298 INTRODUCTION TO PLANT GEOGRAPHY 



' world groups ', such as podzols (developed chiefly in cool regions 

 of high precipitation relatively to evaporation), brown earths (with 

 generally lower rainfall and higher temperatures), chernozems (with 

 low rainfall in continental regions), prairie soils (with higher rainfall), 

 chestnut-brown soils (in warmer and drier places), laterites (with high 

 rainfall in the tropics), red loams (with lower rainfall in warm- 

 temperate regions), tundra soils (of polar regions where the subsoil 

 remains frozen and organic decomposition is retarded), and so on. 

 Fig. 84 shows three characteristic soil profiles and Fig. 85 indicates 

 the distribution of the primary soil groups of the world. 



As we shall see towards the end of the next chapter when dealing 

 with plant succession, soil development and vegetational develop- 

 ment are intimately connected, both being largely controlled by 

 climate. Meanwhile the essential constituents of most soils may 

 conveniently be treated in five categories : 



(i) Mineral fragments of various sizes resulting from the dis- 

 integration of rocky materials by physical and chemical weathering ; 

 the parent material may be either local or otherwise, as may be the 

 weathering. These mineral constituents form the inorganic frame- 

 work and diflter widely according to the physical and chemical 

 nature of the parent material. They afl'ect plants particularly by 

 bringing about variations in soil water and aeration, as water reten- 

 tion is much affected by mechanical composition (the relative 

 proportions of different-sized mineral particles present). The 

 mineral constituents may also aflFect the composition of the soil 

 water in important ways {see below). 



Soils are mechanically analyzed by separating into ' fractions ' the 

 particles whose sizes lie within definite arbitrary limits, ranging from 

 larger stones and gravel (more than 2 mm. in diameter) down 

 through various sizes of sand and silt to clay (less than 002 mm. 

 in diameter). The majority of mature soils consist largely of silica 

 and silicates which are relatively insoluble and form a more or less 

 permanent basis, any calcareous material tending to become dis- 

 solved (' leached ') out of the surface layers and many of the finer 

 insoluble particles being carried down mechanically to lower levels 

 (this process is termed ' eluviation '). Particularly important in 

 leaching and chemical weathering is the acid-forming carbon dioxide 

 dissolved in soil water. The particles forming the soil's inorganic 

 framework tend to be coated with colloidal material of very fine 

 clay or of organic origin, which may help cement them into com- 

 pound particles or grains and increase the ' crumb structure ' of the 



