14 



pose the tendency inaugurated by Ramann to classify the humus 

 alone, which is only one constituent of the biological unit. 

 Miiller's two main types or groups are retained. They are char- 

 acterized morphologically, as the Scandinavian school has al- 

 ways done, contrary to the tendencies in Germany, and some 

 types with unincorporated humus are included in the mull 

 group. Specific types listed are crumb mull, grain m., twin m., 

 detritus m., root duff, leaf d., greasy d., and fibrous d. This 

 list is not supposed to cover every variation possible, but is just 

 an enumeration of conditions found to occur within the region 

 studied sufficiently regularly and characteristically enough de- 

 veloped to warrant their being recognized as types. The crumb 

 mull is the classical prototype of the mull group, inhabited by 

 large earth worms. The types greasy and fibrous duff have been 

 taken over from the Danish forester Juncker. 



The distribution of the types within the region is discussed. 

 Ground-water conditions seem to be a particularly important 

 factor locally. Some plants are listed as indicators of mull and 

 of duff. The most valuable hardwood species of the region seem 

 to be among the mull preferring plants. 



Data are presented on nitrification, pH and lime content of 

 the different types. Contrary to European experience, nitrifi- 

 cation was found in the laboratory within all types, even pro- 

 nounced duffs, and down to a pH of 2.9 which was close to the 

 lowest pH value encountered in any sample, whether nitrifying 

 or not. Still, a great difference was found between the types, 

 the mull samples being practically all nitrifying, whereas the 

 majority of samples of pronounced duffs did not nitrify. Stor- 

 age tests yielded surprisingly high values for root duff and other 

 intermediate forms, as compared to the crumb mull, while inoc- 

 ulation tests gave results agreeing better with the expectations 

 from previous experience and with the indications furnished by 

 the vegetation. The puzzling results of the storage tests are as- 

 cribed to a "sampling effect" to be discussed in a later paper. 



The main data are given in concentrated table form on eight 

 pages. A mimeographed Appendix of 29 pages, distributed by 

 the authors, gives descriptions of 17 chosen localities including 

 vegetation and soil notes, Bouyoucos analyses, etc. 



L. G. Rom ell 



