Abstract
Micromorphological observations have been made for different types of wet lowland soils to obtain information on their characteristics and genesis. 1. Anthraquic soils (Paddy soils) Genesis of anthraquic soils involves unique pedogenic processes including the development of iron accumulation horizons and plowpan, and a subsurface greyization which spreads from the surface downwards. 1) Thread-like iron mottles along root channels are common to iron accumulation horizons. These mottles consist of channel ferran associated with neoferran. The horizons are reducing under a flooded condition. After drainage Fe^<2+> in the soil matrix moves toward root channels and is oxidized when it meets the oxygen which comes down through the channel. 2) Greyizeed subsurface soil is characterized by iron-depletion cutan (alban of neogleyan) on ped-faces and along conducting channels and by ferruginous mottles with diffuse boundaries in the matrix , which indicates a reductive eluviation along macro-voids and in situ segregation of iron in the matrix. 3) Migration of suspended matters is evident. Clay coatings on ped-faces, and channel walls are often observed in greyized subsurface soils. Flood coatings (gleyan) or poorly sorted illuvial materials are also common on ped-faces and channel walls. 4) In anthraquic soils a plowpan develops beneath the plowlayer to keep the lands flooded. Micromorphologically it is well expressed in the amount of macro-pores, which is at a minimum in plowpan. 2. Groundwater aquic soils (Gley and Gray Lowland soils) Tube-like iron mottles are characteristic of Gley and Gray Lowland soils. Tube-like mottle consists essentially of neoferran with or without ferran. Film-like iron mottles (ped-ferran) are also common in aquic lowland soils. Fe^<2+> in the matrix moves by diffusion toward macro-pores and meets oxygen at or around the surface of the pores. 3. Organic soils Fibric, hemic and sapric materials were used for observation. Degree of decomposition of organic matter is well reflected in the ratios among undecomposed weakly birefringent plant residues, partially decomposed opaque plant debris and humified massive or granular colloidal materials.
