On the basis of the schema presented in the preceding paper that the soil humus consists of many parts continuously differing in "the lowest extraction pH", a method for characterizing humus composition among soils was devised. In the method, all the humus in a soil is considered a mixture of four humus fractions : Fr. I (acid-extractable), Fr, II (acid-inextractable, but neutral-extractable), Fr. III (neutral-inextractable, but alkali-extractable), and Fr. IV (alkali-inextractable). For determining the fractions, separate soil samples are extracted with three extractants, pH 1.2, 7.0, and 13, each containing pyrophosphate ion, for 48 hours at room temperature. The extracts obtained are subjected to carbon determination by a permanganate oxidation method. The content, in dry soil, of C in the form of Fr. I is calculated directly from the content of acid-extractable C, and those of Fr.'s II, III, and IV are calculated by the difference between the contents of acid-, neutral-, and alkali-extractable C, and total organic C (CA, CN, CAL, and CT, respectively). Fr. I is divided into two subfractions unadsorbable and adsorbable onto active carbon from acid solution (Fr.'s I-1 and I-2, respectively), and Fr. I-2 is divided further into two, elutable and unelutable with alkali from the active carbon (Fr.'s I-3 and I-4, respectively). Content of C in the forms of these subfractions is obtained from the determination of C in Fr. I, Fr. 1-1, and Fr. 1-3. On Fr. 1-3, its absorbance is measured at the wavelength of 270 nm with 0.1N NaOH as the solvent ; absorbance of the solution with a fixed concentration is represented by K270. From the analysis of 34 soil samples belonging to nine soil groups by this method, it was found that most of fractional C contents, ratios among them, and optical values of Fr. 1-3 were correlated to each other, CT, and soil pH also, and among them C_N, (C_T-C_N)/C_N, and K270 of Fr. 1-3 were the best values about the above correlation. PQ and optical values of alkali-extracted humic acid, however, had no correlation with soil pH. Most of the soil groups examined could be well separated from each other by plotting soils with the values of C_N and K270 of Fr. I-3. Result of a experiment in which a quantity of Japanese pampas grass straw was mixed into a soil and whose humus composition was analysed at intervals during 1.5 years by this method showed that small change in humus composition is possible to be caught by this method.
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