CHEMICAL CONSTITUENTS OF THE SOIL 163 



precipitation with acid from ammonia solution, gives the follow- 

 ing results : l 



Per cent. 



Carbon 44.09-63.58 



(usually about 55 per cent. ) 



Hydrogen 3- 2 7~ 5-45 



Nitrogen 3-36- 6.22 



Ash 1.57-15.74 



The substance is probably a mixture and not a single definite 

 compound. 



According to Hilgard and Jaffa, 2 the humus of humid soils 

 (extracted with caustic potash) contains about 5 to 5.5 per cent, 

 of nitrogen, while that of arid soils may contain as much as 18.5 

 per cent, nitrogen. Hilgard believes that the nitrogen content of 

 humus should not fall below 4 per cent., if the soil is to be pro- 

 ductive. 



Attempts have been made to separate definite chemical com- 

 pounds from humus, the success of which is doubtful. Detmer 3 

 extracted humus from the soil with ammonia, and after repeated 

 purifications obtained a compound said to be of the formula, 

 C 20 H ]8 O 9 . It was a black acid substance which reddens litmus, 

 expels carbonic acid from carbonates, and forms salts with lime, 

 silver, iron, ammonia, potassium, etc. These salts are all in- 

 soluble with the exception of salts of the alkalies. Other acids 

 in addition to this one are claimed to have been separated from 

 the soil. There is doubt, however, whether the bodies in question 

 are really definite chemical compounds or more or less impure 

 mixtures. 



There is no evidence that the ammonia-soluble humus of the 

 soil consists entirely of acids, or that it is formed by decomposi- 

 tion in the soil. Various bodies known to be non-acid are found in 



1 Fraps and Hamner, Texas Station Bulletin 129. 



'* Rep. Cal. Exp. Sta., 189, p. 2-4. 



3 Jahresber, f. Agr. Chem., 1870-2, p. 68. 



