116 JOURNAL OF FORKSTRY 



earths containing but little humus, and there is no evidence to show 

 that the dark-colored, sour humus of coniferous forests contains the 

 same compounds. The reason for the acid reaction of such soil is a 

 matter that has provoked much discussion. Baumann (1909-13) and 

 Gully (191 5) at Munchen have sought to establish that humus is com- 

 posed of substances that are neutral and of colloid nature, and that the 

 acid reaction is caused by the absorption of the positive ion from min- 

 eral salt solutions found in the ground water. This theory has, how- 

 ever, been very strongly contested by Rindell (1911) and Oden (1916), 

 the latter (1912-16) having isolated from peat and other acid soils a 

 tribasic organic acid of high molecular weight. Hesselman observes 

 that as colloids flocculate when a mineral salt is added to the solution, 

 humus lying on soils containing a plentiful supply of water-soluble min- 

 eral salts is less apt to give an acid reaction, for the humus particles 

 become aggregated and granular, permitting the penetration of roots, 

 insects, and earthworms with the consequent aeration of the humus. 

 To illustrate this point, he cites that the water found in the bogs and 

 sump land in the forests of Norrland, where the soil is of basaltic 

 origin, is brown and discolored, while similar water in the limy forest 

 soils of Jamtland is fresh and clear. 



In reviewing former researches on the composition of forest soils, 

 Hesselman reaches the following conclusions : 



1. From earth containing relatively little humus, and to a lesser ex- 

 tent from peat, it has been possible to isolate organic compounds of 

 known composition. In certain cases the isolated and identified com- 

 pounds constitute a considerable proportion of the humus found in the 

 soil. 



2. Many forms of humus, especially those of dark color, are com- 

 posed to a considerable degree of chemical compounds of undetermined 

 characteristics. 



3. The acid reaction of certain humus soils is due to the presence of 

 free organic acids. 



4. In the main, the constituents of humus are colloidal in nature, and 

 the physical structure of the humus soil is therefore greatly influenced 

 by the amount of mineral salts or electrolytes in the soil and ground 

 water. 



Hasselman distinguishes between two types of humus soils — "mild 

 humus" soil, which has been well aerated through the action of worms 

 and insects, and "raw humus," a mixture of decomposed and decom- 

 posing leaves and litter. Alild humus is characteristically found in 

 hardwood forests, and less commonly in coniferous forests on earths 



