Abstract
It is observed that some of the salty paddy fields in coastal areas form very acid soils after drainage. The authors made the experiments in order to ascertain whether such acid marine soils exist or not in coastal areas in Japan. The soils investigated in this study were taken from salty paddy fields which are distributed along the seacoast of Okayama, Hiroshima, Yamaguchi, Tokushima, Kochi and Ehime Prefectures. Soil reactions and soluble salts were determined after the incubation of soils during three weeks at 30℃ under the condition of the moisture content of field capacity. The results obtained may be summarized as follows : (1) The reaction of soils lies within the range of pH from 1.3 to 6.6. Most of these samples show strong acidities, ranging from pH 1.3 to 5.0. (2) Soluble salt contents of these soils are very high, ranging from 0.4 to 1.9 percent. It is found that sulphates are dominant in the soluble salts and come to be increased with the increase of total salts. A relatively good correlation is found between pH value and sulphate content. (3) It may be assumed that the strong acidities found in incubated soils were brought about by the oxidation of sulphides which had been formed in an anaerobic condition. The damage of rice plant in these paddy field is considered to be caused by the accumulation of relatively large amounts of sulphides, especially soluble sulphides and its oxidized acidic compounds. Concerning the reclamation of the depressed land, it is a noteworthy fact that a good many of soils from coastal areas become extremely acidic after drainage.
