and climate can be seen through an analysis of the 

 development of the great soil types of the world. For 

 the purpose, we will here adopt the simple, recent 

 classification of Simonson (1957). 



PodzoUc soils are formed in humid temperate 

 climates, under forest vegetation. The A2 horizon is 

 moderately well developed, for there is sustained 

 leaching. Soils are more or less acid and only mod- 

 erately fertile. Podzols develop under coniferous for- 

 est and have a mor type of humus. Gray-brown and 

 broivn podzolic soils are found under hardwood for- 

 ests and have a mull humus. 



LatosoUc soils develop in humid tropical or semi- 

 tropical forested regions. Humus is quickly oxi- 

 dized by action of microorganisms and hence does not 

 accumulate. Chemical weathering of the parental ma- 

 terial is intense. Water drainage through the po- 

 rous soil is rapid, so leaching is extensive. In early 

 stages of its formation, the soil is neutral or slightly 

 alkaline, but as leaching continues, it becomes acidic. 

 The soil has a thin organic layer (Aq and Ai hori- 

 zons) on a reddish, leached soil (A2 horizon) that 

 extends to great depths below the surface. 



Chernozemic soils occur in humid to semiarid 

 temperate climates under grass vegetation. The 

 grasses on dying return considerable organic matter 

 to the soil. The Aj horizon is consequently dark in 

 color and of great thickness. The soil contains more 

 bases and hence is less acid than in the two types 

 above. The B horizon in humid regions is indistinct, 

 but where there is less rainfall, calcium salts may ac- 

 cumulate to form a hardpan. Prairie soils in temper- 

 ate climates are among the most fertile soils of the 

 world, but fertility decreases in the tropical and 

 desert climates. 



Desertic soils are characteristic of arid climates 

 and contain very little organic matter. A profile is 

 poorly developed. The surface soil is brownish gray, 

 and grades quickly into the calcium carbonate horizon 

 which usually forms a hardpan just below the sur- 

 face. Wind erosion removes the finer soil particles, 

 leaving the coarser material to form a hard pavement. 

 The soils are but slightly weathered and leached : 

 lacking nitrogen, they are infertile. 



Mountain and mountain valley soils vary from 

 shallow layers on eroding rocks to deep organic soils 

 of valleys and swampy areas. 



Tundra soils occur in cold northern areas where 

 the substratum remains continuously frozen and the 

 vegetation of lichens, mosses, herbs, and shrubs 

 makes a peaty surface layer. The region is poorly 

 drained and characterized by many scattered shallow 

 ponds. 



Alluvial soils, may be important locally. These 

 soils are mostly without a developed profile and are 

 the result of deposition by streams. They are usually 

 very fertile and support luxuriant vegetation. 



Saline soils are found in dry climates where rapid 

 evaporation of water results in surface deposition and 

 accumulation of salts leached from surrounding up- 

 land areas. 



These are only the main types of soils, but they 

 are sufficient, however, to show considerable corre- 

 lation with the geographic distribution of the major 

 ecological communities, biomes. In a detailed classi- 

 fication, many subdivisions and intermediate cate- 

 gories would be recognized (Kellogg 1936, Lutz and 

 Chandler 1946). 



REACTIONS IN WATER 



Considerable attention has already been 

 paid to the reactions of animals and plants in streams 

 (Chapter 5), lakes (Chapter 6), and ponds (Chap- 

 ter 7 ) . These involve changes both in the chemical 

 and physical characteristics of the habitat and are 

 fundamentally the same as occur on land. 



Water plants, especially those such as water 

 lilies and water -hyacinths which float, and surface 

 concentrations of both zoo- and phytoplankton reduce 

 light intensities like forest canopies. There is ac- 

 cumulation of plant and animal remains and fecal ma- 

 terial on the bottom of the water bodies just as on 

 land, and this material is worked over by bacteria and 

 a large variety of micro-organisms which differ only 

 in taxonomic composition, not in activities, from 

 those on land (Henrici 1939). Water plants obstruct 

 the flow of water and cause deposition of suspended 

 materials simulating the reduction of wind velocities 

 inside forests. An important physical reaction is the 

 damming of streams by beavers so that ponds are 

 formed. Such beaver activity can sometimes be use- 

 fully co-ordinated with waterflow management 

 (Beard 1953). These ponds eventually fill with silt 

 and organic matter, succession occurs, and so-called 

 beaver meadows are produced (Van Dersal 1937). 



The nitrogen cycle (Cooper 1937), and carbon 

 cycle in aquatic ecosystems are essentially the same 

 as on land. The absorption of oxygen by organisms 

 and by the decomposition of organic matter in some 

 lakes and ponds causes in the habitat a seasonal 

 change of profound importance. 



Water conditioning 



All modifications in the habitat produced 

 through the reactions of organisms represent a condi- 

 tioning effect, whether the habitats are terrestrial or 

 aquatic. The term has been used most commonly, 

 however, in respect to changes produced in small 

 bodies of water, especially under experimental con- 

 trol. Water is conditioned when physical or chemical 



172 Ecological processes and dynamics 



