Determination of phosphorus saturation level in relation to clay content in formulated pond muds

Abstract

An experiment was conducted to determine the amount of P needed to saturate simulated fish pond sediments, formulated to contain six levels of clay (0, 30, 41, 64, 73 and 81% by weight). A series of cylindrical cement tanks were filled to 20 cm depth with the six sediment types and triple superphosphate (TSP) solution was added to reach P saturation in sediment. Results showed that all sediment types reached constant inorganic-P concentration in the upper 5 cm after 12 weeks of TSP application, and P adsorption capacity of sediment increased with increasing clay content. Sediment P adsorption was slower and not significant ( P > 0·05) below 5 cm depth except in the sediment type containing 0% clay. Regression analysis showed that the rate and adsorption capacity of P in sediment are primarily governed by clay content and its dominant minerals. While organic-P and loosely bound-P are commonly deposited in sediment, most inorganic-P is adsorbed by cations to form cation-P complexes. The linear relationship between cation-P saturation level and the percentage of clay in sediment is highly significant ( r 2 = 0·84, P < 0·001) and, therefore, maximum adsorption capacity of cation-P in pond sediment can be approximated by Y = 0·019 X (where Y represents the 100% saturation level in mg P g −1 soil, and X is the percentage of clay in the sediment). In practice, the level of P saturation in sediment can be approximated by the initial cation-P and clay contents in the top 5 cm of pond mud using the equation: P saturation (%) = initial cation-P (mg g −1 soil) × 100/P adsorption capacity (mg g −1 soil).