What are the functions of and precautions for fertilizing fish ponds?

Generally, fertilizing aquaculture ponds is to increase the quantity and quality of fish feed, and at the same time, it can also provide nutrients to the algae in the fish pond and maintain water quality and water body conditions. Because the fertilizer itself affects the pond environment, there are many precautions and taboos for fertilizing fish ponds. Only by choosing the right fertilizer type and method can the fertilizer achieve its proper effect and improve the quality of the fish in the pond.

I. Why fertilize in pond aquaculture

1. The plankton bred through fertilization can be filter-fed by fish, increasing the yield of filter-feeding fish such as silver carp and bighead carp; the bred plankton such as rotifers are suitable first feed for fry.

2. Through fertilization, sufficient nutrients for algae growth are provided, appropriate algae abundance is maintained, and photosynthesis is used to provide sufficient dissolved oxygen for the pond.

3. Through fertilization, the stability of pond water quality is maintained. Organic fertilizer has a certain chelating effect and can reduce the harm of heavy metal ions to fish, such as the amino acid water-fertilizing series.

4. Through water fertilization, maintaining a certain fertility in the water body can prevent the spread of aquatic plants and benthic filamentous algae, adsorb and reduce the sediment content in the water body, and help fish grow healthily.

5. Through fertilization, the water body maintains a good water color. Fish in water bodies with suitable water color have strong stress resistance and a sense of security.

II. Types of fertilizer

1. Organic fertilizers: organic fertilizers mainly include manure, green manure, barnyard manure, compost, etc. These fertilizers contain various inorganic salts such as nitrogen, phosphorus, and potassium as well as organic substances such as proteins, fats, and sugars. Their fertilizer effect is good and lasting, but they take effect relatively slowly after application, so they are also called “slow-release fertilizers”. However, organic fertilizers rot and decompose in water, consume oxygen in the water, and release toxic gases (hydrogen sulfide, ammonia, carbon dioxide, etc.) that have a certain impact on fish growth. When applying them, it is not suitable to apply large amounts of fresh organic fertilizer directly into the fish pond; instead, it should first be sealed, fermented, and decomposed before application.

2. Inorganic fertilizers are also called chemical fertilizers. The characteristic of this type of fertilizer is that the nutrients it contains are relatively single; most are one kind of chemical fertilizer containing only one or two nutrient components. When applied into water, it is easily decomposed and takes effect quickly, so it is also called “quick-acting fertilizer”. It includes nitrogen fertilizer, phosphorus fertilizer, potassium fertilizer, calcium fertilizer, etc.

3. Most fertilizers currently sold on the market are amino acid water-fertilizing liquids or water-fertilizing pastes, or biological bacterial fertilizers composed of inorganic fertilizers, organic fertilizers, and probiotics in proportion.

III. Precautions for fertilization

1. In the early stage of farming, organic fertilizer is mainly used, mainly as base fertilizer, and its fertilizer effect is lasting. In the later stage, inorganic fertilizer is mainly used, because a large amount of feed is put in during the later stage, and fish feces are a very good organic fertilizer. Applying inorganic fertilizer causes relatively light water pollution and has a quick fertilizer effect.

Fertilizer should be dissolved in water and splashed over the whole pond, following the principle of small amounts and multiple applications, appropriately controlled according to whether the pond is fertile or lean, and should be carried out on sunny mornings. Fertilization is not suitable when the water temperature is too high. The nitrogen-to-phosphorus ratio should be controlled at about 3:1. Organic fertilizer is prohibited during high-temperature periods to reduce oxygen-consuming factors.

2. The main factors affecting fertilizer efficiency are water temperature, pH, dissolved oxygen, bottom substrate, weather, and other factors, which should be adjusted according to specific conditions. For example, below 20°C water fertilization is slower, fertilizer efficiency is best at pH 7-8.5, fertilizer efficiency is good when dissolved oxygen is sufficient, loam soil is easier to fertilize than sandy soil, and sunny days are easier for water fertilization; do not fertilize in stuffy and rainy weather. Do not fertilize when fish disease occurs; intensively cultured ponds may not need additional fertilization.

3. In July-August, after the water color becomes darker, stop using nitrogen fertilizer and use phosphorus fertilizer alone. Ponds with relatively lean water quality and high ammonia nitrogen content should be applied with phosphorus fertilizer, using phosphorus to promote nitrogen and promote the growth of phytoplankton, absorbing ammonia nitrogen in the pond.

4. Change the nitrogen-to-phosphorus ratio of the water body through fertilization to control the reproduction of harmful algae (when the nitrogen-to-phosphorus ratio is close to or equal to 16:1, the outbreak of nitrogen-fixing blue-green algae can be effectively controlled. After killing blue-green algae, supplementing phosphorus fertilizer in small amounts and multiple times can also reduce blue-green algae outbreaks). Apply silicate fertilizer to promote the reproduction of beneficial algae such as diatoms and inhibit the outbreak of harmful algae such as blue-green algae.

5. When the water body ages, supplement fertilizer in time to restore algae vitality, and appropriately supplement insufficient carbon sources in the water body to promote the reproduction of beneficial bacteria, thereby promoting the fertilizer effect of the water body. The transparency of the water body should be maintained at 20-40 cm.

6. Apply calcium fertilizer in shrimp and crab ponds to promote molting and growth.

7. Grasping the timing and sequence of the peak of plankton reproduction after fertilization has important guiding significance for aquaculture production. The general sequence is: phytoplankton (4-5 days) ----- protozoa ----- rotifers (4-7 days), nauplii --- cladocerans (6-24 days) --- copepods (7-32 days). According to the peak appearance of plankton, fish fry can be stocked into the pond at the right time, with sufficient first feed, increasing the survival rate and saving feed.

IV. Ten taboos of fertilization

1. Fertilizing on rainy days. On rainy days, photosynthesis by phytoplankton in the water body is not strong, and the ability to absorb elements such as nitrogen and phosphorus is relatively poor; on rainy days the water volume is relatively large, and the effective concentration of fertilizer will decrease; during overflow, fertilizer loss is great.

2. Fertilizing on stuffy hot days. During stuffy hot weather, dissolved oxygen in the water is low. After fertilization, the oxygen consumption of organic matter in the water increases, which can easily cause fish to surface for air and cause pond turnover due to oxygen deficiency.

3. Fertilizing in high-temperature seasons. Fish pond fertilization should be carried out from April to October each year at noon on sunny days when the water temperature is 25°C-30°C. If the water temperature exceeds 30°C, fertilization should be stopped or reduced.

4. Fertilizing in muddy water. If the water body is excessively turbid, it indicates that there are too many clay mineral particles in the water body. Some fertilizer ions are easily adsorbed, fixed, and precipitated by clay particles, so the fertilizer effect cannot be released for a long time.

5. Fertilizing when fish feeding is not vigorous or when an epidemic disease breaks out. Fertilizing when fish feeding is not vigorous means that the large amount of plankton cultivated by the fertilizer cannot be used by the fish in time and easily damages water quality; when an epidemic disease breaks out, the resistance of the fish body weakens, and if more ammonium nitrogen fertilizer is applied, it is easy to poison the fish.

6. Dry application of solid fertilizer. Nitrogen and phosphorus fertilizers applied dry are granular, stay on the water surface for a relatively short time, and easily sink to the bottom, affecting fertilizer efficiency. Generally, when applying solid nitrogen and phosphorus fertilizers, they should be dissolved and then splashed over the whole pond with water.

7. Applying chemical fertilizer alone. The main purpose of fertilization is to cultivate plankton that fish can easily digest, and plankton generally requires a nitrogen, phosphorus, and potassium ratio of 4:4:2. If a certain chemical fertilizer is applied alone, it will restrict the full exertion of fertilizer efficiency.

8. Blind mixed application. Not any two or more fertilizers can be mixed for use. For example, if quicklime is applied, it is best to wait 10-15 days before applying superphosphate, so as to avoid a chemical reaction between the two that causes loss of fertilizer efficiency.

9. Applying too much fertilizer at one time. Excessive application of ammonium nitrogen fertilizer at one time will cause excessive accumulation of ammonia in the water body and poison the fish; excessive application of organic fertilizer at one time will increase the oxygen consumption of organic matter in the water body and cause fish hypoxia and pond turnover, so when fertilizing, never apply too much at one time. Generally, fertilization is required once every 3-5 days.

10. Releasing surface water after fertilization. After fertilizer is applied into the water body, it generally undergoes a series of physical and chemical reactions and can only be fully utilized by plankton after 3 days. Plankton populations are generally evenly distributed 1-2 cm deep on the water surface. If surface water is released after fertilization, fertilizer efficiency will be reduced.

V. Three methods of fish pond fertilization

1. Organic fertilizer topdressing method. Organic fertilizers include various human and livestock manures. Apply 100 kg per mu each time, once every 7-10 days, and either pile them in small heaps or spread them over the whole pond. A rack fertilization method can also be used. That is, use wooden poles as supports and willow twig mats as the base, build several racks in the deep-water area of the pond, place manure on the racks, and let it stay close to the water surface so that the fertilizer decomposes and diffuses in the middle and upper layers of the water. Bag hanging can also be used, that is, put the fertilizer in rot-resistant bags and hang them on wooden posts in the deep-water area of the pond close to the water surface to let them decompose and diffuse on their own. The rack and bag hanging fertilization methods can speed up fertilizer decomposition and allow the fertilizer to evenly diffuse throughout the fish pond. In particular, they can keep the fertilizer decomposing in the water layer of the pond, so although the fertilizer decomposes quickly, it will not cause oxygen deficiency and make fish surface for air.

2. Chemical fertilizer topdressing method. Fish ponds mainly use nitrogen and phosphorus fertilizers, with an application ratio of 1:2. Under normal circumstances, apply 2.5 kg of urea and 5 kg of phosphorus fertilizer per mu. First dissolve them in water, then evenly splash them over the whole pond, once every 5-7 days.

3. Green manure topdressing method. Green manure includes various terrestrial dry grasses. Sun-dry them until half dry, then mix in a certain amount of manure, pile them in a corner of the pond, press stones or soil on top, and thereafter turn them once every 3-5 days to let them rot and decompose. The liquid gradually diffuses into the pond. In autumn, it generally takes 7-9 days to decompose completely. Then remove the thick, hard-to-rot roots and stems, add new material, and apply 100 kg of green manure per mu each time.