CHAPTER IV 

 CHANGES IN INORGANIC CONSTITUENTS 



' WEATHEEINb PROCESS 



Origin and Formation of Soil.- — -Rock surfaces exposed to the 

 action of rain, sunshine and frost lose their fresh appearance, become 

 pitted and uneven, and gradually crumble into larger and smaller frag- 

 ments. In the course of time the layer of disintegrated material 

 becomes deeper and its constituent particles smaller — thanks to the 

 uninterrupted process of subdivision. Finally, lichens, algae and 

 bacteria make their appearance, the organic debris accumulates, and 

 higher plants begin to find a suitable environment for their development, 

 The rock has changed into soU. 



Influence of Biological Factors. — Soil-formation is not entirely 

 a mechanical or chemical process. Even before the layer of weathered 

 rock acquires any appreciable depth microscopical and macroscopical 

 forms of life gain a foothold on the uneven surface. With the aid of 

 sunlight they build organic compounds and make use of the combined or 

 elementary nitrogen of the atmosphere. Their life activities result in 

 the production of carbon dioxide and of varying organic and inorganic 

 acids which in their turn react with the constituents of the rock particles. 

 In this manner the biological activities become of utmost moment in 

 the transformation and migration of mineral substances in nature. 

 They assume an important r61e in the circulation of calcium and mag- 

 nesium, with the accompanying phenomena that find most striking 

 expression in the formation of caves and canyons in limestone strata. 

 They assume a no less important role in the circulation of sulphur; 

 in the accumulation and removal of a,vailable potash compounds in 

 the soil, as well as in the transformation of phosphorus and its migration 

 from inorganic to organic compounds. 



Lime and Magnesia 



Removal and Regeneration of Carbonates. — Lime and mag- 

 nesia are present in soils in different combinations. They may occur 



3S4 



