HUMUS AS A GEOGRAPHICAL AGENCY. 257 



the mici'O-organisms which air, water, and soil contain in varying 

 amount, or to soluble ferments. Most of these micro-organisms 

 are lower plants, such as bacteria, Mucorinea^, and, to a less 

 extent, yeast. The Mucorinepe have a considerable oxidising 

 power, whereas a great many bacteria, by nature or circum- 

 stances anaerobic, act as reducing agents. In putrefaction, 

 for instance, these bacteria bring about the first stages of 

 reduction. All these organisms live exclusively in the super- 

 ficial layers of the soil, though protected from the light. At 

 a depth of one metre their numbers have greatly decreased ; 

 at two metres they have practically disappeai-ed. 



Quite a number of animals contribute to the eremacausis 

 process of decomposition. These are rhizopods, earth-worms, 

 Anguillulpe, crustacete, snails and slugs, myriapods, many 

 insects, both larval and adult. They woik by tearing up the 

 material, or swallowing and digesting it.^ 



Various agencies are at work, modifying both the rate at 

 which the transformation proceeds and its natui-e. On the one 

 hand, the physical and chemical conditions of the matter itself ; 

 and on the other, the external conditions — air, heat, moisture, 

 light, and chemical agencies. 



Thus, the general process of decomposition is the outcome 

 of a number of simultaneous minor processes. Apart from 

 purely chemical forces, each of these is controlled by the 

 activity of certain micro-organisms and soluble ferments, this 

 activity being in its turn controlled by the combination of 

 the conditions of the matter and of external circumstances. 

 The decomposition thus varies its aspects according to the 

 prevalence of one or another process. But, as a rule, the whole 

 is coL trolled in quantity and quality by the factor which is at 

 a maximum or at a luinimum. 



In nature these factors at once depend upon the climate and 

 the soil, with its living and dead plant covering. We may 

 thus approach the problem from this standpoint, and investigate 

 the influence of each component of the climate, soil, and vege- 

 table carpet upon each of the main processes, especially on the 

 freeing of carbon dioxide, which may be used as a criterion 

 of the intensity of decomposition. We are thus enabled to 



1 C. Keller, Humusbilduiuj unci BodencuUur unter clem einflusstierischer 

 Thatigkclt, 1887. 



