干拓地土壤に関する研究 (第16報) 海底土及び干拓地土中の酸化性硫黄の含量と土壤反応の変化の迅速測定法について

Sea muds and saline polder soils frequently contain appreciable amounts of sulfur in various forms, and in polder soils and transported soils derived from sea muds acid condition sometimes develops through aeration of anaerobic soil. A description is given of practical rapid methods of determining the oxidizable sulfur and change of soil reaction of these soils. Recommended experimental procedures are as follows: A weighed quantity of soil (about 1 - 2 g) is mixed with 10 ml of 30 % H2O2, adjusting the pH to 6.0 - 6.5 with N/10 NaOH and removing the sulfuric acid when contained with Ba (OH)2, and heated for about 1 hour on water bath. After cooling, the contents are filled up to 100 ml and filtered. Soil pH value is determined in this soil suspension by means of the pH meter and glass electrode. Titrable acidity is determined by titrating 50 ml of this filtrate with N/10 NaOH and the result is given in terms of ml N/10 NaOH per 100 g soil. Active oxidizable sulfur is calculated from sulfuric acid equivalent to titrable acidity and given in terms of S mg per 100 g soil. Easily oxidizable sulfur is determined from the difference between the water soluble sulfate of the H2O2-treated soil and that of original soil and given in terms of S mg per 100 g soil. Sulfate is determined volumetrically by Benzidine sulfate method. The reliability of this method was examined with sea muds collected from sea bottom, saline pond soils, transported soils derived from sea muds and saline polder soils. The data obtained showed that saline pond soils and some of polder soils contained appreciable amounts of easily oxidizable sulfur and the pH values of these soils became extremely acid after oxidation with H2O2 and easily oxidizable sulfur of fresh soils amounted to over 80 % of total sulfur determined from the difference between the sulfate content of the aqua regia extract and that of the water extract of the soils. (See Table 1 and 2) A considerable difference was frequently found between active oxidizable sulfur and easily oxidizable sulfur and it was shown that the relative amounts of oxidizable sulfur and easily soluble base compounds determine titrable acidity, active oxidizable sulfur and soil pH value of H2O2-treated soil. (See Table 3) The pH values of transported soils derived from sea muds were found to gradually approach in natural fields to the pH values determined by H2O2-treatment. (See Table 4)