Magnesium Budget for Inland Low-Salinity Water Shrimp Ponds in Alabama

Abstract

A magnesium budget was prepared for a commercial low-salinity shrimp farm in the Blackland Prairie region of Alabama for one production cycle. Ponds had previously been used for production and fertilized with magnesium; two ponds (S-5 and S-6) for four previous years and one pond (N-9) for one previous season. Fertilization with sulfate of potash magnesia (K2SO4·2MgSO4 or K-Mag®) was applied to these ponds to obtain the concentrations of 20 mg/L, averaging 1274 kg Mg2+/ha. Additional inputs of magnesium included groundwater, rainfall, and runoff averaged 441.5 kg/ha. A water budget for ponds indicated that 292.6 kg/ha of magnesium in outflows. The difference in inputs and magnesium outputs resulted mainly from adsorption of magnesium by pond bottom soils. However, the increase in exchangeable magnesium in the upper 15-cm layer of pond bottom soils was not great enough to account for the difference in total magnesium inputs and magnesium outputs in water and shrimp. Possible explanations for this discrepancy are magnesium precipitation, nonexchangeable fixation of magnesium by clay minerals, incomplete extraction of magnesium, and analytical error. The decreased uptake of magnesium by older ponds, S-5 and S-6, indicate that the soils had a diminishing affinity for the cation or an equilibrium concentration is being established.