On the two kind of soil sequence, i.e. hydro-catena and permeability of the solum, in paddy soils of lowland area. : in relation to the genesis and classification of lowland paddy soils

Abstract

Classifications of lowland paddy soils have, hitherto, been made mostly on the basis of the sequence of hydro-catena (strength of natural hydromorphism), and ground water level has been regarded as the major function of drainage condition. However, in case that there is no or little influence of ground water, drainage condition of paddy soils is regulated by the properties of soil body itself, particularly by texture and porosity. Difference in permeability of the solum gives rise to different types of paddy soils, i.e. Brown lowland paddy soils, Gray lowland paddy soils and Stagnant-water type lowland paddy soils in order of decreasing permeability. Therefore, various types of lowland paddy soils are proposed to be understood on the basis of combined two kinds of soil sequence, i.e. hydro-catena and the permeability of the solum, as illustrated in the following schematic table. Brown lowland, Gray lowland, and Stagnant-water type lowland paddy soils are the genuine "paddy soils" in its pedological meaning. Characteristics of these soils are briefly outlined as follows. 1. Brown lowland paddy soils. Coarse textured, excessively drained paddy soils. These soils have brownish subsoils, which indicates oxidative condition throughout the year. Iron and manganese leached from the A horizons deposit quantitatively in the B horizons. Brown lowland paddy soils include "Brown oxidation type" and "Mottled gray and brown type," the latter being a transitional type toward Gray lowland paddy soils. 2. Gray lowland paddy soils. Well to poorly drained paddy soils. These soils are characterized by the grayish subsoil, formed by seasonal alternation of reduction and oxidation. Leaching of iron and manganese from the solum is characteristic of these soils. Gray lowland paddy soils are tentatively subdivided into "Irrigation water type," "Fluctuating ground water type," and "Ground water gley type" in order of strengthening the influence of ground water. 3. Stagnant-water type lowland paddy soils. Very fine textured, very poorly drained paddy soils. Beneath the plowlayer lies a thick gley or gley-like subsurface horizon formed by the stagnation of irrigation water, which in turn is underlain by rusty mottled B horizon. This type of soils is subdivided into "Strong gley type" and "Weak gley type" according to the strength of reduction.