by C. Wesley Wood, Bernard M. Meso, Karen L. Veverica, and Nancy Karanja
ertilizers are often applied to ponds to increase nutrient concentrations that favor phytoplankton growth, enhancing production of fish and crustaceans. During harvesting, ponds are drained to levels where fish can be recovered via nets. A result of pond draining is effluent that is often allowed to run into natural waterways. Effluents from fertilized ponds have relatively high nutrient concentrations, particularly of nitrogen (N) and phosphorus (P), and can be potential sources of pollution and eutrophication for receiving waters. Thus, pond effluents present an environmental management challenge, but they may also present an agricultural opportunity in the form of nutrient-laden irrigation water for crops.
Given the challenge and opportunity for managing pond effluent, CRSP researchers Wes Wood and Karen Veverica began a project at the Sagana Fish Farm in Kenya looking at the use of tilapia pond effluent for producing French bean (Phaseolus vulgaris) and kale (Brassica oleracea). Nancy Karanja, Head of the Soil Science Department at the University of Nairobi, collaborated on the project. A masters student in the Soil Science Department at the University of Nairobi, Bernard Meso, used this research for his thesis project. Karanja and Wood served as comajor professors for Mesos thesis work.
A limited amount of previous research indicated that CRSP researchers would likely get a favorable crop response to applied pond effluent. Previous reports suggested that crop yields could be doubled with pond water when compared to well water. Improved water use efficiency (measured in kilograms of grain per hectare per millimeter of water) for aquaculture effluentirrigated crops in comparison to well water treatments has also been reported. Other past research showed that pond effluent could supply much of the N requirement for grain crops.
At Sagana, CRSP researchers studied the effects of irrigation with pond effluent and its interaction with applied fertilizer in a field experiment using French bean and kale over two growing seasons. Comparisons were made with fresh canal water that had not been cycled through tilapia ponds. Drip irrigation was chosen as the irrigation application method owing to its water conservation benefits in an arid climate such as Kenyas. Fresh and dry matter yields of the crops were recorded at harvest, and samples were collected for determination of tissue nutrient concentration.
In the first season, French bean fresh pod yield for plots receiving canal water and fertilizer at recommended rates had the highest yield (9.1 metric tonnes of fresh pod per hectare) while those receiving no fertilizer or irrigation had the least yield (1.3 metric tonnes fresh pod per hectare). In the second season, differences were observed among treatments in fresh bean pod and fresh kale leaf yield. The highest (4.4 metric tonnes per hectare) fresh pod yield was observed in pond effluentirrigated and fertilized plots while the lowest (1.3 metric tonnes per hectare) was observed in non-irrigated, unfertilized plots. The highest fresh kale leaf yield (11.5 metric tonnes per hectare) was obtained with irrigation with canal water combined with fertilizer application while the lowest (4.2 metric tonnes per hectare) was observed in non-irrigated, unfertilized plots.
Low nutrient status in the tilapia pond water together with inadequate water supplied to some crops due to emitter clogging was responsible for low yield where pond water was substituted for canal water. Emitter clogging was due to algae contained in pond effluent, and filters to eliminate clogging are necessary when pond effluent is used in a drip irrigation system. Unlike other research previously mentioned, pond water from the Sagana Fish Farm supplied low amounts of N and P for crops, indicating that recommended rates of mineral fertilizers should be used when pond water is used for irrigation.
In summary, application of chemical fertilizers in ponds and activities of fish increase nutrient concentration of pond water. Application of pond water to crops during fish grow-out is feasible, but filters capable of removing particulates will be required if it is to be delivered through a drip irrigation system. CRSP research suggests that nutrient enrichment of pond water during aquaculture production is insufficient to meet crop nutrient demand, and fertilizer recommendations for crops should not be altered when pond water is used as an irrigation source.
Wood is a Professor in the Department of Agronomy and Soils at Auburn University; Meso is a former graduate student in the Department of Soil Science at the University of Nairobi; Veverica is a Research Associate in the Department of Fisheries and Allied Aquaculture at Auburn University and was formerly located at the Sagana Fish Farm in Sagana Kenya; Nancy Karanja is Head of the Department of Soil Science at the University of Nairobi.