Constraints of Acid Sulfate Soils Converted from Rice to Shrimp Culture in Coastal Areas of Ca Mau Province, Vietnam.
Guong Vo Thi, Quang Tri Le, and Truong Giang Thai. Can Tho University, Campus II, 3/2 street , Ninh Kieu, Can Tho, Vietnam
The high benefit gained from shrimp compared to rice has led to rapid conversion of rice to shrimp culture in saline acid sulfate soils in coastal areas in the Mekong Delta. Adverse conditions of soil and water in acid sulfate soils were considered as one of factors leading to failure of shrimp cultivation and increasing poverty. This study compared water and soil quality of shrimp and shrimp- rice systems on alluvial, potentially acid sulfate and acid sulfate soils in Ca Mau province. The soil pH of potentially acid sulfate and acid sulfate soils in saline areas ranged from 7.0-7.4, but dropped rapidly to 4.5 when the soil dried during land preparation. By contrast, the pH of alluvial soil in saline areas was about 7.5. There was no significant drop in pH below this value when the soil dried. Soil organic matter content increased gradually only in potentially acid sulfate soils. Exchange sodium percentage and Sodium Adsorption Ratio (SAR) values were determined below 15 in shrimp- rice systems of acid sulfate soils. For the shrimp system in potentially acid sulfate soil and alluvial soil, the SAR ranging from 17-25 was obtained, indicated of soil sodification. Water alkalinity was ranged from 58-70 mgCaCO3.l-1. This value was below the optimum level for shrimp growth during the first half shrimp culture period. Hydrogen sulfide reach a range of 0.03-0.71 ppm which was harmful for shrimp growth. Highest level of H2S was found in acid sulfate soil. Water salinity varied in large extent (from 6ppt- 37 ppt). Dissolved nitrogen in water ranged from 0.2-0.4 ppm. Phosphorus available in water was low ranging from 0.01- 0.04 ppm. This low nutrient status was a factor related to low chlorophyll-a content and low natural food in water column. Other factors as biological oxygen demand, chemical oxygen demand, dissolved oxygen content were in an optimum level for shrimp. Keywords: acid sulfate soils, potentially acid sulfate soils, shrimp-rice and shrimp system.