674 PRINCIPLES OF SOIL MICROBIOLOGY 



mature plants are rich in pentosans, celluloses and lignins. This 

 is true also of leaves. 21 It is natural, therefore, to find 22 that green plants 

 decompose much more readily than mature plants and that the nitrogen 

 in the former is changed much more rapidly into nitrates than the 

 nitrogen of mature plants, due to differences in the relative composition, 

 in the rates of decomposition and in the nature of the organisms bringing 

 these processes about. 



A part of the organic matter, consisting of monosaccharides, pento- 

 sans, hexosans, and of the proteins and their derivatives, is completely 

 decomposed, with the formation of C0 2 ,H 2 0, NH 3 and various minerals, 

 under aerobic conditions; CH4 and H 2 are also formed under anaerobic 

 conditions. A part of the materials decomposed is reassimilated by the 

 organisms and synthesized into microbial protoplasm; under aerobic 

 conditions this amounts to as much as 20 to 40 per cent of the decom- 

 posed materials. A part of the original materials is left in the form of 

 intermediate products, due either to the greater resistance of these to the 

 action of the specific microorganisms, or to the formation, under certain 

 conditions such as excessive moisture and excessive acidity, of products 

 which hinder further development of the organisms. A part of the 

 original organic matter, consisting largely of fats, waxes, tannins, resins, 

 and lignins, is left undecomposed. This mass of undecomposed, partially 

 decomposed and transformed materials makes up the soil organic matter, 

 which is being modified constantly. A large part of this organic matter 

 is soluble in alkalies and is commonly referred to as "humus" or the 

 "humified" fraction of the organic matter. 



The chemical ingredients of the organic matter added to the soil are 

 decomposed at various rates and to varying degrees. Of the non-nitrog- 

 enous substances, the mono-saccharides are the first to disappear; 

 these are followed by the starches and pectins and then by the celluloses 

 and pentosans. The lignins, fats and waxes are decomposed only very 

 slowly. The more ripe and mature the original plant from which the 

 organic matter is derived, the greater is the degree of its lignification and 

 the more slowly does the lignified portion decompose. The pentosans 



21 Fricke, K. Beitr;'ige zur Kenntnis der Bestandteile einiger Laubholzblatter. 

 Ztschr. physiol. Chem., 143: 272-289. 1925. 



22 Martin, T. L. Decomposition of green manures at different stages of 

 growth. N. Y. (Cornell) Agr. Exp. Sta. Bui. 406. 1921, 139-169; Whiting, A. L., 

 and Schoonover, W. R. The comparative rate of decomposition of green and 

 cured clover tops in soil. Soil Sci., 9: 137-149. 1920. 



