CHAPTER II 

 THE DECOMPOSITION OF ORGANIC MATTER IN THE SOIL 



CARBOHYDRATES 



Origin. The sugars, starches, vegetable gums and allied pec tine 

 substances, as well as the cellulose, contained in roots and other crop 

 residues add large quantities of carbohydrates to the soil. The crop 

 residues are augmented still further by green manures and animal 

 manures whenever these are used. A good growth of timothy, for 

 example, may increase the content of organic matter in the surface 

 soil by 250-500 kg. (500 or 1,000 pounds) per acre, and three-quarters 

 of this consists of carbohydrates. In the same manner, a green ma- 

 nure crop, or an application of barnyard manure may add to the land 

 as much as 1,500 pounds, or even more, of carbohydrates per acre. 

 These carbohydrates contain a large proportion of cellulose. 



The Decomposition of Cellulose. Pure cellulose (page 237), 

 (C 6 HioO 5 ) x is a rather inert substance, as exemplified by the resistance 

 of cotton and flax fiber to decomposition processes. It is well known, 

 nevertheless, that even cellulose is in the end decomposed and resolved 

 into simple compounds. Plant roots, leaves and stems, as well as the 

 trunks of fallen trees, gradually disintegrate and vanish. Under favor- 

 able conditions this may proceed rapidly, as is indicated by the process 

 in septic tanks, or in manure heaps on the one hand, and in open 

 sandy soils on the other. The disappearance of cellulose may be ac- 

 complished by (a) anaerobic organisms, (b) by aerobic organisms, (c) 

 by denitrifying bacteria, (d) by molds and (e) by actinomycetes usually 

 classed as higher bacteria. 



The Production of Methane and Hydrogen. The decomposition 

 of pure cellulose and the formation of methane and hydrogen mixed 

 with other gases was first noted by Popov in 1875. Some years 

 later Tappeiner and also Hoppe-Seyler confirmed Popov's observa- 

 tions that nearly pure cellulose in the form of Swedish filter-paper, or 

 cotton fiber may be fermented by bacteria with the evolution of 

 methane, carbon dioxide and occasionally also of hydrogen. These 



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