256 TRANSACTIONS OF ROYAL SCOTTISH ABBORICULTL'RAL SOCIETY. 



XXX. Humus as a Geographical Agency} By Marcel Hardy, 

 University Ccllege, Dundee. 



The excellent translation by M. Ed. Henry - of E. Wollny's 

 book on the decomposition of organic matter and kinds of soils, 

 has again drawn attention to this fundamental work. E. Wollny 

 greatly added to, arranged methodically, and criticised the 

 researches on this subject which were scattered throughout 

 scientific publications ; and from these he draws conclusions 

 which ai*e of interest to the botanist, the agriculturalist, and 

 the forester, no less than to the geographer. Thus it is neces- 

 sary to give a concise account of this important work, not only 

 in order to direct the attention of specialists to the original, 

 but also to give the general public a grasp of the whole question, 

 that they may utilise practically the results. 



Organic remains lie either on the surface of the ground or 

 in its more superficial layers, where they are continually under- 

 going physical and chemical changes, which vary with the 

 nature and state of the material and with the external conditions. 

 From a chemical standpoint these processes may be reduced 

 to two main types : oxidation, when there is a sufficient supply 

 of air to the organic matter, and reduction when otherwise. 

 Slow oxidation or eremacausis is chiefly characterised by a 

 somewha*" abundant formation of volatile products, such as 

 carbon dioxide, ammonia, water, leaving a residue of mineral 

 matter, most of which is available for immediate use to the 

 higher plants. 



On the other hand, if oxygen is not supplied to the decaying 

 matter in sufficient amount, the mode of decomposition becomes 

 a process of reduction or putrefaction. The gaseous products 

 are then much less abundant. They are principally carbon 

 dioxide, marsh-gas, hydrogen, sulphuretted hydrogen, and some 

 others. The solid residue is consequently greater, mainly 

 consisting of nitrogenous compounds, volatile fatty acids, and 

 mineral matter, and is unfit for immediate use by the higher 

 plants. It is dark in colovir and very resistant to any further 

 decomposition changes. In both cases, however, these phenomena 

 are chiefly of a biological nature, and due mainly either to 



' Reprinted from the Scottish Gco(jr(q}hical Jfagazine. 



- E. Wollny, La decomposition des matures onjuniques ct Us formes d'hiivias 

 dans leurs rapports avec Vaffriculturc. Traduction E. Henry, 1902. 



