MINERAL TRANSFORMATION AND SOIL FORMATION 179 



manures, plant stubble, and microbial cells all contain potassium. 

 When these organic substances are decomposed by microorganisms 

 in the soil, the potassium is liberated in forms available to higher 

 plants. A small part of this potassium may be reassimilated by 

 the microbes which bring about the decomposition processes, thus 

 part of the potassium may be temporarily removed from circula- 

 tion. The ash of bacteria and fungi usually contains between 

 5 to 40 per cent of potassium calculated as K2O. 



The inorganic forms of potassium added to the soil as fertilizers 

 are generally soluble, but the minerals in the soil which contain 

 potassium are quite insoluble. Solution of potassium from these 

 minerals is accelerated by interaction with various acids produced 

 by microorganisms, as shown in the case of orthoclase : 



Al203-K20-6Si02 + 8HNO3 



= 2A1(N03)3 + 2KNO3 + 6Si02 + 4H2O 



The products of both nitrification and sulfur oxidation, as well 

 as the carbonic acid produced by the microbial population in gen- 

 eral, aid in such solution of potassium. 



Importance of Mineral Transformation by "Microbes in 

 Soil Formation. — The various inorganic constituents of the sur- 

 face of the earth are subject to a number of changes as a result of 

 the activities of the numerous soil-inhabiting microbes, especially 

 in the presence of organic substances. These changes cause con- 

 tinuous modifications in the nature of the physical and chemical 

 composition of the surface of the earth, which for convenience is 

 divided into layers, called the surface soil and subsoil, or horizons 

 A, B, and C. 



There is evidence which tempts speculation regarding the role 

 played by microorganisms in the development of this soil layer 

 from the rocks, which were formed subsequent to the cooling of the 

 earth's surface. The presence of carbon dioxide in the atmos- 

 phere, the formation of small amounts of ammonia and nitrous 

 acid by electrical discharges, the presence of sulfur and of various 

 minerals containing phosphorus, potassium, calcium, magnesium, 

 iron and other necessary elements in the earth's crust, was sufficient 

 to allow the growth of autotrophic bacteria. These were capable 

 of obtaining their required energy for growth from the ammonia, 

 nitrous acid, and sulfur, and could satisfy their requirements for 

 carbon from the carbon dioxide of the atmosphere. As a result of 



