. SOIL FORMATION 79 



nesium, and pota^ium in the plants and animals are mainly in the 

 form of organic compounds, and as such are not available to 

 other plants. Bacteria decompose them liberating carbon dioxid, 

 ammonia, hydrogen sulfide, sulfuric acid, and the mineral con- 

 stituents which are 

 rendered available 



as plant-food. In 

 this manner, the 

 biological activi- 

 ties become of the 

 utmost importance 

 in the transforma- 

 tion and migration 

 of mineral sub- 

 stances in nature. 

 The bacteria act, 

 therefore, as the 

 link between the 

 living and the 

 dead. 



Soil bacteria are 

 continually p r o - 

 ducing large quan- 

 tities of carbon di- 

 oxid, nitrous, ni- 

 tric, and sulfuric acids, together with organic acids which are 

 in the main combined with calcium or magnesium of the soil with 

 the formation often of a soluble compound. The waters carry 

 these to the lakes, seas, and oceans — there to be taken up by ma- 

 rine life. In the course of time these are deposited as coral reefs, 

 chalk cliffs, and marl beds. At times the speed with which the 

 lime is taken from the waters by marine life is faster than it is 

 carried into a lake by its tributaries. The result is that in spite of 

 the evaporation and concentration which is going on, the main 

 body of water contains less lime than does its tributaries. This 

 is the case with Bear Lake, Utah, the tributaries of which have an 



Etching op' Marble by Plant Roots 



Fig. 23. — The picture on the left shows the solvent 

 action of plant roots on a marble slab in the absence 

 of bacteria; the one on the right the action in the 

 presence of bacteria. The latter have formed acids 

 from the material given oif by the plant and have 

 increased the solvent action on calcium carbonate. 

 (After Fred.) 



