A dissertation submitted in partial fulfillment of the requirement for the degree of Doctor of Technical Science, School of Environment, Resources and Development

Thesis (Ph.D.) - Asian Institute of Technology, 1994

A series of six experiments was conducted to study the effects of pond bottom soil types on phosphorus (P) saturation level and its relation to P fertilization; to study the nutrient recovery potentials from bottom soils by rooted macrophytes and upland crops. Pond muds accumulate P in two forms; (1) adsorbed inorganic P and (2) deposited organic P. P adsorption in mud is related to cations (A13+ , Fe3+ , Ca2+) content in clay and clay content (%) in mud. can represent its adsorption capacity. P adsorption is slower below 5-cm depth and cation P adsorption capacity is a linear function of clay content (%) in mud. Therefore, level of P saturation (%) in mud can be calculated by initial cation P and clay content (%) in the top 5-cm of pond mud using the equation: P saturation (%) = initial cation P (mg/g soil) x 100 / P adsorption capacity (mg/g soil). The results show positive linear relationship between soluble reactive P (SRP) availability and mud P saturation level. and negative correlation between added P loss from water column and mud P saturation level. Biogenic P was a major source of P sedimenter, With a rate twice higher than that in direct adsorption of inorganic P in mud. Bioturbation facilitates release of loosely bound P (loose-P) and aluminum bound P (A1-P) but it also leads to adsorb iron bound P (Fe-P) and calcium bond P (Ca-P) in mud at pH 7-9. Increasing the SRP concentration in the water Column up to 0.3-0.4 mg/L is sufficient for the fish pong as higher SRP concentration will increase primary productivity, nor fish yield. The concentration of SRP can be achieved with a P fertilization rate of 0.2 g, 0.1 g and 0.05 g/m3/day and N:P ratios of 2:1, 4:1 and 8:1 when the level of P fertilization rate for tropical ponds in a semi-intensive system should be 0.2 g, 0.1 g and 0.07 g/m2/day (2 kg, 1 kg and 0.7 kg/ha/day) with a N:P ratio of 2:1, 4:1 and 6:1 for below 50%, 50-80% and above 80% P saturated muds, respectively, to compensate the P adsorption below 5-cm depth which is slow but continuous. Organic P deposited in pong bottom can be recovered by enhancing oxidation to mineralize organic matter by drying the pond at an optimal 20-40% moisture for two weeks. oxidation of pond mud certainly enhances the adsorption of mineralized P but, its rate and amount depend on P saturation level in mud. Once the partial saturation of P in mud increases, mineralized P released from organic matter to water column will be greater than mud adsorption, Rooted plants can mobilize the adsorbed P in mud. Due to frequent liming in pond preparation, a considerable amount of P is adsorbed by Ca ion. Plant roots are capable of extracting Ca-P from mud. Roots of semiaquatic taro crop is efficient to utilize Fe-P in mud in reduced environment P fertilization strategy based on mud P saturation level has been discussed. Procedure for estimating mud P saturation level and corresponding P fertilization relate to tropical semi intensive fish pong is suggested.