36 The Soils of the British Isles 



7. Salty soils containing a considerable percentage 

 of sodium cUoride (common salt) or certain other soluble 

 mineral salts, are very unfavourable for the majority of 

 plants. They are characteristically inhabited by a special 

 ecological class of plants — the halophytes. In this country 

 they occur mainly in the mud fiats of tidal estuaries which 

 are covered by the high spring tides. Sand in immediate 

 proximity to the sea, and spray-washed cliff s, are also often 

 impregnated with salt and bear a halophilous vegetation. 



8. Bock, i.e. hard rock not covered by loose earth, 

 can hardly be called a soil in the ordinary sense. Where 

 it forms horizontal or comparatively slightly inclined 

 surface, the products of weathering, largely through the 

 agency of lichens, mosses and algfe, quickly cover it 

 with soil in most cases, but vertical or very steep rock 

 faces which cannot hold soil are inhabited only by 

 these plants, which can adhere to the bare rock face, and 

 by an occasional higher plant which has succeeded in 

 colonising a small thin patch of soil formed by a lichen 

 or alga. Eock clefts in which soil accumulates often 

 have a special vegetation of their own. The vegetation 

 of rocks is influenced very much by the chemical and 

 physical constitution of the rock, and also by such factors 

 as available moisture, exposure, etc. 



The talus of cliffs and rooks and "mountain top 

 detritus " (p. 302) form other classes of substratum allied 



work of Baumanu [Mitteil. d. bayrischen Moorkulturanstalt, 1909 and 

 1910) has shown that humus is to be regarded as a colloid complex, and 

 that its properties are determined by the special laws governing the 

 behaviour of colloids to other bodies. The old conceptions of "humin" 

 and "humic acid" do not, apparently, correspond with real chemical 

 bodies, and the most important character of humus in relation to other 

 chemical substances in the soil and to plants is whether it is " absorp- 

 tively saturated" (mild humus) or "unsaturated" (acid humus). On 

 these lines we may expect a real increase in our knowledge of the all- 

 important relations of humus to plant life, a subject which has hitherto 

 been very obscure. 



