72 REACTION: THE INFLUENCE OF COMMUNITY ON HABITAT 



Soil Reactions 



Reactions upon the substratum may be arranged in three major 

 categories, namely: (1) those that give rise to soil or a new layer of 

 it, or contribute materials that will become soil ultimately; (2) those 

 that protect the soil against erosion, trampling, etc.; and (3) those 

 that change the texture, structure, or character of the soil in an appre- 

 ciable degree. A new substratum may be produced by four different 

 types of reaction: (1) by the accumulation of organic bodies them- 

 selves, regularly under conditions that retard or prevent decay, and 

 usually also of excreta to some degree; (2) by the concretion of min- 

 eral matter into shell, marl, or rock as a consequence of physiologic 

 activity; (3) by the weathering of rock into fine soil, chiefly through 

 the excretion of acids; (4) by the resistance that organisms, especially 

 plants, offer to wind and current, resulting in the deposition of particles 

 in transport. 



Soil Formation 



Reaction by Accumulation. As has been indicated, reaction by the 

 accumulation of organic materials becomes possible only in the absence 

 of processes that remove or rapidly decompose them. It is conse- 

 quently at a minimum in open communities where wind and sun 

 are constantly at work, but increases steadily with the height and 

 density of organisms. Accumulation is naturally most pronounced in 

 small water bodies without currents and, by comparison with the 

 atmosphere, provided with a low content of oxygen for producing 

 decomposition. The maximum effect is attained in peat mosses, which 

 possess unique powers of thriving in pools with little oxygen and low 

 pH, optimum conditions for accumulation. Somewhat similar condi- 

 tions as to oxygen deficit occur in reed swamps, and these are likewise 

 sites of rapid accumulation. Marshes are also built up by the accumu- 

 lation of marl or of diatom shells, but these in addition are products 

 of certain chemical activities of the organisms concerned. 



Usually, plants and animals share in the formation of biogenous 

 soils, plants commonly assuming the leading role on land. The chief 

 exception is that of deposits of guano, which are not only relatively 

 rare but are likewise to be regarded only as potential soils. In the 

 initial stages of the hydroscre, the bodies of minute animals and 

 excrement may play a large or even predominant part, as in the case 

 of coprogcnons deposits. Such sediments are more characteristic of 

 large ponds and lakes; they have been extensively studied and classi- 



