HUMUS AS A GEOGRAPHICAL AGENCY. 263 



the surface of the water. They are constantly associated with 

 very pure waters, especially poor in lime. They arise on very 

 diflferent subsoils, but more particularly on impermeable clay, 

 peaty pastures, or on infra-aquatic marshes, if the nature of 

 the water happens to change. These sphagnum moors often 

 form in the forests on mountain slopes or plateaux, owing to 

 the thick layers of raw humus which accumulate there, and 

 the moorpan which it produces. ^ 



Between these two types there is every stage of transition ; 

 they may even be intermixed, 



Injluence of Humus on the Fertilihj of the Soils. — Humus exerts 

 a different influence, according to whether it forms part of the soil 

 itself or is spread over it as a covering. As a constituent of the 

 soil, its effect on the vegetation varies with the amount contained 

 in the soil. A moderate proportion of humus is a considerable 

 help to the fertility, but an excess of humus is extremely unfavour- 

 able. Humus matter enriches the soil by contributing soluble 

 food-stuffs, and protects it against percolating water which 

 would otherwise dissolve and wash away its salts. From the 

 researches of Petermann, P. P. Deherain, Lawes, Gilbert, and 

 Breai, we may conclude that higher plants can utilise some 

 organic matters supplied to them, but it has not yet been 

 ascertained whether those substances can be directly absorbed, 

 or whether they have to undergo a chemical change brought 

 about by the action of certain substances secreted by the roots. 

 Besides, quite a number of plants have, symbiotic fungi or 

 mycorhiza in their roots, which can supply to their hosts 

 carbon and nitrogen from the organic detritus. 



The formation of carbon dioxide in humus furthers the disinte- 

 gration of the underlying rock and the solution of certain othei-- 

 wise insoluble mineral constituents, and thus helps to increase 

 the fertility of the soil. Humus has also the power of loosening 

 compact soils, thus making them permeable ; whereas with sand, 

 it diminishes permeability. Consequently evaporation is furthered 

 in sand, but checked in clays and compact subsoils. Finally, 

 humus moderates extremes of soil temperature arising from 

 variations in extei-nal conditions. 



The quality and quantity of the dead covering sjjread over 

 the ground in forests depends upon many factors, but chiefly 



1 P. Graebner, Die Heidc Norddeutschlands, Leipzig, 1901. 



