Abstract
The authors studied the soil-forming process of the halogenetic polder soils developed along the seacoast of the inner Kojima Bay and investigated the distribution of soil types and the characteristics of these soils. As polder soils of Kojima Bay have been developed under the influence of alkali salts, we grouped these soils as halogenetic soils. After being diked, it was found that polder soils were subjected to process of desalinization and solotization. The greater part of polder soils recently diked belongs to the younger halogenetic polder soils. These soils contain a large amount of the soluble salts of sea water and of the exchangeable bases, among which Mg and Na are more prominent. The base-exchange complex is nearly saturated with bases and the hydrolytic acidity is very small. Hence the reactions of these soils are neutral or slightly alkaline. These soils show the characteristics of the saline alkali soils of DE'SIGMOND. Younger acid halogenetic polder soils are found on poorly drained region along the dike. Air-dried surface soils show extremely strong acidities, ranging from pH 3 to pH 4,under the influence of iron sulfides formed under an anaerobic condition. Soluble salt content is also much, especially sulfates constitute the dominant salt among the salts. The V values are very low. Slightly leached halogenetic polder soils are found close to the non-leached soils above mentioned. When a part of the sodium salt have been washed out from the upper soils by the rain or irrigation water, the salt content of the surface soils is decreased but that of subsoils is 0.1 per cent or more as Cl. The exchangeable mono-valent cation (Na and K) decreases slightly in surface soils but remains unchanged in subsoils. The pH values of surface soils are 6〜7,and those of subsoils, 7 or more. Moderately leached halogetic polder soils distribute in the greater part of Fujita-Son and in a part of Kojo-Son and Nadasaki-Cho, which had been embanked from 1820 to 1900. Soluble chloride content of the surface soils is below 0.04 per cent. The pH values of surface soils, are 5.5-6.0,and those of subsoils, 7 or more. The hydrolytic acidity of the surface soils increases to as much as 20 or more. Strongly leached halogenetic polder soils distribute in the background, which had been diked from 1610-1820. With a continued supply of non-saline water, the leaching process advances greatly and the salts of all horizons are thoroughly washed out. Hence the solube chloride content of all horizons is below 0.04 per cent. Of exchangeable bases, Na and Mg are replaced by hydrogen and V values decrease greatly. The pH values of surface soils are 5.0-5.5,and those of subsoils, below 7. These ultimate products of leaching process, named strongly leached halogenetic polder soils, show the characteristics of the degraded alkali soils of DE'SIGMOND. A close relationship is found between the date of enclosure and the distribution of each soil types in this region.
