Abstract
Five different experiments were conducted in the Faculty of Fisheries Field Laboratory ponds of Bangladesh Agricultural University at different durations in between March 1999 to April 2001. The main objective was to determine the effectiveness of poultry manure (PM), cow manure (CM), and urea plus triple super-phosphate (UP) on soil-water quality and biological productivity in carp polyculture system. The three fertilizers were tested alone, with either of the nitrogen (N) and phosphorus (P) nutrient content similar, and in different combinations and fertilization intervals with similar content of both N and P.While the iso-nutrient combinations of PMUP (poultry manure and urea plus TSP) and CMUP (cow manure and urea plus TSP) were tested for their effects in pond productivity without stocking of fish, the values of N nutrients (NH₃-N, NO₃-N, NO₂-N) in water were significantly lower, but that of PO₄-P were significantly higher with PM-UP. This might be due to the combined effects of efficient mineralization capacity of P and higher rate of N uptake by the significantly higher production of chlorophyll-a (389.25±55.15 μg/l) and phytoplankton (78.25±6.33x10⁴ cells/l). Though not significant, total N and available P in pond sediment were lower with PM-UP. The phytoplankton abundance was more or less steady throughout the experimental period, possibly that, bypassing the phytoplankton link to some extent in food chain, the zooplankton sustained feeding directly on small solid pieces of manure, digesting bacteria from outer surfaces and absorbing dissolved organic matter in water.PM, CM and UP with either of iso-N and iso-P content did not result in any significant difference in nitrogenous nutrient concentrations in water, but did in PO₄-P concentrations. Despite the highest (P<0.05) mean concentrations of PO₄-P and a similar mean concentrations (P>0.05) of NO₃-N with either of iso-N and iso-P UP, PM resulted in significantly highest levels of chlorophyll-a (253.16±46.51 μg/l for iso-N and 86.70±12.66 μg/l for iso-P) concentration and phytoplankton (244.93±54.24x10⁴ cells/l for iso-N and 72.00±6.60x10⁴ cells/l for iso-P) abundance. The highest fish yields of 2,067 and 1,136 kg/ha/4 months were obtained with iso-N and iso-P PM, respectively.
While PM, CM and UP were tested in different combinations with a similar N and P content, the mean concentrations of PO₄-P (1.47±0.18 mg/l) and NO₃-N (1.88±0.08 mg/l) were significantly higher with PMUP. The chlorophyll-a concentration of 97.63±16.29 μg/l was significantly lower with CMUP, while that of 213.16±31.43 μg/l with PMUP was higher, though not significant than that of 198.14±25.91 with CMPM (cow manure and poultry manure). The plankton abundance was also significantly higher with PMUP (126.25±8.65x10⁴ cells/l) than with CMUP (80.21±8.22x10⁴ cells/l), similar with CMPM (112.38±10.76x10⁴ cells/l). The PMUP iso-nutrient combination yielded the highest quantity of fish production of 1,773±18.65 kg/ha/4 months.Among the different application intervals (twice a week, weekly, fortnightly, and once a month) of iso-nutrient PMUP fertilizer, the water quality parameters did not vary significantly, except the chlorophyll-a concentration. The chlorophyll-a concentration in pond water, receiving fertilizer at fortnight intervals, increased more or less steadily with significantly higher mean value of 114.24±8.52 μg/l. The fortnight fertilization also resulted in significantly higher phytoplankton (62.49±3.17x10⁴ cells/l) and zooplankton (45.25±4.37x10⁴ cells/l), compared to other fertilization frequencies. The increment in growth parameters of stocked fish species was higher, resulting in the highest (P<0.05) total fish yield of 2,301±20.60 kg/ha/4 months, with the fortnight fertilization frequency.Chlorophyceae, in all experiments, was the dominant group of phytoplankton, both in number of genera and relative abundance, followed by Bacillariophyceae, Cyanophyceae and Euglenophyceae. The zooplankton group of Crustacea always dominated over Rorifera, in terms of both number of genera and relative abundance. There had been a similar trend of increment in different pond sediment parameters, except pH, at the end of all experiments.The overall results indicate that PM either alone or in combination with UP and CM at a similar loading rate of either N or P or both was superior to either of CM, UP and their combination in creating favourable limits of physico-chemical factors, nutrient mineralization and possibly in releasing growth promoting soluble salts in water column for increased primary production and thereby the increased fish production. The results also suggest that comparatively better PMUP combination could be highly effective in pond fish culture with an application interval of twice a month.