THE CHEMICAL COMPOSITION OF SOILS 1 03 



accumulated animal and vegetable remains in various 

 stages of decay. The organic matter of soils is a 

 mechanical mixture of a large number of organic com- 

 pounds, many of which have not yet been studied. 



112. Classification of the Organic Compounds. Vari- 

 ous attempts have been made to classify the organic 

 compounds of the soil. An old classification by 

 Miilder 29 was humic, ulmic, crenic, and approcrenic 

 acids. None of these contain more than 4 per cent 

 nitrogen, while organic matter with 8 to 10 per cent 

 and in some cases 18 per cent 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) those 

 of medium nitrogen content, 5 to 10 per cent; and (3) 

 those of high nitrogen content, 1 1 to 20 per cent. 



113. Humus. The term ' humus ' is employed to 

 designate the most active of the organic compounds ; 

 it is the animal and vegetable matter of the soil in 

 intermediate forms of decomposition. From different 

 soils, it is extremely varied in composition ; in one soil 

 it may have been derived mainly from cellulose, while 

 in another from a mixture of cellulose, proteid bodies, 

 and other organic compounds. The term 'humus,' unless 

 qualified, is a very indefinite one. Humus is obtained 



