Physical and chemical characteristics of paddy soils in a toposequence

Guilan province is one of the agricultural centers due to rice production. Paddy soil is important in this province because it is the base of agriculture. Hence، it is necessary to recognize and analyze paddy soil for understanding its limitations and its optimum usage. One of the methods of soils surveying is determination of physical and chemical characteristics of soils. Sefidrood plateaus and upper terraces، river alluvial plain، lowland and Caspian sea coastal plain physiographic units in a toposequence in the South of Sangar Dam in adjust of Rasht city to Caspian sea shore were recognized. Five profiles were studied on each physiographic unit، one of which was the reference profile. Physical and chemical characteristics of paddy soils formed on mentioned physiographic units were studied by providing disturbed soil samples from the horizons of described soils. Results showed that soil structure was weak granular in all surface horizons and single grain in the subsurface horizons of the Coastal plain physiographic unit. Soil structure was subangular blocky and angular blocky in the subsurface horizons of other units. Organic matter was high in surface horizons because of rice plant and weed tissue accumulation due to high moisture. Organic matter was high in lowlands due to high ground water table and lower decomposition rate. Cation exchange capacity was high in lowland and river alluvial plain due to high organic matter and clay content in soils. Clay particles in plateaus lands were lower than other units because of surface runoff. Electrical conductivity in subsurface horizons of lowlands and coastal plain was high because of deep lands، high ground water table and high rate of sea water seepage، respectively. Exchangeable Na+ was high in subsurface horizons of coastal plain and lowlands due to sea water seepage from depth in these soils. Soil reaction was nearly neutral because of natural and artificial submerged effect.