144 SOIL BACTERIOLOGY 



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: 



Where X = Number of milligrams of N as NO 3 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 NO 3 in i c.c. oi standard solution as 

 diluted for reading. 



Standard Nitrate SolutionDissolve 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 250 or 500 c.c, of aqueous soil extract in an 

 8oo-c.c. Kjeldahl flask 5 c.c. of a 50 per cent, sodium 

 hydroxid solution ; partially close the mouth of the flask with 

 a small funnel to prevent spattering and boil for half an 

 hour. 



