64 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



tropics and sub-tropics true humus soil occurs only in shady forest ; ^ 

 peat soil is very rare, but still it occurs where the cHmate is sufficiently 

 moist ; ^ typical moors are wanting.^ 



In steppe and desert likewise the soil is mostly poor in humus, because 

 plant-remains are scanty, though the soil is sometimes sufficiently moist. 

 Only in most richly clad grass-steppes is humus {' black earth ' occurring 

 in South Russia) formed, especially upon closely deposited loess soil.* 



In cold-temperate lands vegetable mould is the most frequent 

 kind of soil. Only in sunny open localities exposed to wind, such as 

 dunes, do we find the soil with scanty humus. Raw humus is frequent 

 wherever decomposition is restricted from any cause, which according 

 to Ramann ^ may be lack of nutriment, exclusion of air, excess or lack 

 of water, or lowness of temperature. The formation of raw humus is of 

 specially wide distribution in the heaths of maritime Western Europe 

 where the summers are cool, as well as in alpine and arctic situations.^ 



Different species of plants demand very different amounts of humus 

 in the soil. Accordingly Kerner has ranged plants into three groups : 



(i) Plants which can settle upon bare rock, the most barren, sandy, 

 or gravelly surfaces, and other spots where there is not a trace of humus ; 

 their seeds or spores are mainly transported by wind : sub-glacial plants, 

 many tundra-plants, desert-plants, and the like. 



(2) Plants requiring a moderate amount of humus : for instance, 

 Gramineae and Cyperaceae. 



(3) Plants thriving only in rich humus, in the remains of a previous 

 vegetation : many Orchidaceae, species of Pyrola and Lycopodium, Azalea 

 procumbens, Vaccinium uliginosum, a number of other moorland plants, 

 hemisaprophytes, and finally the highly modified holosaprophytes, 

 Monotropa, Neottia, and others. 



We may regard it as certain that there is a correlation between the 

 unusual forms of the last-named plants and their method of nutrition, 

 and thus between their forms and the kind of soil upon which they live ; 

 but beyond this we know nothing of the matter."^ 



6. Saline soil is soil of varied constitution (sandy, clayey, and so 

 forth) that is heavily charged with sodium chloride. It will be treated 

 in detail in Section VII. 



Soils at the bottom of water. Deposits and varieties of soil are 

 formed here. In the sea, fine particles of mud are accumulated by the 

 action of animals and Blue-green Algae, in places where the water is 

 calmest, and they form the foundations of the fertile marshes on the 

 coasts of the North Sea.^ Mud of another kind is raised up in mangrove- 

 swamps. On many coasts and at the mouths of many rivers there arise 

 masses of mud that are rendered deep-black by sulphide of iron ; accord- 

 ing to Beijerinck and Van Delden,^ anaerobic bacteria play a part in the 

 production of iron sulphide. 



^ See Warming, 1892 a; Vahl, 19046. ^ Ule, 190 1. 



' See Friih and Schroter, 1904, p. 143. " Albert, 1907. 



* Ramann, 1893, 1895. ' Kerner, 1863 ; Warming, 1887. 



' For further information on moors and peat, readers should consult the great 

 work by Friih and Schroter (1904), which has already been cited. 



* Wesenberg-Lund and Warming, 1904, Chapter 58. 



' Beijerinck, 1895 ; A. van Delden, 1903; Wesenberg-Lund and Warming, 1904. 



