decomposition of organic matter in the soil 3 19 



Organic Acids 



Source. — The cleavage products of proteins include large quantities 

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

 variety of fatty acids. The carbohydrates being present in larger 

 quantities 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, succinic and lactic are common. The extent 

 of acid production 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 occasionally 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 

 bacteria 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 compounds 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 acQumulation 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 



