HUMIFICATION AND HUMATKS 87 



ganic compounds of the soil, but those which have 

 been described are without doubt mixtures of various 

 bodies, and not distinct compounds. An old classifi- 

 cation by Miilder 29 was humic, ulmic, crenic, and ap- 

 procrenic acids. This classification does not include 

 any nitrogenous matter containing more than 4 per 

 cent, nitrogen, while organic matter with 8 to 10 per 

 cent, and in some cases 18 per cent, of nitrogen is 

 quite frequently met with ; hence this classification is 

 incomplete as it includes only a part of the organic 

 compounds of the soil. For practical purposes the 

 organic compounds of soils may be divided into three 

 classes : (i) Those of low nitrogen content, i to 4 per 

 cent, of nitrogen ; (2) medium nitrogen content, 5 to 

 10 per cent. ; (3) high nitrogen content, n to 20 per 

 cent. 



103. Humus. The term humus is employed to 

 designate the most active parts of the organic com- 

 pounds. Humus is the animal and vegetable matter 

 of the soil in intermediate forms of decomposition. 

 From different soils, it is extremely varied in compo- 

 sition ; in one soil it may have been derived mainly 

 from cellulose, while in another it may have been de- 

 rived from a mixture of cellulose, proteid bodies, and 

 other organic compounds. The term humus, unless 

 qualified, is a very indefinite one. The humus as given 

 in the analyses of soils is obtained by extracting the 

 soil with a dilute alkali as ammonium hydroxide, 

 after treating the soil with a dilute acid to remove the 

 lime which renders the humus insoluble. 



104. Humification and Humates. When the ani- 



