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Rice is the most important agricultural product in Gilan Province. Nutrient availability in wetland soils is influenced by redox potential. This study was conducted in Langroud area, located in eastern Gilan Province, Northern Iran. During the study year changes in redox potential were studied on four sites in rice paddy fields, located on mountain, basin, marsh and seashore landforms. The Eh values were measured through permanently installed platinum electrodes and a voltmeter at 25, 50 , 100 cm depths, but redox potential for the depths < 5 cm and 10-15 cm were recorded by intermittent measurments. Redoximorphic features were evaluated by both measurments of redox potential and reaction to ex., ex.' - dipyridyl dye. Soils on the mountains (Aeric Epiaqualfs) are submerged by irrigation water during the growing period and may be saturated for 1 to 2 months during the year by rain water. In these soils the reduced layer extends to the base of the top soil. The soils in the basins and seashores (Aeric Epiquepts) exhibit two reduced layers, one at the base of tillage layer
and the other within about 75-100 cm. Excluding the grwoing period, the surface of
these soils are saturated for about 2 to 4 months during the year. The soils in the marshy areas (Fluvaquantic Endoaquolls) were permanently saturated, thus reduced all year round. Toward the end of the growing period, the Eh varied, but lower valdes were obtained for all soils studied on different landforms. The Eh variations in different landforms could be related to redoximorphic features of the soils being developed under the influence of ponded water at soil surface and fluctuations in water table. During the growing period the lowest Eh values were recorded for the months of Khordad and Tir (June and July). During these months the recorded Eh Values, were + 170, + 1 00, +30, -100 and +60 m V in mountain soils; + 110, +80, -50, 0 and -120 m V in basin soils_ +50 , +30 , -180 , -210 and -130 m V in marsh soils; + 105, +65, -50, +96 and +5 m V in seashore soils for depths 5, 10 , 25 , 50 and 100 cm respectively.