Formation of soils with fragipan and plinthite in old beach deposits in the South of the Caspian Sea, Gilan province, Iran

Abstract Rice cultivation in the mountainous and sloping areas of Gilan province by leveling and terracing imposes special conditions upon the soils of these areas. Following the retreating Caspian Sea, the beach deposits left in the vicinity of the Elborz Northern foot slopes were covered by fine texture deposits. In this research, macro and micro-morphological characteristics of the soils formed in the Holocene and Pleistocene beach deposits in eastern Gilan, province, were studied. Qualitative and semi-quantitative testing of the soil reduction was carried out by using α,α′-dipyridyl indicator. Free and amorphous sesquioxides were extracted and measured by dithionite-citrate bicarbonate, and oxalate ammonium, respectively. Total Fe, Al, and Mn were measured by digestion. Polished and thin sections were used for micro-morphological description. The most important soil forming process occurring in the Apg and Bg horizons is the formation of depleted, amorphous and cryptocrystalline pedofeatures. The microstructure present in the fragipan and in the horizon characterized by the plinthite properties is mainly pellicular associated with simple packing voids. Coarse/fine (C/F) related distribution shifts from closed chitonic to gefuric. Polished sections show that the opaque minerals of the soils are mostly magnetite (60%) and hematite (40%). Magnetite weathering and its transformation to hematite (martitization process) were also observed. The results of energy dispersive analysis X-ray (EDAX) analyses showed that 40% of the sand grains coatings are iron. Fragipans have been formed in the Late Holocene beach deposits, which are more recent. This occurred when the soils with plinthite characteristics were developed in the Pleistocene beach deposits. Plinthite formation in the Iranian soils was not expected and had not been reported before. Rice cultivation and Anthric saturation may play an important role in plinthite and even fragipan formation in the studied area.