Gypsiferous soils occur in xeric, ustic, and aridic moisture regimes. Environmental conditions affect the mode of gypsum formation. Gypsiferous soils in Fars Province, southern Iran, are found in piedmont plains, flood plains, and alluvial plains. The objective of this work was to investigate the micromorphology of gypsum crystals formed under different soil moisture regimes. The results indicate that lenticular crystals of gypsum have been frequently found in more developed soils, whereas under aridic soil moisture regime such form is rare and they are frequently found in the subsurface horizons. Columnar, prismatic, and blade forms of gypsum are found in areas with aridic moisture regime, where soils are highly leached. Formation of gypsum pendant under gravels is dominant in piedmont plains with limited moisture in the profile. Complex gypsum crystals were found in low rainfall regions. It seems that surface runoff, as well as hydrological system of the region, transfers gypsum from geological sediments in higher elevations to coarse-textured soils of flood plains. In landscapes with xeric and xericaridic soil moisture regimes, lenticular, euhedral and subhedral crystals of gypsum were abundant. The results of this study indicate that, in addition to soil moisture, texture and landscape position play a significant role in the formation of pedogenic gypsum. Well crystallized gypsum was observed in soils with silt loam, sandy loam, and loamy texture. Observation of gypsic horizons suggests that the accumulation of gypsum took place under per descendum process in the soils studied.
