Phosphorus Fractions in Soil and Water of Aquaculture Ponds Built on Clayey Ultisols at Auburn, Alabama

Phosphorus fractions in soil and water were determined for a 22-year-old, 400m² fish pond constructed on clayey Ultisols at Auburn, Alabama. This pond had been used in pond fertilization and fish feeding experiments each year. Total phosphorus concenlrations in bottom soil were greater in deep water than in shallow water areas. Highest concentrations of phosphorus occurred in the 5–10 cm soil layer, but phosphorus had accumulated above its original concentration to depths of 20 to 45 cm ( x = 36.8 cm). The soil phosphorus accumulation rate was 2.68 g/m² per year. Less than 1% of the total phosphorus in soils from three ponds was extractable in distilled water or 0.5 M NaHCO₃. Sequential extraction with 1 M NH₄Cl, 0.5 N HCl, and 0.1 N NaOH removed less than 25% of the total phosphorus. The loosely-bound phosphorus fraction (1 M NH₄Cl extractable) was 0.4 to 5.2% of the extractable phosphorus. The ability to adsorb phosphorus decreased and the capacity to release phosphorus increased in pond soils as total phosphorus concentration increased. After 22 years of aquacultural production, phosphorus adsorption sites in a pond soil were only about half-saturated. Although soluble phosphorus accounted for 37% of the phosphorus in pond water, only 7% of the total phosphorus in pond water was soluble reactive phosphorus. The phosphorus pool in pond soil was over 500 times greater than that of pond water, but most of the soil phosphorus was strongly adsorbed and unavailable.