TRANSFORMATION OF ORGANIC MATTER 691 



through a weighed Gooch crucible or filter paper; a brown to yellow brown solution 

 (II) is obtained. The precipitate is washed with dilute acid, then with distilled 

 water, dried and weighed; this precipitate or fraction a of the soil organic mat- 

 ter is equivalent to the portion commonly referred to as "humus" or "humic 

 acid." When properly washed with acid, the a fraction contains only 1 to 4 per 

 cent ash and about 3.0 to 3.5 per cent nitrogen. The filtrate (II) from this 

 fraction is then treated with 5 per cent NaOH solution, until the reaction reaches 

 approximately pH 4.8. At this point a heavy precipitate (fraction /3) is formed. 

 This is filtered off; the solution (III) should be straw to yellowish colored; if the 

 solution is brownish, more alkali or acid solution should be added to adjust to 

 the proper reaction and the precipitate is filtered off and added to the rest of 

 fraction /?. This precipitate (J3) is well washed with distilled water, dried to 

 constant weight and then ashed. This fraction is found to contain about 40 to 

 60 per cent ash and about 1 per cent of nitrogen. It consists of either a chemical 

 or physical complex of aluminum hydroxide and organic materials. These two 

 fractions contain 60 to 80 per cent of the soil organic matter (the undecomposed 

 part is left in the soil and a part is left in the final solution III). The amounts of 

 these two fractions vary with the soil and its treatment and can serve as an index 

 of the amount and nature of the organic matter in the soil. 56 



Nature of soil "humus." Since the chemical nature of "humus" 

 is not known and the very methods for measuring it are not exact, 

 there is little wonder that a number of theories have been suggested 

 to explain the formation of this more or less stable substance or group 

 of substances in the soil. It was at first supposed that "humus" is 

 derived chiefly from carbohydrates. 56 Even at present various theories 

 are frequently suggested to explain how celluloses may change, through 

 the oxy-cellulose stage, into "humic acids." 57 However, as far back as 

 1889, Hoppe-Seyler suggested that cellulose decomposition does not 

 contribute to the formation of "humus" under anaerobic conditions. 

 Cellulose was found to be decomposed completely to CO2 and CH 4 , 

 according to the following tentative reactions: 



C 6 H 12 6 = 3C0 2 + 3CH 4 



The greater the amount of oxygen, the less methane and the more 

 carbon dioxide was formed. The direct sources of "humus" in the soil 

 are looked for among the lignins, proteins, tannins, chlorophyll, pig- 

 ments, certain fats, and resins. The celluloses, hemi-celluloses, mono- 

 and di-saccharides, glucosides, organic acids and various alcohols 

 do not appear to form "humus" directly. 58 The protoplasm of fungi 



57 Marcusson. Zur Kenntnis der Huminsauren. Ztschr. angew. Chem., 31: 

 237-238; 34: 437; Ber. deut. Chem. Gesell., 54: 542. 1918-1021. 



68 Trusov, A. G. Contributions to the study of soil humus. I. Processes of 

 forr. ation of "humic acid." Materials on the study of Russian soils. XXVI- 

 XXVII. 1-210. 1917. 



