78 BACTERIA AND SOIL FERTILITY 



years to the same amount in 500 years. Water also has an ap- 

 preciable solvent action on feldspar, granite, and even quartz. 

 Sandstones with calcareous cements are disintegrated by the solu- 

 tion of the cement, causing the rock to fall to pieces and form 

 sand. 



Moreover, many minerals combine with water. This is known 

 as hydration and plays a very important part in weathering. Dur- 

 ing hydration the mineral increases in volume and often becomes 

 soluble. Building stones which are uniform in color when iirst 

 quarried sometimes become discolored on exposure to the rain and 

 air. The discoloration is due to the action of water and oxygen 

 upon the iron pyrite in the stone with the formation of sulfuric 

 acid which readily reacts with the rock. 



BIOLOGICAL AGENCIES 



Bacteria as Soil Formers. — Early in the history of soil for- 

 mation bacteria appear and play an essential part in rendering the 

 soil fertile. Their life activities result in the production of car- 

 bon dioxid, organic and inorganic acids, and alkalies. These 

 react with the constituents of the rock particles, changing their 

 solubility. When water becomes charged with these acids pro- 

 duced by bacteria its solvent powers are greatly increased. 



Common limestone is one of the rocks most actively attacked 

 by carbonated water; noile 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 bene- 

 ficial or injurious soil constituents, depending on the kind and 

 the concentration left in a particular soil. In a similar manner 

 the sulfur, iron, and phosphorus of the soil are changed to 

 available forms. 



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

 zation of plant and animal residues which continually find their 

 way into the soil. The phosphorus, sulfur, iron, calcium, mag- 



