SOIL BACTERIOLOGY AS A FACTOR TX CROP PRODUCTION. 5 



CARBOHYDRATE-FERMENTING BACTERIA. 



The group of organisms fermenting carbohydrates, such as sugar, 

 starch, and especially cellulose, which is the substance forming the 

 firm or fibrous portions of plants, has many representatives, but their 

 functions and their relations to crop plants are in most cases not well 

 understood. It is known, of course, that many species of bacteria 

 will ferment the different sugars, forming some organic acid and fre- 

 quently carbon dioxid and hydrogen or carbon dioxid and methane. 

 It is highly probable that the constant production of these gases, in 

 minute quantities it is true, in the chemically powerful condition of 

 the nascent or freshly liberated gas exerts a much greater influence in 

 soil weathering and the solution of plant food from small particles of 

 minerals than any of the much-discussed agencies of the plant roots 

 themselves. Both the carbon dioxid and the organic acids produced 

 by these organisms are presumed to be of importance in controlling 

 the availability of phosphates. No satisfactory methods of con- 

 trolling this action have been developed, however, and no accurate 

 estimate of this function has been possible. 



The most resistant carbohydrate found in the soil and the type of 

 carbohydrate which is most important because of the enormous 

 quantities added to soils annually is plant fiber or cellulose. This is 

 a substance so resistant to even moderately strong acids and alkalis 

 that it appears at first glance almost indestructible. Obviously, 

 however, active agents for cellulose destruction must be constantly 

 operating; the decay of wheat stubble, of cornstalks, and of green 

 manure turned under, as well as the rapid rotting of dead trees, are 

 perhaps the best illustrations of this. It should be noted that humus 

 has long been recognized as an essential constituent of a fertile soil, 

 and more recently it has become apparent that many of the processes 

 constantly going on in the soil which result in the formation of avail- 

 able plant food are controlled in a large measure by the quantity and 

 quality of the humus compounds. In the formation of these com- 

 pounds the cellulose, which is decomposed in the soil in such large 

 quantities, must play a prominent role. 



Extensive studies in our laboratory during the past two years 

 have shown that there are numerous organisms endowed with the 

 power of actively fermenting cellulose. The cellulose is made soluble 

 and becomes available as carbohydrate food for other organisms, 

 especially the nitrogen fixers, which will be referred to later. There 

 is another phase of cellulose destruction which appears to be of 

 great economic importance from the soil-fertility standpoint, and 

 that is the action of filamentous fungi. Several investigators from 

 time to time have found one or more species of molds to which they 



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