation. . 

 d = Number of cubic centimeters to which A was diluted. 

 K == Reading on scale of standard solution. 

 U = Reading on scale of unknown solution. 

 M = Milligrams of N as NO3 in i c.c. ol standard solution as 

 diluted for reading. 



Standard Nitrate Solution. — ^Dissolve 0.722 gram of pure 

 dry potassium nitrate in 1000 c.c. of water. Of this strong 

 solution dilute 10 c.c. to 100 c.c, and from this take 10 c.c. 

 for a standard. Evaporate to dryness in a porcelain dish 

 on a water-bath and treat as described above. Make up 

 volume to 100 c.c. Each cubic centimeter of this standard 

 is equal to o.ooi milligram of N as nitrate, or 100 c.c. of 

 this standard is equal to o.i milligram of nitrogen. 



(5) Nitrates (Reduction): 



I. Add to 2 £50 or <oo c.c, of aqueous soil extract in an 

 800-c.c. Kjeldahl flask 5 c.c. of a 50 per cent, sodium 

 hydroxid solution; partially dose the mouth of the flask with 

 a smaU funnel to prevent spattering and boil for half an 

 hour. 



