EFFECTS OF FLOOD PROTECTION ON SOIL FERTILITY IN A RIVERINE FLOODPLAIN AREA IN BANGLADESH

The study compared the nutrient status of flood-protected and -exposed soils in the Chandpur Irrigation Project (CIP) southeast Bangladesh. Significant differences in nutrient concentrations between protected a nd exposed soils were correlated with differences in deposited river-b orne sediments, irrigation water quality, local topography, cropping p ractices and fertilizer applications. Significant differences in nutri ent concentrations were measured within sails before and after the mon soon season. Sediments deposited in agricultural lands following river flooding were significantly higher than local topsoils in organic mat ter, nitrogen, potassium, calcium, sulphur, magnesium and manganese, b ut significantly lower in copper and zinc. Nutrients contributed in di ssolved form by floodwaters were low by comparison to those introduced by sediment deposition. Nitrogen-fixing blue-green algae were abundan t in surface water and attached to vegetation in both flood-exposed an d -protected areas. We concluded that river-borne sediments are potent ially valuable sources of soil nutrients. Depending on the topography of agricultural lands in relation to sediment-bearing rivers and on th e physical and chemical nature of the sediments, their exclusion from agricultural lands by embankments could be a potentially negative impa ct of flood control developments in some areas in Bangladesh.