CHANGES IN CLAY MINERALOGY IN A CHRONOSEQUENCE OF POLDER PADDY SOILS FROM KOJIMA BASIN, JAPAN

Changes in the clay mineralogy in a chronosequence of polder soils cultivated with paddy rice for 15, 75, 150, and 265 years in Kojima Basin were studied. Parent materials of each profile appeared to be homogeneous. Prolonged paddy cultivation (150 and 265 years) brought about some modification in the clay mineralogy of Ap horizons, while the mineralogical changes were not evident in the profiles less than 75 years of age. Characteristic changes consisted of the formation of 18-25 A minerals, which were composed of interlamellar humus-smectite complexes and interstratified minerals. The interstratification showed intermediate stages from smectite to kaolin minerals along with partial chloritization of smectite. The interstratification and partial chloritization of smectite caused the CEC of the clay fractions of Ap horizons to decrease partly.