Larry W. Dorman
A proper fertilization program is essential for maximum fish production in ponds. Fertilization feeds phytoplankton which causes a "bloom" when it grows. Fish eat the "bloom," thus production, carrying capacity and fish yields increase.
A good fertilization program is costly in terms of labor and materials. The various forms of granular fertilizers vary in effectiveness, cost and ease of application. Some inconvenience can perhaps be eliminated by the use of liquid fertilizer. The liquid form is more available for plant uptake compared to the less soluble granular fertilizer.
Liquid vs. Granular Fertilizer
Recent research comparing granular and liquid fertilizers reveals the following:
1. Phosphorus dissolves from granular fertilizer very slowly. Only 15 percent of the phosphorus dissolves from granules through a six and one-half foot column of water. The remaining 85 percent dissolves on the pond bottom, is absorbed by the muds and is unavailable for phytoplankton production. Solubility is reduced by cold temperatures and fertilization is not recommended below 65 degrees Fahrenheit.
2. Nitrogen is not as important as phosphorus in fish ponds.
3. Liquid phosphate fertilizer is superior to granular triple superphosphate as a fertilizer because all the phosphorus from the liquid immediately dissolves in the surface water and is directly available to phytoplankton.
4. Liquid phosphate fertilizer provides higher concentrations of available orthophosphate in ponds than granular triple superphosphate at the same rate of P2O5 (phosphorus pentoxide) equivalent. Orthophosphate is the form of phosphorus that phytoplankton use for growth.
5. Liquid phosphate fertilizer provides higher initial levels of inorganic phosphorus in water than does diammonium phosphate or triple superphosphate at the same application rate.
Grades of Liquids Available
The most available grade of liquid phosphate fertilizer in Arkansas is 10-30-0. In other states 10-34-0 and 13-37-0 are available, as well as liquid ammonium orthophosphate as 13-38-0.
The first number in the grade is the percentage of nitrogen as N. The second number is the percentage of phosphorus as P2O5 (phosphorus pentoxide). In liquid form, the nutrients are in a 1:3 ratio. Any grade of liquid fertilizer can be used as long as 3 to 4 pounds per acre of phosphorus as P2O5 are used per application. This is equivalent to one gallon per acre per application.
Application Methods
Liquid fertilizers are heavy, weighing about 12 pounds per gallon. Water weighs 8.3 pounds per gallon. It is essential to dilute liquid fertilizer with water or to apply into turbulence. Otherwise the fertilizer sinks to the bottom and is absorbed by the mud. This diluting takes much less time than dissolving granular fertilizer would. Granules always sink and a large portion is lost to the sediment (mud) at the pond bottom.
Methods of dispersal include the following: Pour the fertilizer into a container at the pond's edge, dilute with at least 5 parts water, and splash onto the pond's surface.
Also, a pump can be used. Pump the water into a container to dilute the fertilizer, then pump solution out onto the pond.
Another method is to drip the fertilizer from the front of a boat and let the turbulence from the motor mix it.
Also, boat bailers are available to siphon the fertilizer mixture from a container into the propeller wash.
Fertilization Schedule
1. Make the first one gallon per acre application in mid-March when water temperature is consistently over 65 degrees Fahrenheit. Follow this with two additional applications at two-week intervals.
2. Make three more applications at three-week intervals.
3. Continue applications at monthly intervals or whenever the depth of light penetrations are greater than 18 inches. (Note: To measure light penetration, attach a white disk to a yardstick then lower the yardstick into the water until the disk disappears. This represents the depth of light penetrations.)
4. Discontinue fertilization by September. If pond is supplementally fed, then by late spring the feeding rates are increased and fertilization won't be necessary. The nutrients necessary for the plankton are in the fish's waste and the uneaten food.
Special Considerations
1. If you are going to fertilizer, do it continuously. Fertilizing once or twice a year is worse than not fertilizer at all. It's like alternately feeding, then starving fish.
2. If you fertilized the previous year, then fertilize this year. The fertilizer the year before increased the pond's carrying capacity. Those extra fish will starve if the pond is not fertilized.
3. If there is a vegetation problem in the pond, kill the vegetation with a recommended herbicide before beginning a fertilization program. Don't broadcast fertilizer over the vegetation, it will only grow larger.
4. If you can't get a "bloom" after fertilizing, check the following:
a. Lime requirement – sometimes lime is needed to activate fertilizer.
b. Excess outflow of water – this may dilute your fertilizer.
c. Weeds – these absorb the fertilizer and it's not available to the "bloom."
d. Muddy water – particles will bind to the fertilizer and tie it up, so the "bloom" can't use it.
5. Sometimes the "bloom" is difficult to initiate or reestablish even after potential problems are eliminated. Temperature, inadequate light and other weather-related factors may be at fault. Continue fertilizing every two weeks until the desired "bloom" occurs.
LARRY W. DORMAN is Extension fisheries specialist, University of Arkansas at Pine Bluff. His office is in Lonoke.
FSA9005R-3M-7-93-S493
Issued in furtherance of Extension work, Act of September 29, 1977, in cooperation with the U.S. Department of Agriculture, Dr. Mazo Price, Director, Cooperative Extension Program, University of Arkansas at Pine Bluff. The Arkansas Cooperative Extension Program offers its programs to all eligible persons regardless of race, color, national origin, sex, age, or disability, and is an Equal Opportunity Employer.