V. THE ANALYSIS AND COMPOSITION OF SOILS 85 



about 150 or even to low redness for a minute or two, again cooled 

 and weighed. The loss is equal to the organic matter and combined 

 water. The object of the treatment x with ammonium carbonate is to 

 restore any calcium carbonate (which would be decomposed by the 

 heating into carbon dioxide, which would escape, and quick-lime) back 

 into its original form. Otherwise the loss on ignition would include 

 the carbon dioxide thus expelled. 



Care must be taken that the temperature be as low as is consistent 

 with the oxidation of the carbon, or loss, due to volatilisation of alkaline 

 chlorides, may occur. 



Determination of nitrogen. The nitrogen in a soil may exist in 

 three states of combination : 



1. As nitrates. 



2. As ammonium compounds. 



3. As organic compounds of complex but little known constitution, 

 associated with the " humus ". 



The nitrogen existing at any given time in a soil in the state of 

 nitric acid or ammonia is usually very small in amount and in most 

 cases does not require separate determination. 



Total nitrogen. Several methods are in use for the determination 

 of nitrogen in soil, but, in recent years, the well-known Kjeldahl pro- 

 cess for the determination of nitrogen in organic substances gener- 

 ally, has, with various modifications in detail, been adopted in soil 

 analysis. 



Broadly speaking, the method is based upon the behaviour of strong 

 sulphuric acid towards organic matter ; by continued heating with 

 strong acid the carbonaceous matter is oxidised into carbon dioxide 

 and water, the nitrogen which it contains being converted into am- 

 monia, which, in the presence of the large excess of acid, remains be- 

 hind as ammonium sulphate. A large proportion of the sulphuric acid 

 is reduced with the evolution of sulphur dioxide. It is found that the 

 oxidation of the organic matter is facilitated by the addition of small 

 quantities of certain metallic salts, e.g., of mercury or copper. They 

 apparently act as carriers of oxygen from the acid to the organic 

 matter. 



The following are the details of the method which the author 

 uses : 



10 to 15 grammes of the air-dried "fine soil" (i.e., which has been 

 crushed and passed a 1 millimetre sieve) are introduced into a 16 oz. 

 spherical flask and treated with 20 to 25 cc. of pure sulphuric acid,- 

 heated for some time over the bare flame, care being taken that the 

 soil is completely wetted by the acid and that no dry places are left 

 in the lower part of the flask. When the frothing has ceased (usually 



1 Several errors in the determination of organic matter are not adequately corrected 

 for by this treatment ; magnesia left from magnesium carbonate on ignition only 

 very slowly takes up carbon dioxide again ; so, too, lime present as calcium hum ate 

 will be converted into carbonate. These errors and others inherent in the method 

 are small and can usually be ignored. 



2 Which must be free from ammonia ; the re-distilled acid of commerce is usually 

 pure enough. 



