Use of Pond Effluents for Irrigation in an Integrated Crop/Aquaculture System

3) Produce integrated water and nutrient budgets for fish ponds/crop systems.

Significance
The second planting season in Kenya begins in about October and continues through December. Rains can be quite unreliable and in the last decade, second season crops have failed 40 to 60% of the time. Farmers typically plant early-maturing maize which matures in 90 days to reduce their risks of crop failure. These varieties typically provide only 50% of yields of the later maturing varieties but they are less prone to losses from drought. Irrigation can provide the water needed to allow farmers to plant high-yielding crops. Farming fish in irrigation reservoirs can provide additional revenues. The majority of irrigation in Kenya is practiced by small scale irrigators (16,700 ha) versus 9,023 ha for large irrigation schemes. Eighty percent of Kenya’s land falls in the ASAL regions (arid and semi-arid). The Ministry of Land Reclamation, Regional and Water Development seeks to promote multiple uses of irrigation waters (Government of Kenya, 1996).

Aquaculture in Kenya is practiced in water retention reservoirs and in fish ponds. While the former make up a greater surface area, the latter tend to be much more productive. One reason is that farmers who use reservoirs for crop irrigation are hesitant to add fertilizers to reservoir ponds because there are no guidelines for this and no benefit to crops has been demonstrated. On the other hand, farmers who wish to intensify their aquaculture production may be interested in knowing that the water from their ponds can have beneficial effects on their crops in addition to the advantages arising from having a source of water. Soils in much of Kenya and eastern and central Africa tend to be very limited in available phosphorus and have high phosphorus fixing ability. Effluents from fertilized ponds can have high phosphorus concentrations and be potential sources of pollution. It is therefore advantageous to use one system’s waste as another system’s resource.

Fisheries Department staff in Kenya tend to function on their own with little input and advice from the agriculture sector because each answers to a different Ministry. A collaboration between agriculture researchers and fish-farming researchers can help each sector understand and incorporate the other into their line of thinking. Working through university professors assures that experiences gained by professors will be transferred to numerous students along the way. Kenyan counterparts will be instrumental in carrying out the experiment, interpreting the results, and transferring the information to user groups.

Anticipated Benefits
The value of this research is in the demonstration that fish farming and irrigation can be complementary as opposed to competitive. Water requirements for ponds at Sagana will be known from previous work on water budgets. This trial will demonstrate some possible beneficial effects of fertilizing water retention reservoirs that are used solely for irrigation. It may also show the benefits of passing pond effluents through farm land. Comparative water budgets and cost/benefit analyses for fish culture and irrigation will allow irrigation planners to assess the benefits of adding fish culture activities to irrigation schemes. (A rough estimate of cost of fertilizers and anticipated fish yields was made to make sure fish yield can more than cover the cost of fertilizers applied to ponds.)

Activity Plan
Location: Sagana fish farm, Kenya; Faculty of Agriculture, Dept. of Soil Science, University of Nairobi (greenhouse trials)

Methods: The experimental design will be a split plot with levels of P in pond effluents as main plots.The main treatment will be P added to water used for irrigation. Different P levels in the pond water will be obtained through fertilization with P at different rates. Although pond soils at Sagana are over 80% clay and have a high P-fixing capacity, they will have been saturated with P prior to use for this experiment.

This will result in 6 x 3 reps = 18 plots, of about 25 m² each. Each main plot will be split into subplots consisting of crop varieties recommended for the Sagana area:

This results in 18 plots x 3 subplots/plot = 54 subplots. Each subplot will consist of four rows of
5 meters each. The two outer rows will be guard rows and will serve for pre-harvest biomass sampling. The two inner rows will be used for yield measurements.

Pre-harvest biomass sampling: maize ear leaf samples at silking, and uppermost mature leaves of soy at the R1 (first flower) stage of growth. Maize whole plant samples and soy whole plant samples at R1 will also be collected. Leaf samples of maze and soy will be analyzed for P and other nutrients to determine sufficiency/deficiency. Whole plant samples will be used to assess mid-season nutrient uptake.

Harvest Sampling: grain yield and grain subsamples for protein and nutrient analyses.

Soil Sampling: surface soil (15 cm) will be sampled prior to planting and after harvest for analysis of organic matter, extractable P (Olsen method, depending on pH), and extractable nitrate and ammonium.

Subplots (except Control 1) will be irrigated by the furrow method, following standard irrigation recommendations determined by weekly sampling in all subplots of that treatment. Tensiometers will be used to assess soil moisture. However, if earlier trials on drip irrigation demonstrate that it is feasible for these crops and with algae-rich water, drip irrigation will be used instead of furrow irrigation. The reason for favoring drip irrigation is that the irrigation scientists currently recommend drip methods as the most water efficient. Planting and weeding will be done according to accepted methods for this area.

Pond Fertilization: Ponds will be limed prior to filling at the rate determined necessary from following the general lime requirement (Boyd, 1994). DAP will be the sole source of P. Nitrogen will be provided at a rate determined to be optimal for this site and for warm season from previous CRSP research, probably about 20 kg N per ha per week. Nitrogen will be provided by DAP and urea.

Water Movement: Water required for irrigation is anticipated as follows: At 500 mm water added as irrigation, and 75 m² total plot area per treatment use 0.375 m³ or 375 liters per treatment. If furrows are used, more than double this amount will be needed. The current drip irrigation method recommended involves use of a 70-liter plastic barrel (cost $8 each) and hose pipe. The barrel can be filled by siphoning from the ponds. Given the total pond volume (3 ponds x 800 m2/pond x 0.8 m average depth = 1,920 m³), water use will be minimal.

Sampling: Water, soils, fish, and crops will be sampled in order to make nitrogen and phosphorus budgets for the pond/crop system. Data needed for N and P budgets will also serve to make water budgets.

A complete weather station with dataloggers will be in place at the site to provide max and min air temperature, relative humidity, solar radiation (as PAR and watts), precipitation, and wind speed data. An evaporation pan is in use at Sagana station.

Fish sampling: Fish will be sampled by seine net at biweekly intervals to measure growth; approximately 10% of the initial stock will be seined up, counted, and weighed en masse. Ponds will be harvested by draining upon completion of the trial.

Concurrent greenhouse trials using different soils from Kenya will be used to assess effects treatments used in the field experiment could have on other soils. Water from the ponds will be used.

Regional Integration
Making water budget information available to policy makers is an area of activity recommended in the regional plan for Africa. The test crops are grown throughout southern, eastern and central Africa; maize is a staple from southern Africa to Kenya and soy is a rather new but promising crop being promoted by several agriculture programs. A soy irrigation scheme has just started about 30 km from Sagana. Soy is an important ingredient of animal feeds and its low production level has been a leading cause of high costing animal feeds in Africa.

Schedule
Pond filling and stocking will be done in early November 1998. It is anticipated that ponds will be drained after about 5 to 6 months (April 1999) Planting of crops will occur in October and crop harvest will be in January to February 1999. This schedule should provide the water necessary at the most critical times for crop production. The experiment will be repeated from May to October 1999. This is often not a rain-limited season but expectations are for a drought in 1999. Planting will be done in mid-May and harvest is usually in August. Report on first harvest due May 1999. Report on second harvests due February 2000.

References
Government of Kenya, 1996. National Development Plan 1997-2001. Government Printer, Nairobi, Kenya.