CHEMICAL ANALYSIS OF SOILS 313 



procedure that it is worth while to consider the principles in- 

 volved.^ About 10 grams of dry soil are placed in a Kjeldahl 

 flask with about 30 c.c. of strong sulfuric acid and 0.7 gram 

 of mercuric oxide or its equivalent in metallic mercury. The 

 mixture is boiled vigorously until the solution is clear. The 

 flask is then removed from the flame and, while hot, potassium 

 permanganate is added in small quantities to complete the 

 oxidation until, after shaking, the liquid remains a green or 

 purple color. The nitrogen of the soil, no matter what has 

 been its combination, is now in the form of ammonium sulfate 

 [(NH4)2S04], the mercury acting as a catalytic agent and 

 the permanganate as an oxidizer. 



After cooling, the contents of the flask are diluted with 

 about 200 c.c. of water, zinc dust or a few pieces of granu- 

 lated zinc are added to prevent bumping and 25 e.e. of 

 potassium sulfid are poured in with shaking. Next a sodium 

 hydroxide solution, sufficient in amount to neutralize the acid, 

 is carefully poured down the side of the flask. The flask is 

 then connected with a condenser and the contents cautiously 

 mixed by shaking. The ammonia set free by the alkali is dis- 

 tilled over into a standard acid, the excess acid being titrated 

 with a standard alkali, using a suitable indicator. When the 

 amount of standard acid neutralized is known, the amount of 

 nitrogen, which has passed over in the form of ammonia, may 

 be calculated and expressed as a percentage, based on the 

 original dry sample of soil. (See Fig. 53.) 



* The method described above is the Kjeldahl method. See Official 

 and Tentative Methods of Analysis of the Assoc, Official Agr, Chemists, 

 p. 314, 1920. 



This method does not determine the nitrogen in the nitrate form. 

 If this is desired a modified procedure must "be followed. As the 

 nitrate nitrogen in most soil is low compared to the nitrogen in other 

 combinations, the objection just made to the regular Kjeldahl method 

 is not serious. 



Snyder, E» 8., Determination of Total Nitrogen in Soils Containing 

 Bather La/rge Amounts of Nitrates; Soil Sci., Vol. VI, No. 6, pp. 487- 

 490, 1918. 



