54 jenny. ORIGIN OF SOILS [Ch. 2 



ganic matter amount to a few percent. In other words, in a stabilized 

 organic matter profile annually 1 to 2 percent of the soil organic matter 

 is lost and, eo ipso, replenished by decay of organisms. 



In grassland soils the bulk of soil organic matter is derived from 

 the decomposition of the root system. In forest soils a considerable 

 portion may be acquired by infiltration of humus from decomposing 

 leaf layers lying on top of the mineral soil. 



Pedocals and the Formation of Lime Horizons 



Many soils in arid and semi-arid regions are characterized by lime 

 horizons. Throughout the profile there are seams and nests of lime 

 concretions, the individual concretions varying in size from pinheads 

 to pea and nut size. The number of concretions per unit soil mass is 

 greatest in the subsoil (lime horizon). Chemical analyses of such soil 

 profiles reveal a high CaO content of the soil in the lime horizon and 

 relatively low contents in the horizons above and below. Such soils 

 were designated by Marbut as pedocals. 



If a uniform parent material containing some calcium carbonate is 

 assumed, the formation of the lime horizon may be visualized as the 

 consequence of calcium carbonate-bicarbonate equilibria which are 

 regulated by the carbon dioxide pressure of soil air. 



Root respiration and decay of vegetable matter which are very active 

 in the surface soil produce large amounts of carbon dioxide. It con- 

 verts the relatively insoluble calcium carbonate to the much more 

 soluble calcium bicarbonate. Percolating rain water translocates the 

 bicarbonates from the surface soil to the subsoil. There, owing to 

 reduced C0 2 pressure of the soil air, which is the result of a low biologi- 

 cal activity, calcium carbonate is precipitated as lime concretions. In 

 areas of low annual rainfall the carbonate horizon is close to the 

 surface. As annual rainfall increases, the lime horizon moves to 

 greater depth and, finally, above 40 inches of rainfall — in the tem- 

 perate region— completely disappears from the soil profile. 



On uplands and high terraces, lime horizons will develop only if the 

 parent material is high in bases. Thus, in semi-arid California, soils 

 derived from basic igneous rocks frequently possess calcareous sub- 

 soils; but soils derived from acid igneous rocks (for example, granite) 

 rarely do. Likewise, non-calcareous sandstones do not produce cal- 

 careous profiles. In these soils Ca exists as Ca-clay rather than 

 CaC0 3 . 



On the other hand, soils of arid regions which are under the influ- 



