144 THE SOIL: ORGANIC MATTER 



will be produced small amounts of carbon dioxide and water, 

 considerable humus, and in addition methane and hydrogen 

 sulphide. In both cases there will be varying amounts of 

 organic acids, alcohols, higher hydrocarbons, waxes, etc. 



The classification holds only in a general way. Of course, 

 where there is an excess of air, organic matter is largely 

 oxidized to carbon dioxide, water, residual inorganic salts, 

 and nitrates. As the amount of air available to the bacteria 

 becomes less, oxidation to carbon dioxide, water, nitrates, 

 and mineral salts, materially lessens. More acids and alcohols 

 form, as well as more of that black, amorphous product 

 called humus. As the air supply continues to decrease, less 

 and less carbon dioxide and water are produced, although 

 their formation never ceases entirely, for intermolecular 

 oxidation sets free small quantities. The production of 

 nitrates practically ceases in the absence of air, and products 

 that result from reduction, like methane and hydrogen 

 sulphide, begin to form. The reduction of one compound is 

 accompanied by a simultaneous oxidation of another com- 

 pound and the production of energy. Considerably more 

 acids, alcohols, and waxes are formed, since intermolecular 

 decomposition results in the formation of these compounds 

 rather than of the completely oxidized forms. At the 

 same time there is produced more and more of this curious, 

 amorphous mixture called humus. In soils which are 

 absolutely anaerobic, however, little humus is formed. In 

 fact under conditions where no air at all is present decom- 

 position is very slow. Organic matter is maintained in a 

 fairly well preserved condition. In peat bogs, for example, 

 the structure of the original plants, sphagnum moss in many 

 cases, is not destroyed. 



The bacteria exist in soils for the most part in the upper 

 eight to ten inches, just under the surface. There are very 

 few on top of the ground, for direct sunlight kills most 

 bacteria. 



121. Factors Affecting the Rate of Decomposition. — The 

 extent to which decomposition of organic matter will take 

 place theoretically agrees with the above classification, but 

 the amount or rate of decomposition depends on two prin- 



