What Is Humus? 125 



This bioatl concept ot huinus must l)c tUllcrcntiatcd iroin its nar- 

 row definition, whereby only certain co'nstitutents ot the organic 

 matter of soils, composts, and peats, possessing characteristic prop- 

 erties, such as dark pigment, solubility in alkalies, and insolubility in 

 certain oxidizing agents, are designated as humus. Frequently, the 

 narrow definition does not difi^erentiate between "humus" and "humic 

 acid," another ill-defined term, occasionally used to designate the 

 alkali-soluble or the alcohol-soluble humus constituents. A highly 

 complex terminology has been introduced for distinguishing a num- 

 ber of "humic acids" on the basis of their solubility in certain re- 

 agents. In view of the great confusion that has resulted from this 

 definition, this concept of the "humic acids" can no longer be ac- 

 cepted in classifying humus types and humus constituents. 



By the use of selective adsorption techniques, Forsyth separated 

 four fractions from humus: (1) a fraction that is usually present in 

 small quantities and contains water-soluble organic compounds, such 

 as sugars and amino acids; (2) a fraction containing phenolic glyco- 

 sides or tannins; (3) a polyuronide of the glucuronic acid type con- 

 taining d-glucose, rf-xylose, /-rhamnose, and another sugar; this frac- 

 tion seems to have a composition independent of the soil from which 

 it has been extracted, and may be of bacterial origin; (4) a fraction 

 rich in nitrogen and containing pentose sugars and organic phos- 

 phates. 



Bremner used neutral pyrophosphate for dispersing the humic 

 fractions of the soU humus. The proportions of the fractions ap- 

 peared to depend on the treatment. These results indicate that 

 artifacts are produced by hydrolysis of the humus material, espe- 

 cially when sodium hydroxide solution is used for dispersion. Other 

 side reactions may also be brought about. 



The so-called humins in the soil are not dispersed in caustic soda. 

 They appear to be polymerization products of some of the humus 

 constituents and also contain some of the undecomposed or partially 

 decomposed plant and microbial residues. 



Certain constituents of humus or even certain types of humus are 

 frequently designated as "true humus" or "pure humus," especially 

 in speaking of organic materials in advanced stages of decomposi- 

 tion. This is true of various peats, forest fitter, well-composted plant 

 materials, and the humus of mineral soils. These must not be con- 

 sidered superior forms of humus, but merely types of humus, or 

 designations of certain humus forms. Under different conditions, a 

 variety of different humus types is produced. One can thus speak of 



