effects of rice straw incorporation on some physical characteristics of paddy soils

the prevalent method of irrigation in paddy fields in iran is continuous ponding, which changes to intermittent method at the time of water shortage. soil cracks are the main problem in intermittent irrigation. in this study, effects of adding rice straw on some soil physical characteristics and cracks were studied in paddy soils of the guilan province of iran. the experiment was performed as split-split plots based on a complete randomized blocks design. treatments included four soil textures (silty clay, silty clay loam, clay loam, and sandy loam), seven rates of rice straw (0, 2, 3, 4, 5, 6 and 7% by weight), and three soil moisture stages of drying conditions [primary stage (t1), initial crack stage (t2), final crack stage (t3)]. results revealed that soil texture, rice straw rate, crack treatments, and their interactions had significant (p< 0.01) effects on soil moisture content, bulk density, and time to crack formation. during the drying stage (from t1 to t3), moisture content decreased, however, bulk density and time to crack formation increased. in silty clay soil, addition of 2-3% rice straw expedited crack formation, and an addition of 4-7% straw delayed crack formation. in silty clay loam soil, addition of 3-5% straw enhanced crack formation, and addition of 6-7% delayed crack formation. the addition of rice straw in clay loam and sandy loam delayed crack formation. in general, addition of rice straw increased soil moisture content, decreased bulk density, and delayed crack formation.