vi.] DETERMINATION OF SOLUBLE HUMUS 165 



able. The conversion of the nitrogenous matter of 

 the soil into soluble nitrates, in which form nitrogen 

 enters the plant, is a biological process which is influ- 

 enced by a number of conditions, such as temperature, 

 degree of moisture and aeration of the soil, the mechani- 

 cal treatment it receives, all impossible to predict. 



Some idea of the condition of the organic matter 

 and the readiness with which it is likely to change, may 

 be obtained by a determination of the humus soluble in 

 dilute ammonia and the percentage of nitrogen in this 

 humus, or again by a study of the ratio of carbon to 

 nitrogen in the organic matter as previously indicated 

 (p. 46). In order to determine the soluble humus, 20 

 grams of the soil are digested with enough 1 per cent, 

 hydrochloric acid to dissolve all the calcium carbonate, 

 thrown upon a filter, washed with a little more of the 

 hydrochloric acid and then with water until neutral. 

 The soil is then washed off the filter into a bottle with a 

 4 per cent, solution of ammonia and shaken for twenty- 

 four hours, after which the bottle is left to stand until 

 the bulk of the inorganic matter of the soil has settled. 

 150 c.c. are pipetted off and evaporated to dryness over 

 the water-bath, weighed and ignited, a deduction being 

 made of the inorganic matter remaining after ignition. 



To determine the nitrogen content of the humus a 

 second 1 50 c.c. are placed in a Kjeldahl flask. Two or 

 three grams of magnesia are added and the whole 

 evaporated to dryness to get rid of the ammonia in the 

 solution, after which the contents of the flask are 

 digested with sulphuric acid in the usual manner. 



Valuable as these determinations may become in 

 judging a soil, a sufficient body of data do not as yet 

 exist to enable them to be interpreted with precision. 



In the analysis of a soil, without doubt the most 

 important figure is the proportion of calcium carbonate, 



