Impacts of shrimp culture on some chemical properties of soil in Amphoe Ranot, Songkhla

Expansion of shrimp farming, tiger prawn, into rice growing areas has adverse impacts on socioeconomic aspects and natural resources of the areas, particularly soil resources. Therefore, it is necessary to investigate the impacts of shrimp farming on the chemical properties of soils. Three sites of Bangkok soil series (Bk) were selected from different locations at Amphoe Ranot, Songkhla, namely a paddy field, new ponds (one-year old shrimp ponds), and old ponds (three-year old shrimp ponds). Soils samples were collected at the depth of 0-10, 10-20, 20-30, 30-40 and 40-50 cm from the bottom of the ponds respectively, and then analyzed for pH, electrical conductivity, organic matter, Na, K and S after they had been air-dried and sieved. Statistical analysis comparisons among the same depth of the soil samples obtained from the paddy field, new ponds and old ponds revealed that pH levels of the shrimp pond soils decreased with cultivation time. Organic acid from the decomposition of organic matter and sea water employed in the shrimp ponds were probably responsible for the decline in the soil pH. The electrical conductivities of the shrimp ponds soils considerably increased when compared with paddy soils; however, they decreased with depth. The much higher electrical conductivities of shrimp ponds (new ponds) indicated that vertical salinity penetration was at least 40 cm per year. The results also suggested that vertical intrusion of Na and K were 30 and 20 cm per year. Moreover, the electrical conductivities of the shrimp pond soils had positive correlations with the amounts of Na and K and their coefficient of determination were 0.83-0.99 and 0.77-0.81 respectively. The amounts of organic matter of the shrimp pond soils also had positive correlations with the amounts of S with correlation coefficient of 0.93-0.94 and both amounts declined with depth. It was concluded that organic matter could have been a source of S in the shrimp pond soils. The information obtained from this study may be employed for land reclamation, management and land use planning for minimizing the adverse impacts of shrimp culture on the nearby paddy fields.