ORGANIC COMPOUNDS OF SOILS 91 



per cent, and in soine cases eighteen 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. 



98. Humus. — The term humus is employed to 

 designate what are considered to be the most active 

 principles of the organic compounds. Humus is the 

 animal or vegetable matter of the soil in intermediate 

 forms of decomposition. From different soils, it is ex- 

 tremely varied in composition : in one soil it may 

 have been derived mainly from cellulose, while in an- 

 other it may have been derived from a mixture of cel- 

 lulose, proteid bodies, and other organic compounds. 

 The term humus, unless qualified, is a very indefinite 

 one. The humus given in the analyses of soils is ob- 

 tained 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. 



99. Humification and Humates. — When the ani- 

 mal and vegetable matter incorporated into soils un- 

 dergoes decomposition there is a union of the organic 

 compounds and the base-forming elements of the soil. 

 The decaying organic matter produces organic prod- 

 ucts of an acid nature. The organic acids and the 

 base-forming products unite to form humates or or- 

 ganic salts, which are neutral bodies. This process is 

 humification. 17 



