Pond Construction, Soil and Water Quality Management
Site and design rules
Choose a site with clayey loam soil holding 20-30% clay so that it retains water, with a gentle slope allowing gravity drainage, a perennial water source and no history of flooding. Standard dimensions: water depth 1.5-2.0 m, freeboard 0.3-0.5 m above the water line, dyke top width 1.0-1.5 m, inner side slope 1:1.5 to 1:2, and separate inlet and outlet with a screen and a monk or sluice for draining. Nursery ponds are small, 0.05-0.1 ha, so that seed can be harvested easily; rearing and stocking ponds are larger.
Soil management
Test soil pH before the first liming. Typical agricultural lime doses are 400-500 kg/ha below pH 5.5, 200-300 kg/ha at pH 5.5-6.5 and 100-200 kg/ha above pH 6.5. Remove excess bottom silt each cycle, because accumulated organic sludge consumes oxygen. Dry and plough the bed to oxidise the soil between crops.
Water quality standards to quote
- Temperature 26-32 degrees C for carps.
- pH 6.5-8.5, with the best growth near 7.5.
- Dissolved oxygen above 5 mg/L; below 3 mg/L fish gasp at the surface.
- Total alkalinity 50-200 mg/L as CaCO3, hardness 50-200 mg/L.
- Unionised ammonia below 0.1 mg/L; nitrite below 0.5 mg/L.
- Secchi transparency 30-40 cm, indicating healthy plankton.
Worked example
A pond of 40 m x 25 m has water depth 1.5 m and soil pH 6.0. Water area = 1000 m2 = 0.1 ha; water volume = 1000 x 1.5 = 1500 m3. For pH 6.0 the lime dose is 200-300 kg/ha, so use 300 x 0.1 = 30 kg of lime, spread on the dry bed and dykes about a week before filling, not on the same day as manuring.
Traps that cost marks
- Liming and manuring simultaneously, which knocks out the phosphate and can raise pH into the ammonia-toxic range.
- Reading a high Secchi value, above 60 cm, as good water; it means sparse plankton.
- Ignoring that ammonia toxicity rises with rising pH and temperature.