380 MICROBIOLOGY OF SOIL 



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 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 humic acids and acid humates are changed into neutral com- 

 pounds and are then subject to more rapid decomposition by micro- 

 organisms. According to the investigations of Blair the average acid 

 soil in Florida requires 1,500 pounds of lime (CaO) per acre to neutralize 

 the acidity to a depth of 84 mm. (9 inches), This means an acidity 

 equivalent to more than one ton of hydrochloric acid per acre. In 

 peat and muck soils the acidity is equivalent to many times this 

 amount of hydrochloric acid. 



PROTEIN BODIES 



Amount and Quality. The protein content of farm crops that 

 leave residues in the soil is variable, but in all cases quite considerable. 

 Dried corn stalks contain 5 per cent of protein, timothy hay 6 per cent, 

 red clover hay 12 per cent or more, alfalfa hay 15 or 1 6 per cent. Even 

 wheat and rye straw may contain as much as 3 per cent of protein. 

 Cotton-seed meal and other oil cakes, tankage, ground fish, hair and 



