TRANSFORMATION OF ORGANIC MATTER 671 



precipitated by an excess of acid. 6 The second group has commonly 

 been known as "humus" or "humic acid," and has been frequently fur- 

 ther subdivided into a number of fractions by the use of various solvents, 

 as the alcohol soluble fraction ("hymetomelanic acid" of Hoppe-Seyler) 

 and the alcohol insoluble fraction ("humic acid" of Oden). 7 



By special manipulations, various definite chemical substances have 

 been isolated from one or another of the above fractions. Some of 

 these substances are not characteristic of soil organic matter but are 

 constituents of the original materials added to the soil, while some are 

 products of the cell substance, synthesized by the soil microorganisms. 

 Among these isolated substances are those found by Schreiner and 

 Shorey, 8 namely pentosans, arginine (C 6 Hi 4 2 N4), histidine (C 6 H 9 2 N3), 

 cytosine (C^HvONs-HoO), hypoxanthine (C 5 H 4 ON 4 ), xanthine 

 (C5H4O2N), picoline carboxylic acid (C 7 H 7 2 N), dihydroxystearic acid 

 (C18H36O4) (found also in fungus mycelium), paraffinic acid (C24H48O2), 

 lignoceric acid (C24H48O12), agroceric acid (C21H42O3), agrosterol (C 2 e 

 H44OH2O), phytosterol (C26H44OH2O), glycerides, resin acids and 

 resin esters. Some of these may become toxic to plants and micro- 

 organisms and have been held responsible by some investigators for 

 the injurious influence of straw upon plant growth. 9 Some of the 

 resins, pigments and other substances frequently found among the 

 constituents of the plants added to the soil may be more or less detri- 

 mental to the germination of seed. 10 



Various aldehydes, 11 nitrogenous compounds, 12 fats, waxes, and cho- 

 lesterols 13 are commonly found in the soil. The organic matter of peat 

 and muck soils may contain 3.3 to 6.2 per cent of fats and waxes; 13 



6 Waksman, S. A. What is humus? Proc. Nat. Acad. Sci., 11:463-468. 

 1925. 



7 Oden, S. Die Huininsauren. Kolloid Chem. Beihefte, 11: 75-260. 1919. 



8 Schreiner, O., and Shorey, E. C. Chemical nature of soil organic matter. 

 Bur. of Soils, U. S. Dept. Agr. Bui. 74. 1910. 



9 Collison, R. C. The presence of certain organic compounds in plants and 

 their relation to the growth of other plants. Jour. Amer. Soc. Agr.. 17: 58-68. 

 1925. 



10 Sigmund, W. Uber die Einwirkung von Stoffwechsel-Endprodukten auf 

 die Pflanzen. IV. Einwirkung N-freier pflanzlicher Stoffwechsel-Endprodukte 

 auf die Keimung von Samen: Harze, Farbstoffe. Biochem. Ztschr., 154: 399- 

 422. 1924. 



11 Skinner, J. J. Soil aldehydes. Jour. Frankl. Inst., 186: 165. 1918. 

 12 Lathrop, 1917 (p. 474). 



13 Ramann, E. Die von Postlischen Arbeiten liber Schlamm, Moor, Torf, 

 und Humus. Landw. Jahrb., 17: 405^20. 1888. 



