vi.] DETERMINATION OF SOLUBLE HUMUS 187 



as the nature of the plant, the texture of the soil, 

 the supply of water, and other necessaries of nutrition. 



No method akin to solution in dilute citric acid 

 has yet been devised for determining what proportion 

 of the nitrogen reserves in the soil is likely to be avail- 

 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 influenced 

 by & number of conditions, such as temperature, degree 

 of moisture and aeration of the soil, the mechanical 

 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. 51). 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 150 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, 



