MINERAL ELEMENTS AS SOURCES OF ENERGY 155 



excluded as a result of the saturation of the material with water, 

 complete oxidation of the organic matter is made impossible. 

 The activities of the aerobic fungi and bacteria are thereby ex- 

 cluded, and the decomposition of organic matter is dependent 

 largely upon the activities of anaerobic bacteria. Under these 

 conditions, some of the constituents of the organic substances, 

 such as the waxes and the lignin, are scarcely decomposed at all, 

 and others, like the cellulose and hemicelluloses, are decomposed 

 very slowly. The formation of peat is thus a result of the con- 

 tinuous accumulation of certain plant residues in habitats deprived 

 of free oxygen by saturation with water. The peats formed in 

 prehistoric times have finally given rise to coal, as a result of 

 various natural phenomena, including physical and chemical 

 processes. 



When organic materials are decomposing under anaerobic con- 

 ditions, considerable amounts of hydrogen are produced. When 

 it reaches an aerobic environment , the hydrogen is utilized as food 

 by a number of bacteria, and becomes oxidized to water. 



Most of the other elements which enter into the composition 

 of the soil are acted upon by microorganisms in one way or another. 

 Mineral substances become transformed from one inorganic form 

 into another, or are brought into solution, or are precipitated as a 

 result of the direct or indirect action of microorganisms. Some 

 elements as sulfur and phosphorus are frequently constituents of 

 organic compounds and consequently pass through a more varied 

 series of changes. Utilization of these organic compounds as 

 sources of energy by microorganisms results in the mineralization 

 of the elements. Autotrophic microorganisms are capable of 

 using several mineral elements or their simple compounds as sources 

 of energy. Various microorganisms produce a number of organic 

 and inorganic acids, which increase the solubility of many mineral 

 materials. Compounds of potassium, magnesium, calcium, and 

 iron pky important roles in the decomposition and synthetic 

 processes which are carried on by the soil organisms. Zinc, man- 

 ganese, chlorine, sodium, etc., are required only in mere traces 

 by the soil organisms, and their role in microbial processes is still 

 imperfectly understood in most instances. 



Mineral Elements and Their Inorganic Compounds as 

 Sources of Energy. — Some of the elements or their simple inor- 

 ganic compounds, which enter into the composition of the living 



