CHAPTER XVII. 



MINERALIZATION AND SOLVENT ACTION OF 



BACTERIA. 



SOILS are the earthy material in which plants have their anchorage, 

 and from which they obtain their water and part of their food. 

 They are in reality disintegrated rock intimately mixed throughout 

 with varying quantities of decaying plant and animal residues. 

 They are derived from the native rocks by a complex process known 

 as weathering. The agents at work to bring this about are changes 

 of temperature, the action of air, water, ice, and plant and animal 

 life. 



Bacteria as Soil Formers. Early in the history of soil formation 

 bacteria appear and play an essential part in rendering the soil 

 fertile. Their life activities result in the production of carbon 

 dioxid, organic and inorganic acids, and alkalies. These in turn 

 react with the constituents of the rock particles, thereby changing 

 their solubility. When water becomes charged with substances 

 from the soil, its solvent powers are greatly increased. Especially 

 is this true when it becomes filled with carbon dioxid either from the 

 atmosphere or from the decay of plants and animals. 



Common limestone is one of the rocks most actively attacked by 

 carbonated water; none are wholly resistant to its action. Even 

 quartz is slowly dissolved. Granite and related rock are rather 

 quickly acted on by water due to the feldspar minerals which it 

 contains. The bases potash, soda, lime, and alumina are 

 dissolved out. The last is deposited as clay, the first as beneficial 

 or injurious soil constituents, depending on the kind and the con- 

 centration left in a particular soil. In a similar manner the sulphur, 

 iron, and phosphorus of the soil are changed to available forms. 



Moreover, bacteria play a very important part in the mineraliza- 

 tion of plant and animal residues which continually find their way 

 into the soil. The phosphorus, sulphur, iron, calcium, magnesium, 

 and potassium in the plants and animals are mainly in the form of 

 organic compounds and as such are not available to other plants. 

 Bacteria act upon them, liberating carbon dioxid, ammonia, hydrogen 

 sulphid or sulphur with the liberation of the plant-food. In this 

 manner, the biological activities become of the utmost importance in 

 the transformation and migration of mineral substances in Nature. 

 The bacteria act merely as the link between the living and the dead. 



