90 REACTION: THE INFLUENCE OF COMMUNITY ON HABITAT 



exudates by roots is slender, if not altogether wanting. Moreover, 

 the presence of deleterious substances in soils is to be explained chiefly 

 by deficient aeration and resulting anaerobic conditions, as has been 

 emphasized elsewhere (Clements, 1921, b). Even animal excreta ac- 

 cumulate only exceptionally to the extent of becoming deleterious. 



Reaction and the Soil Profile. The profile of a soil is characterized 

 by the evident differences shown by a cross-section from the surface to 

 country rock or other undifferentiated matrix. Normally, a soil pro- 

 file consists of three horizons or layers, more or less clearly distin- 

 guished by texture, color, structure, and so forth. The uppermost or 

 A horizon is marked by darker color and lighter texture, the middle or 

 B horizon by relatively brighter color and heavier texture. The 

 C horizon is usually set off by a color difference also, owing to the 

 fact that it is the parent rock or sediment, little affected by weather- 

 ing. In a large number of soils, both A and B exhibit a further but 

 slighter differentiation into subhorizons, known as Ai and A2, Bi 

 and B2. 



In terms of reaction, these three layers show both quantitative and 

 qualitative differences. As the uppermost, A is the level of major 

 reaction, B of minor reaction, and C of little or no effect from the biotic 

 community. The A horizon, as a result of direct contact with the 

 plant cover, is influenced, in some degree, by nearly every one of the 

 reactions described. However, the addition and incorporation of or- 

 ganic material are the characteristic features of it, evoking the chief 

 distinctions between it and the B horizon. In the latter, the outstand- 

 ing process is the concentration of fine particles and of mineral salts, 

 often leading to the formation of hardpan. Because of its depth, 

 horizon C is beyond the reach of practically all reactions, and hence 

 it is not really a component of the soil in the strict sense. Thus, from 

 the standpoint of reaction, A may well be termed the level of accumu- 

 lation, B of concentration, and C of inaction. However, it is necessary 

 to recognize that all three horizons regularly shade into one another, 

 with respect both to processes and to visible criteria. 



The soil profile further possesses certain important relations to air 

 reactions and consequently to climate, indirectly as well as directly. 

 In fact, from the ecological viewpoint, a soil may well be regarded as 

 a mass of parent rock, more or less consolidated, in which the external 

 portion has been differentiated by climate, biome, and topography. 

 Of these, the influence of the plant matrix is most direct and imme- 

 diate; that of climate is indirect through its control of climax and 

 direct by virtue of rainfall and temperature in particular. Topog- 

 raphy may exert the most striking effects in connection with erosion 



