250 MICROBIOLOGY OF SOIL. 



ORGANIC ACIDS. 



SOURCE. The cleavage products of proteins include large quantities 

 of amino-acids. The latter are still further transformed and yield a 

 variety of fatty acids. The carbohydrates being present in larger quan- 

 tities than the proteins are still more important as a source of organic 

 acids. Finally, the fats, gums, and higher alcohols contribute additional 

 quantities of the latter. Among the more simple acids, acetic, propionic, 

 butyric, oxalic, succinic and lactic are common. The extent of acid pro- 

 duction was already indicated in connection with cellulose decomposition 

 by the methane and hydrogen bacilli. Apart from these organisms, 

 organic acids are formed by nearly every important species of soil bacteria; 

 moreover, the tissues of dead plants and animals are not the sole source 

 of organic acids in the soil. According to Stoklasa conditions may occa- 

 sionally occur in the latter, especially when atmospheric oxygen is 

 excluded, that favor the excretion by plant roots of appreciable quantities 

 of acetic acid. 



TRANSFORMATION AND ACCUMULATION. Salts of organic acids are 

 suitable as food for a wide range of soil bacteria. Azotobacter will 

 readily make use of acetates, propionates and butyrates. A number of 

 denitrifying bacteria will grow vigorously with citrates as the only source 

 of organic nutrients. The fermentation of lactates by butyric bac- 

 teria has been known for a long time. The decomposition of malates, 

 succinates, tartrates and valerates may be accomplished by various 

 species, and even simple compounds like formates may yield food and 

 energy to certain soil bacteria, among them B. methylicus studied by 

 Loew and his associates. It is evident, therefore, that organic acids are 

 not liable to accumulate in well ventilated soils. Molds, as well as bacteria, 

 destroy them rapidly, and carbonates, carbon dioxide and water are the 

 final products of the decomposition of non-nitrogenous organic matter. 



Notwithstanding the ready decomposition of the more simple organic 

 acids in the soil, it is well known that arable soils are frequently acid. 

 This acidity is largely due to the so-called "humic acids," organic com- 

 pounds whose composition is not well understood. They are composed, 

 to some extent, of rather complex organic acids or of their acid salts. 

 Continued cultivation seems to favor the accumulation of these acid 

 compounds, partly on account of the diminished supply of lime and of 

 other basic materials in older soils. When these soils are limed the 



