252 PLANT SOCIOLOGY 



high concentration of hydrogen ions. The organic remains are 

 thoroughly decomposed and frequently well mixed with mineral loam. 

 The organic matter ranges around 30 per cent. 



Mull and Duff. — Romell and Heiberg (1931) have discussed the 

 formation of humus in the northeastern United States and have 

 proposed a classification of the resulting forms. They distinguish two 

 main classes, which are designated "mull" and "duff" and are in 

 turn subdivided. 



Mull is defined as a more or less friable humus layer of crumby or 

 granular structure, with diffuse lower boundary, slightly or not at all 

 matted. Some types of mull are inhabited by earthworms and possess 

 a rich herbaceous vegetation in which spring geophytes such as 

 Dicenira, Dentaria, and Hydrophyllum are a characteristic element. 



Duff, in contrast, is a strongly matted layer of unincorporated 

 humus, distinctly delimited from the mineral soil. It is quite lacking 

 in plants typical of the mull soils. 



Humus Determination. — For some time quantitative determina- 

 tions of humus have been carried out, humus being understood to 

 consist of organic matter in the process of decomposition. Scil 

 samples from different root layers should always be examined sepa- 

 rately. The simplest process for determination of humus is that used 

 for soils with no lime and little clay. About 3 to 10 g. of air-dried soil 

 are heated to white heat in a platinum crucible for 30 to 60 min., 

 and the amount of loss determined. At the same time a sample is 

 dried for 3 hr. at 110°C. The resulting loss of water is calculated for 

 the ignited soil and deducted from the loss due to ignition. 



The customary process in the case of soils which contain lime and 

 carbonates is somewhat more complicated. The organic matter is 

 ignited in an ignition tube, and the carbonic acid evolved is measured 

 and multiplied by the factor 0.471, derived by the assumption that 

 1 g. of humus represents 0.58 g. of C.^ 



Degree of Humification. — The degree of humification of a soil is 

 determined colorimetrically according to Melin and Oden (1917). 



The humus soils are first leached out with HCl, in order to decom- 

 pose the calcium compounds, then with alkah, to dissolve the humus 

 acids. The more or less dark-brown coloration of the solution is 

 compared with commercial Acidum huminicum and given a humifica- 

 tion number according to the relation of its color to that of the com- 



1 V. Anderson (1927, p. 84) has given the following method: 5 g. of soil, dried 

 at 100°C., is treated with 5 per cent HCl, till all carbonates are removed, then 

 washed and dried. The amount lost by ignition is then determined and reckoned 

 as humus; the residue gives approximately the clay fraction (admixture of clay); 



