Changes in soil properties of abandoned shrimp ponds in southern Thailand

Chemical soil properties between active shrimp ponds and abandoned ones on the Bangkok soil series were compared, at Ranote District, Songkhla Province in southern Thailand. Soil samples were collected at depth intervals of 0-10, 10-20, 20-30, 30-40 and 40-50 cm from pond bottoms at the same ponds used in a former study conducted in 1994, for a total of 6 ponds with 3 sampling sites for each pond. These ponds were active during the previous study in 1994, abandoned in 1996 and investigated by this study in 1999. All the samples were analyzed for exchangeable Ca, Mg, K and Na electrical conductivity (EC), organic matter, S, P and pH, and statistically compared with the analytical results of the previous study. An increase in amounts of Ca, Mg, K, Na and EC in the abandoned ponds as compared with the active ones by 1.3-3.4, 1.4-2.1, 7.0-30.0, 1.2-6.3 and 1.3-10.9 times respectively was observed. That more of these elements were gained than lost each time the seawater was introduced into the ponds, is explained by the 'Element Input/Output Consideration' as proposed herein. Furthermore, a decrease in organic matter, S and P was also observed in the abandoned pond soils, and attributed to the absence of shrimp food and shrimp excreta following the cessation of shrimp raising activities. An unexpected decline in the soil pH of the abandoned ponds was found as well. Aerobic decomposition of organic matter during the absence of shrimp raising activities caused by soil microorganisms triggering SO2 and H2SO4 formation probably played a more significant role than the increase in the amounts of the basic elements (Ca, Mg, K, and Na) eventually reducing soil pH in the abandoned ponds. The significant depletion of the amounts of organic matter in the abandoned pond soils also supports this observation.