A Morphological and Mineralogical Study of the Gray Hydromorphic Soils of the Hawaiian Islands!

Gray hydromorphic soils are imperfectly to poorly drained soils that occur on the coastal fringes of the Hawaiian Islands on surfaces of Pleistocene to Recent age. Mottling is characteristic of the soils, and gley horizons occur in the more hydromorphic soils in the group . As the soils become hydromorphic, soil color values increase and structures deteriorate. Halloysite is the dominant clay mineral in the less hydromorphic soils and montmorillonite is dominant in the more hydromorphic soils of the group . The montmorillonite is iron-rich and in one soil has the formula (XO.74Ko.1l) (Si7.52Alo.48) IV (AI1.85Fe1.603+MgO.35Tio. lO ) V1020 (OH) 4' Hydrated halloysite occurs in all the soils studied, but it is most abundant in the more hydromorphic soils. Although the soils are derived from different alluvial materials, the trend of increasing montmorillonite and increasing hydrated halloysite with increasing hydromorphism is clearly related to the pedogenic processes operating in the soils. Similar mineralogical trends are found with increasing depth in each soil. GRAY HYDROMORPHIC SOILS in Hawaii are imperfectly to poorly drained soils that occur along the coastal fringes of the islands. They are formed from alluvium derived by erosion from the Low Humic Latosols that occur above them and from the surrounding hills. Cline et al. (1955) classifiedthese soils on the basis of the expression of hydromorphic characteristics in the profile. Gill and Sherman (1952), attempting to explain the poor physical properties of the soils, stated that they are always in a dispersed condition because the percentage of Mg+ + ions in the exchange complex is high. They also noted that the soils had a high amount of 2:1 expanding type minerals. Jackson (1965) , in dealing with the formation of minerals in lowland soils, noted that pedogenic montmorillonite frequently resulted from the i Part of a thesis submitted by the senior author in partial fulfillment of the requirements for a Ph .D. degree at the Uni versity of Hawaii. Published with the approval of the director of the Hawaii Agricultural Experiment Station as Journal Series 1191. Manuscript received Apr il 17, 1970. 2 Former graduate student and profe ssor of Soil Science, respectively, Department of Agronomy and Soils, University of Hawaii. Present address of senior author, University of Dacca, Dacca, East Pakistan. influx into lowland areas of mineral colloids and solutions containing silicon, iron, aluminum, magnesium, calcium, and sodium ions. This paper describes the morphological and mineralogical properties of a sequence of hydromorphic soils formed from calcareous and noncalcareous alluvia. Variations with respect to drainage are examined; the nature of the secondary minerals, their distribution , and the processes of their formation are commented upon.