88 NITEOGEN SILICA 



nitrogen is then easily calculated from the amount of sulphuric acid 

 which has been neutralised by the ammonia carried over in the steam. 



The method gives the nitrogen existing as organic matter and as 

 ammonia in the soil ; it also probably gives some, at least, of that exist- 

 ing as nitrates, but if nitrates are likely to be present in any consider- 

 able quantity, about a gramme of salicylic acid or benzoic acid should 

 be added with the sulphuric acid, when nitro-compounds of these acids 

 will be formed, readily capable of reduction to amides and finally to 

 ammonia. 



An example will show the method of calculation. 10 '868 grammes 

 of soil were treated as described. On distillation into 50 cc. of deci- 

 normal sulphuric acid there were required afterwards 27*0 cc. of 

 decinormal sodium hydrate solution. Hence 50- 27'0 = 23 cc. of the 

 acid must have been neutralised by the ammonia. 1 cc. of decinor- 

 mal sulphuric acid contains "0049 gramme of real H,S0 4 , capable 

 of neutralising -0017 gramme of ammonia, corresponding to -0014 

 gramme of nitrogen. Hence the 23 cc. of sulphuric acid correspond 

 to 23 x *0014: gramme of nitrogen, and this was present in 10'868 

 grammes of soil. 



The percentage of nitrogen is therefore 



asx-oouxioo 



10-868 



Direct estimation of humus. A method originally proposed by 

 Grandeau l is based upon the solubility of humus in ammonia. About 

 10 grammes of the soil are treated with dilute hydrochloric acid (con- 

 taining about 1 per cent pure acid) until all lime and magnesia are re- 

 moved. Then the acid is washed out by water and the soil residue is 

 treated with about 15 cc. of ammonia (strong ammonia diluted with 

 about its own volume of water) for three or four hours ; the whole is 

 then filtered and the residue washed once or twice with dilute ammonia. 

 The dark-coloured solution is then evaporated in a weighed platinum 

 dish, dried at 100 and weighed. The contents of the dish are then 

 ignited and the organic matter thus oxidised. The loss of weight on 

 ignition is the amount of humus. The phosphoric acid in the residue 

 may be determined in the usual way, and, according to Grandeau, 

 affords a good measure of the available phosphoric acid of the soil. 



Determination of the silica, alumina, ferric oxide and total potash. 

 3 or 4 grammes of the finely divided soil are weighed out accurately 

 into a conical flask, 20 cc. of strong hydrochloric acid are added, and 

 the whole boiled on a sand tray for ten minutes, a watch glass being 

 placed on the neck of the flask to prevent loss by spirting ; the flask 

 is then placed on the steam bath and digested at 100 C. for forty-eight 

 hours. The liquid is then diluted and filtered, the residue washed with 

 hot water, dried, ignited and weighed. 



This is reported as silica, though in many cases it doubtless still 

 contains some refractory silicates. The filtrate and washings are eva- 

 porated to complete dryness in a platinum or porcelain basin on the 

 water bath, the residue heated over a flame until thoroughly dry and 

 1 Vide Analyse des Matieres Agrieoles, 1897, Vol. I, 141. 



