In aquaculture, maintaining a balanced ecosystem is crucial for maximizing fish growth and yield. For fish farmers, urea is an indispensable tool. This versatile nitrogen-rich compound plays a vital role in enhancing pond productivity.
However, urea is a double-edged sword. When used correctly, it can significantly increase fish yield; however, when misused, it may lead to the death of the entire fish population within a few hours. How is urea used in fish ponds, and what are its benefits? This article explores the application, benefits, and best practices of using urea in fish pond management.
Urea (NH₂)₂CO) is an inorganic fertilizer with extremely high nitrogen content (about 46%). Its primary function in fish ponds is to provide nitrogen and act as a nutritional supplement to support the growth of phytoplankton.
Phytoplankton are tiny algae that survive on nitrogen and phosphorus. They are the food of planktonic animals, which are then eaten by small fish, forming a natural feeding cycle that sustains larger fish populations. By providing a concentrated nitrogen source, urea stimulates the growth of phytoplankton, ensuring a stable food supply for fish and improving the overall productivity of the pond.
Firstly, we must understand that we are not “feeding the fish”, but rather “cultivating the food for the fish”. Fish ponds are a microcosm, and the foundation of their food chain is phytoplankton—the tiny algae in the water.
Fertilization: Adding key nutrients such as nitrogen (N) and phosphorus (P) to water.
Phytoplankton reproduction: These nutrients, with the help of sunlight, can promote the massive reproduction of phytoplankton. This process is called “algal bloom” or “algal bloom”. The water presents a healthy, green or brownish-green colour.
Growth of zooplankton: Phytoplankton is the primary food source for zooplankton.
Fish feeding: Many fish (especially fry and filter-feeding fish) feed on planktonic animals and some planktonic plants.
By fertilizing, we have significantly increased the primary productivity of the fish pond and created a natural, nutrient-rich ‘self-service restaurant’ for the fish.
This is the most direct and rapid danger.
Chemical process: Urea is broken down by urease in water, and it is quickly converted into ammonia (NH3).
Toxicity principle: Ammonia is highly toxic to fish. It can damage the gill tissue of fish, affect oxygen exchange, damage the nervous system, and damage organs. High concentrations of ammonia can cause large-scale deaths in a short period.
The influence of pH and temperature: The higher the pH and temperature of water, the higher the proportion of toxic free ammonia (NH3) in total ammonia nitrogen, and the stronger the toxicity. Applying urea in high-pH ponds is particularly hazardous on hot afternoons.
This is another common fatal cause, usually occurring within a few days after fertilization.
Process:
Overfertilization leads to excessive reproduction of phytoplankton (“concentrated green water”).
At night or on cloudy days, all algae undergo respiration and consume a large amount of oxygen.
Algae have a life cycle, and after a large number of deaths, they sink to the bottom of the water.
The bacteria that decompose dead algae will consume almost all of the dissolved oxygen in the water.
Result: Fish suffocate and die due to lack of oxygen, usually before dawn when the dissolved oxygen in the water is at its lowest.
If you decide to use urea after fully understanding the risks, you must strictly follow the following procedures:
Not all ponds require fertilization. Use a Secchi disk to determine. Suppose the transparency is greater than 60 centimetres (24 inches). In that case, it indicates that the water is too thin and there is insufficient phytoplankton, suggesting that fertilisation may be a viable option. Suppose the transparency is between 30 and 45 centimetres (12 and 18 inches). In that case, it indicates that the water is moderately fertile and does not require fertilization.
Never fertilize based on intuition!
Initial fertilisation: 2-3 kilograms of urea and 5-8 kilograms of phosphorus fertiliser (such as superphosphate) can be applied per acre (1 meter deep).
Topdressing: Depending on the water quality, apply 1-2 kilograms of urea per acre every 10-15 days (in combination with phosphate fertilizer).
Key principle: Better to have less than more, first halving. Starting from the low or even half of the recommended dosage, observe the changes in water colour and gradually adjust.
Time: Apply on a sunny morning between 9:00 and 10:00 a.m. This ensures that algae have sufficient sunlight for photosynthesis, quickly utilizing nutrients and producing oxygen. Avoid use in rainy or hot weather (prone to hypoxia).
Water temperature impact: Reduce the amount appropriately during high-temperature seasons (above 25 ℃). During low-temperature seasons (below 15 ℃), decomposition is slow, and the frequency of the process needs to be reduced.
Water quality testing: Before fertilization, it is essential to test the water’s ammonia nitrogen and nitrite content. If ammonia nitrogen is greater than 0.5mg/L, the use of urea is prohibited.
Application method: It is strictly prohibited to directly sprinkle urea particles into the pond! This will result in lethal ammonia concentrations in the local area. The correct approach is:
Dissolve the calculated urea in the bucket thoroughly with pool water.
Evenly sprinkle the dissolved fertilizer water over the shallow and upwind areas of the pond to ensure even diffusion.
After fertilization, it is necessary to closely monitor changes in water quality:
Water colour: Observe whether the water colour turns into a healthy light green within a few days. If it turns dark green, blue-green, or has an odour, it indicates excessive fertilization.
Fish activity: Observe whether fish show floating heads (signs of hypoxia) at dawn. If it occurs, immediately start the aerator and prepare to change the water.
PH value: Regularly check the pH value to avoid drastic fluctuations.
Considering the high risk of urea, breeders can consider safer and more stable alternative solutions:
Fermented organic fertilizers such as chicken manure, duck manure, etc. They release nutrients slowly, are less likely to cause drastic changes in water quality, and can cultivate more beneficial bacterial communities.However, it needs to be thoroughly fermented; otherwise, it will consume oxygen and produce harmful substances.
Commercial compound fishery fertilizers: These products are scientifically proportioned to achieve a more balanced nitrogen-to-phosphorus ratio and controlled nutrient release, and are usually safer than using urea alone.
Ecological aquaculture models, such as fish-duck co-farming and fish-grass symbiosis, utilise natural ecological cycles to fertilise water bodies.
The use of urea in fish ponds is a high-return, yet high-risk technology that can improve productivity by supporting phytoplankton growth, enhancing the food web, and increasing fish production. When used appropriately, with the correct dosage, timing, and water quality monitoring, urea is a cost-effective tool for sustainable aquaculture. If you decide to use urea, you must approach every step with a scientific and cautious attitude, keeping the risks to a minimum level.
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