Where the natural pond productivity is enhanced through fertilizer application, reasonable production can be achieved without exogenous feeding. Although yields will be lower than those obtained with exogenous feeding, fertilizer application will reduce the quantity and expense of feeding.
Application of an inorganic fertilizer high in phosphorus should be done prior to stocking fish to create an algal bloom. Tilapia productivity is stimulated mainly by an increase in phosphorus and to a lesser extent by an increase in nitrogen. The inorganic fertilizers used in Kenya are DAP and CAN.
Di Ammonium Phosphate (DAP), Mono Ammonium Phosphate (MAP) and Urea are the cheaper sources of nutrients.
- For Phosphorus; DAP or TSP
- For Nitrogen; UREA
a) Methods of applying chemical fertilizers:
Dissolve the fertilizer in a bucket of water by stirring, and then sprinkle the solution at different points of the pond. If you throw the fertilizer in while dry, it will sink and some of the nutrients, especially phosphorus will be absorbed by the mud.
Application Rates for chemical fertilizers recommended in Kenya:
D.A.P: 2g/m2 every week i.e. 200g per 100m2 per week.
UREA: 3g/m2/week i.e. 300g/100m2/week.
Animal manure is widely used in Kenya in fish production in earthen ponds. The quality of manure as a fertilizer varies depending on the source animal and the quality of feed fed to the animal. Pig, chicken and duck manures increase fish production more than cow and sheep manure.
Accumulations of manure on the pond bottom produce low oxygen conditions (during decomposition) in the sediment resulting to reduced microbial activity and sometimes result in the sudden release of toxic chemicals into the water.
b) Methods and rates of applying manure
- Crib method: A compost crib constructed using wooden sticks at one or more sides of the pond. It helps fertilize the water gradually. The manure in the crib requires frequent turning to facilitate the release of nutrients.
- Bag method: A bag is filled with manure and tied to the corner of the pond. The bag is shaken weekly or daily to release nutrients.
Manure application rates depend on the size of the pond, which is expressed as surface area of the water in the pond. The recommended rate is 50g of dry matter per m2 per week i.e. 5Kg/100m2/week.
The maximum rate depends on the quality of the manure, the oxygen supply in the pond and water temperature. If early morning DO is very low (fish tend to breathe through the water surface) or the water temperatures are less than 18°C, manure application should be discontinued.
At low temperatures the rate of decomposition decreases and manure may accumulate on the pond bottom. A subsequent increase in temperature could then result in oxygen depletion.
c) Agricultural lime.
- Used to improve soil quality, which helps the organic and chemical fertilizers to work better. It also helps to clear up muddy water.
- In red soils; about 20kg per 100m2 can be applied. Black cotton soils may require a little more.
Some characteristics of organic and chemical fertilizers:
| Organic (Farm manure) | Chemical: – DAP, Urea, MAP, TSP |
| Contains trace minerals and vitamins. | Contains only what the label says |
| Uses oxygen to decompose. | Does not use oxygen when dissolving |
| Is highly variable in composition depending on feeds given to the animals and bedding used | Varies little in composition from what is indicated on the label. |
| Can help reduce turbidity due to clay silt in the ponds | Does not reduce turbidity |
| Can help reduce seepage in ponds | Does not act on seepage |
| Some of the ingredients can be consumed directly by the fish | Not directly consumed by the fish |
Article by NAFIS