CHAPTER IV. 

 CHANGES IN INORGANIC CONSTITUENTS. 



WEATHERING 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 be- 

 comes deeper and its constituent particles smaller thanks to the uninter- 

 rupted 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 soil. 



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 foot-hold on the uneven surface. With the aid of 

 sunlight they build organic compounds and make use of 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 role in the circulation of calcium and magnesium, 

 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 available 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 as 



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