ECOLOGICAL RELATIONS OF SOIL 



217 



and atoms. This latter energy may be stored 

 deep in the earth or may be more superfi- 

 cial. Such energy is Uberated in the weath- 

 ering of rocks, to an extent that is suggested 

 when we know that the transformation of 1 

 gm. of granite to clay liberates about 120 

 calories. 



The mechanics of the decomposition of 

 primary rocks in natiire are not fiilly known. 



The depth of soil developed in situ varies 

 from a few milHmeters to several meters; 

 it is usually not more than 3 meters deep. 

 Its tliickness reflects the cUmate, the topo- 

 graphic relief, nature of the source rocks, 

 vegetation, the animals actively present, and 

 the length of time the particular soil has 

 been evolving. Soil equilibrium, when 

 achieved, is dynamic rather than static. 



The sol uin 

 or true soil 



Zone of 

 eluviatioii 



Zone of 

 illuviation 



The weathered parent 

 material 



Aoo_ 



'An 



•-I 



ciiz: 



Cc c 



Loose leaves and organic debris, largely undecomposed. 



Organic debris partly decomposed or matted; frequently divided into 

 subhorizons. 



A dark-colored horizon, containing a relatively high content of organic 

 matter, but mixed with mineral matter. Thick in prairie and thin in 

 forest soil. 



A light-colored horizon, often representing the zone of maximum leaching 

 (or reduction). Absent in prairie and some other soils. 



Transitional to B, but more like A than B. Sometimes absent. 

 Transitional to B, but more like B than A. Sometimes absent. 



A usually deeper-colored horizon, often representing tne zone of maxi 

 mum receipt of transported colloids. Often transitional to C, with 

 definite structure, but not hardened. 



Transitional to C 



G represents the glei layer of the intrazonal soils of the humid region. 



Cc and Ca represent possible layers of accumulated calcium carbonate or 



calcium sulfate found in prairie and other soils; usually occurring 



between B and C. 



bnderlying stratum. 



Important subdivisions of the main horizons are 

 conveniently indicated by extra numerals, thus: A 2 i 

 and A2 2 represent subhorizons within A2. 



Fig. 54. Schematic arrangement and nomenclature of horizons in the soil profile. (Redrawn 

 from the U. S. Department of Agriculture Yearbook of Agriculture, 1938. ) 



Even igneous rocks, if divided finely 

 enough, can be decomposed by water that 

 contains acid. The acid may be furnished 

 by suspended hydrogen clays, acidic organic 

 colloids, or dissolved carbon dioxide. Sur- 

 face waters receive their acidic organic col- 

 loids from biological sources, as they usually 

 do their carbon dioxide. It follows that one 

 of the basic decompositions in the biosphere 

 is now being produced, as it has been dur- 

 ing much of geological history, by the reac- 

 tion of organisms on their nonhving environ- 

 ment (Hutchinson, 1943). 



THE SOIL PROFILE 



Soil consists of several horizons, some oi 

 which are illustrated in Figures 54, 55 and 

 57. A part of a soil that develops character 

 istic physical and chemical properties is 

 called a soil horizon; taken together in 

 natural sequence from the surface down- 

 ward, the soil horizons in a given place 

 make a soil profile. Regardless of their dis- 

 tinctness, the soil horizons in a profile 

 develop together as a more or less harmo- 

 nious system. They may have come directly 



