SOIL TYPES 



255 



Soil Climax.— Since the weathering processes of rocks as well as 

 the decomposition of humus go on at different rates under different 

 climatic conditions, and since the vegetation of each climatic region 

 reacts differently upon soil formation, the resulting mature soil must 

 also bear the stamp of climatic influences. All soils, in fact, have a 

 tendency under certain climatic conditions to develop into a certain 

 type, the soil climax. The soil types of the earth are, in the main, 

 climatically conditioned. The geological substrata influence soil 

 development only to a small degree as compared to the effects of 

 climatic factors. The soils of a climatically uniform region thus 



Fig. 126. — Soil development and plant succession in temperate brown-earth regions 

 (upper) and in humid northern bleached earth regions (lower). Both show pioneer 

 stages with sparse vegetation on soil with high mineral and low humus content (a, 

 a') : intermediate stages with increasing vegetation on soil with increasing humus 

 content (6, V): and optimal stages for both vegetation and soil (c, c'), the latter with 

 moderate humus content. On brown earth the optimal stage is the final climatic 

 climax for both soil and plant communities (c), but on the bleached earth too much 

 acid humus accumulates with a final but degenerate plant community {d') . (After 

 Ludi.) 



present either stages in development of the climax or the ultimate, 

 climatically conditioned climax soil itself. This soil climax may 

 develop from the most varying rock material, provided that the dura- 

 tion of development is not too short. 



Between the vegetational climax — the climatically conditioned 

 climax plant community — and the soil climax there is an unmistakable 

 analogy. Just as climatic regions and vegetational regions are dis- 

 tinguished, so must soil regions be recognized, and they correspond 

 more or less closely to the vegetational regions of the earth (Fig. 

 126). 



This viewpoint, which is slowly gaining ground, is destined to 

 prepare the way for a classification of soil types primarily according to 

 climatic control. Temperature, precipitation, distribution of rain, 



