THE PHYSICAL ENVIRONMENT OF ORGANISMS 561 



(pedalfers). The warm and humid climate of the tropics produces zonal 

 (climatic) soils known as laterites, or latosols, which are particularly high 

 in iron and aluminum. 



It will be noted that there are a number of soil groups in both the 

 calcium-rich and the calcium-poor classifications. This is due largely 

 to the various kinds of ratios between rainfall and evaporation. At the 

 one extreme, evaporation is greatly in excess of rainfall; at the other, 

 rainfall is greatly in excess of evaporation. Moreover, some other factors 

 enter in. The physically active parts of the soil are the humus (derived 

 from organic materials) and clay particles. Altogether, these have an 

 enormous surface area and usually possess electronegative charges. 

 Consequently, they tend to hold the positively charged molecules of 

 calcium and magnesium against the leaching effect of heavy rainfall and 

 so keep them available for the plants. Thus it is possible for certain types 

 of soils to be subjected to heavy rainfall and still produce a luxuriant 

 growth of vegetation year after year. The more productive soils may be 

 thought of as composed of relatively insoluble mineral particles coated 

 with gelatinous organic and inorganic colloidal films to which enormous 

 numbers of soil organisms cling. 



The Prairie, Chernozem, Gray-brown, and Savanna soils are generally 

 rich in soluble raw materials for food manufacture, and because of their 

 generally favorable moisture conditions plants growing upon them are 

 able to utilize great amounts of the sun's energy. The areas of these soils 

 (Fig. 32.11) represent the greatest energy-capturing zones, and it is here 

 that we find the great grain, grass, and cattle-producing regions of the 

 United States. The fertile lands of Russia, Argentina, and Africa lie in 

 belts of very similar climate and soils. The once vast bison herds of North 

 America, the huge cattle production of the Argentine, and the tremendous 

 animal life of Africa were (or are) possible because of the luxuriant grass 

 production in regions where abundant materials for photosynthesis 

 coincide with an adequate supply of soil moisture. 



