Micromorphological properties of salt affected soils in Northeast Thailand

Abstract

A micromorphological study was carried out on salt affected soils from three locations on erosional terraces overlain by wash and alluvium under tropical savanna climate in Northeast Thailand where salinity problem has been acute. This aimed at providing detailed data for a better agricultural land use planning of the area. Five or six profiles at each location were sampled along a linear traverse that included the soils with highest and lowest levels of salt accumulation. Sixteen soil profiles were described, sampled and analyzed using physio-chemical, optical microscope and scanning electron microscope with energy dispersive spectrometer (SEM/EDS) techniques. The highest salt affected profiles show salt crusts, bare surfaces and halophytic plants and these soils have been left idle. All soils are deep, poorly drained, moderately to highly developed and with various salinity levels. They are fourteen Natraqualfs and two Endoaqualfs. Soil textures are sandy for location 1, clayey for location 2 and with a sandy topsoil and clayey subsoil for location 3. The soils at locations 1 and 2 mostly have high values of electrical conductivity (EC) and exchangeable sodium percentage (ESP). Soils at location 3 may have been affected by desalinization and alkalization as is indicated by low EC, high basic cations and high pH. Field morphology, optical micromorphology and SEM/EDS analysis results show clear accumulation of salt in some profiles. Their microstructures are different due to the difference in their texture. Compact grain and bridge grain structures are common in sandy-textured soil whereas channel and subangular blocky structures are common in the clayey soils. Quartz and runi-quartz grains are common as skeleton materials, but are present in different sizes and amounts. Halite occurs in voids in soils at locations 1 and 2 and is abundant in Pedons 1 and 6 where salt crusts exist. Calcite is present only in one profile (Pedon 1). Amorphous pedofeatures are manganiferrous impregnative nodules and iron and manganese oxide impregnated s-matrix. The EDS spectra indicate the presence of barite (BaSO 4) and sylvite (KCl) crystals in voids in some of the soils. The element composition of the soil matrix in a triangle graph of SiO 2: Al 2O 3: Fe 2O 3 is displaced from the ‘kaolin line’ towards the SiO 2 apex, indicating that very fine-grained quartz and/or a Si-rich clay mineral (s) is present in the matrix. The presence of Si-rich clay minerals is supported by XRD results that identified smectite in addition to kaolin and may be an evidence of the resilication process that can occur in salt affected soils. The data points near the Fe 2O 3 apex of the triangle graphs represent iron oxide nodules and their position on the matrix Al 2O 3/SiO 2 line indicates that induration of the matrix with iron oxide has occurred and that the nodules have formed in situ. The normalized element composition of the matrix in triangle graph between SiO 2: Al 2O 3: Na 2O + CaO + MgO indicates that in some instances the salts of Na, Ca and Mg have indurated the soil matrix as data points for salt-rich matrix fall on the clay matrix composition line.