A. G. Norman 175 



as they themselves are readily available sources of energy other organ- 

 isms would immediately accomplish their oxidation. Under anaerobic 

 conditions, on the other hand, soluble organic acids may accumulate 

 since they form characteristic products of the activity of facultative and 

 anaerobic forms. Polysaccharides containing carboxyl groups, such as 

 the polyuronide gums produced by some bacteria which utilize cel- 

 lulose, may represent the only "acids" that accumulate under aerobic 

 conditions (10). 



Continually cropped soils in humid regions become increasingly 

 mom acid. The change in base status in such soils is due not only to 

 cation:, removal in the crop and to the formation of carbonates or 

 bicarbonates; the nitrification process, which is believed to be exclu- 

 sively microbiological, is in part responsible. Ammonia, liberated from 

 organic sources, which at first probably occupies positions on the base 

 exchange complex, may be oxidized to nitric acid and then combine 

 with other bases. The oxidation of 100 pounds of nitrogen, an amount 

 often exceeded per acre per season, would result in the formation of 

 450 pounds of nitric acid. Similarly, the oxidation of the sulfur present 

 in proteins and thio compounds 'results in the formation of sulfuric 

 acid. 



It is unlikely, however, that the direct and indirect effects of microbial 

 activity on nutrient availability that result from the production of car- 

 bon dioxide, carboxyl groups, or inorganic acids rival in importance 

 those transformations that are involved in the decomposition of organic 

 matter and plant residues. 



ORGANIC MATTER DECOMPOSITION AND MICROBIAL SYNTHESIS 



The soil population is predominantly heterotrophic and accordingly 

 the supply of energy is of major significance in determining its over-all 

 activity. Soil organisms constitute an important link in the chain of 

 carbon transformations in nature. The green plant accomplishes the 

 synthesis of a great diversity of organic substances from carbon dioxide 

 and water; the soil organisms carry out the reverse process and oxidize 

 to carbon dioxide and water almost any carbonaceous material that 

 finds, its way onto or into the soil. That these downgrade processes are 

 accompanied by synthesis is often overlooked, yet the synthetic activi- 



