SELMAN A. WAKSMAN 315 



FUNCTIONS OF SOIL MICRO-ORGANISMS 

 DECOMPOSITION OF ORGANIC MATTER 



Considerable quantities of organic matter of plant and animal origin are con- 

 stantly introduced into the soil, in the form of stable manures, green manures, plant 

 roots, plant stubble, and various waste products as well as certain organic fertilizers. 

 The decomposition of these materials is one of the most important functions of the 

 soil organisms. Various bacteria, fungi, protozoa, nematodes, rainworms, and other 

 invertebrate animals take an active part in this process. Some macerate the organic 

 matter mechanically and remove certain constituents for their own nutrition; others 

 decompose a large part of the organic matter, but may leave certain constituents 

 undecomposed. Some attack only certain specific ingredients of the organic matter. 

 All of them build up cell substance, thus tending to replenish the supply of organic 

 matter in the soil. 



The organic matter which is commonly added to the soil consists of water-soluble 

 constituents including sugars and amino acids, of pentosans and other hemicelluloses, 

 of true celluloses, of lignins, proteins, fats and waxes, tannins, pigments, etc., and of 

 ash. Among the fungi, for example, the Mucorales can attack the water-soluble con- 

 stituents, the proteins, and certain hemicelluloses, but not the celluloses, the lignins, 

 the cutins, and the tannins. Other fungi, like various species of Aspergillus, Penicil- 

 lium, Trichoderma, Fusarium, Cephalosporiiim, can readily attack the celluloses and 

 hemicelluloses, but not the lignins and the cutins. Some of the wood-destroying fungi 

 can decompose the lignins in preference to the celluloses, while others are unable to 

 decompose the lignins but can decompose the celluloses. Most of the heterotrophic 

 bacteria, including the numerous spore-forming and non-spore-forming bacteria and 

 cocci, attack only the water-soluble constituents, the proteins, and the hemicellu- 

 loses. A few highly specific organisms are able to decompose celluloses. Certain an- 

 aerobic bacteria decompose celluloses with the formation of organic acids (acetic 

 butyric), alcohols (butyl, ethyl), and gases (hydrogen, methane, carbon dioxide). 

 The aerobic bacteria decompose the celluloses largely to carbon dioxide and water, 

 with the synthesis of considerable protoplasm and frequently with the formation of 

 slimy substances or gums. The decomposition of lignins by bacteria is still un- 

 decided. 



The decomposition of organic matter in soil by the mixed-soil population of pure 

 cultures of organisms can be followed by three distinct methods: 



I. By determining the evolution of carbon dioxide, which is the most important final 

 product in the metabolism of all aerobic micro-organisms. The more easily the organic 

 matter is decomposed, the larger is the amount of CO2 given off both under controlled labora- 

 tory conditions and in the field. Soil itself will give off a constant stream of carbon dioxide, 

 the actual amount depending on the temperature, moisture, and reaction of the soil and the 

 content of organic matter. Under normal temperature, practically all the CO2 is given off 

 as a result of the decomposition of the soil organic matter by micro-organisms. The amount 

 of CO2 given off by the soil itself must be subtracted from the CO2 given off from the soil 

 which has received the particular organic substance, the difference being an index of the 

 decomposition of the latter. 



