SOIL FORMATION. 81 



Kinds of reactions. — Since two or more major reactions regularly occur in a 

 primary sere, and in many secondary ones also, it is impossible to classify 

 them on a strictly developmental basis. It is most convenient to group them 

 in accordance with their nature and effect, an arrangement which is likewise 

 fimdamental because it emphasizes the directive influence of reactions. While 

 it is helpful to distinguish them as primary and secondary with respect to a 

 particular sere, such a general distinction is not feasible, owing to the fact that 

 a reaction may be primary in one sere and secondary in another, or in different 

 periods of the same sere. The main division may well be made upon the seat 

 of the reaction, which results in the two groups, (1) soil reactions and (2) air 

 reactions. The soil as a fixed substratum is much more affected by plants, and 

 the soil reactions are correspondingly much more numerous than those in the 

 air. They do not permit of any precise subdivision, since soil factors are so 

 intimately related. It is helpful in permitting a comprehensive view to group 

 them in accordance with the factor directly affected. This results in the fol- 

 lowing arrangement: (I) soil formation and structure, (2) water-content, (3) 

 solutes, (4) soil organisms. The subdivision of air reactions is less satisfactory, 

 but the following will serve our present purpose: (1) light; (2) other factors 

 (humidity, etc.) ; (3) aerial organisms. 



In the following discussion of reactions in detail, an endeavor is made to 

 indicate the cause of each reaction, to trace its effect upon the habitat, and to 

 relate this to the development of the succession. Some of the recent quantita- 

 tive studies of reactions are also indicated. The exact study of this most 

 diflacult portion of the field of succession has barely begun, and the many gaps 

 in our knowledge are consequently not surprising. 



SOIL FORMATION. 



Manner. — ^The reactions of plants upon the substratum fall into two cate- 

 gories, viz, (1) those which produce a new substratum or soil and (2) those 

 which affect and usually change the texture of the soU. 



A new substratmn may be formed in four essentially different ways : (1) by 

 the accumulation of the plant bodies themselves, usually under conc^tions 

 which retard or prevent decay; (2) by the concretion of mineral matters into 

 rock or marl through the activity of water plants; (3) by the weathering of 

 rock into fine soU by the excretion of acids; (4) by the resistance which plant 

 bodies offer to moving air and water, resulting in the deposition of particles in 

 transport. Plants modify the structure of the soil primarily as a result of the 

 death and decay of plant bodies and parts, a reaction differing from the accu- 

 mulation of plant remains into a new soil, only in the degree of accimiulation 

 and of decay. They also affect soil-texture in consequence of the penetration 

 of their roots and the accompanying Uberation of carbon dioxid, but this effect 

 hardly seems a significant one. The most striking reaction upon soil-structure 

 occurs in the formation of a rocky layer termed "ortstein" from the typical 

 "bleisand" of many heaths. Another group of reactions affect the soil by 

 preventing weathering, or the erosion of the surface by wind and water. 



(1) Reaction by accumulating plant bodies or parts. — The complete decom- 

 position of plants in contact with air prevents any considerable heaping-up 

 of plant remains in ordinary habitats. Accumulation in quantity can occur 

 in consequence only imder water, where oxidation is largely or completely 



