ANR-249, New Dec. 1990. John W. Jensen, Extension Fisheries Specialist, Professor, Fisheries and Allied Aquacultures, Auburn University.

Fertilizing Fish Ponds

Farmers know that proper fertilization can greatly increase crop yields. Similarly, pond fertilization can increase fish yields three to four times. Also fish will be in better condition, and catch by fishermen usually improves. Fertilizers used in ponds stimulate the growth of microscopic plants called algae. Algae are eaten by microscopic animals (zooplankton) and insects which serve as food for bream, which in turn are eaten by bass. Algae also make the water turn green, which helps to shade the pond bottom, preventing growth of troublesome rooted weeds and filamentous algae commonly called pond "moss."

Types and Grades of Fertilizer

Fertilizer manufacturers are required to list the grade on each fertilizer container by the percent of nitrogen (N), phosphorus (P) as phosphoric acid (P2O5) and potassium (K) as potassium monoxide (K2O). Therefore, a 20-20-5 grade fertilizer contains 20 percent nitrogen, 20 percent phosphorus as P2O5 and 5 percent potassium as K2O. "Complete" fertilizers contain N, P2O5, and K2O while "incomplete" fertilizers contain only one or two of these elements. Common incomplete fertilizer sources are normal superphosphate (0-20-0), triple superphosphate (0-46-0), diammonium phosphate (18-46-0) and liquid ammonium polyphosphate (10-34-0). Examples of complete fertilizers are 13-13-13, 20-20-5, and 4-12-12.

Phosphorus usually gives a much greater increase in fish production than either nitrogen or potassium. However, nitrogen in combination with the phosphorus is sometimes better than phosphorus added alone.

Liquid fertilizers are superior to granular fertilizers in promoting rapid growth of algae in farm ponds. This is because the nutrients are already in solution, making them more rapidly available to the algae. Also, smaller applications of liquid fertilizers can be used, which may reduce the cost of pond fertilization.

Common grades of liquid ammonium polyphosphate fertilizer are 10-34-0 and 11-37-0. Liquid ammonium orthophosphate, commonly sold as a 13-38-0 grade, can also be used. Nutrients are found in a ratio of about 1:3 in liquid fertilizers. Any grade of liquid fertilizer can be used, as long as approximately 3 to 4 pounds of phosphorus as P2O5 are applied per application.

Standard Fertilization Schedule

The following standard fertilization schedule can be used with any of the fertilizers and rates recommended in the fertilization table.

1. Make the first application of fertilizer in late February or early March. Follow 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 water clears enough that a white disk attached to a yardstick is visible to a depth of 18 inches.
4. Stop applications by the last week in October.

Because watershed fertility and pond uses vary, the standard fertilization schedule and recommended rates are not necessarily the most efficient for all ponds. The following considerations can help you modify them to meet your needs.

Pond Use

Not all ponds have to be fertilized. Large unfertilized ponds which are fished by only a few people may produce excellent fishing. Heavily fished ponds, such as club ponds, should generally be fertilized. Sometimes less fertilizer is needed in ponds in watersheds where cattle are grazing, due to nutrients from droppings.

Application Methods

Liquid fertilizers weigh about half again as much as water. Because they are heavier than water, they must be diluted with water or applied into turbulence so that they mix with the pond water and don't sink to the bottom.

One method of application of liquid fertilizer is to drip it into the water from the bow of a boat driven by an outboard motor so that the wake of the boat and the action of the propeller will mix fertilizer into the water before it has a chance to sink. It can also be applied by pouring it directly into the turbulence caused by the outboard motor.

Another method of application is to pour the liquid fertilizer into a wash tub, or similar container, at the pond's edge. Dilute the fertilizer by mixing more than 10 parts of water with it and slosh or splash the mixture onto the pond surface. A small pump can also be used to dilute the fertilizer with water and then pump the solution out onto the pond. This method is best for ponds smaller than 2 acres.

Liquid fertilizer can be applied full strength using a garden sprayer. Direct the spray onto the pond surface while walking around the pond. With some sprayers the fertilizer must be diluted with water so that it will pass through the nozzle.

Especially in late spring and summer, liquid fertilizer may be used more efficiently and produce a more stable "bloom" if applied at half the recommended rate, but twice as often as the standard fertilization schedule calls for. For example, you can apply 5 pounds per acre at two-week intervals in summer, instead of 10 pounds at monthly intervals.

Keep granular forms of fertilizer from direct contact with the pond mud, because phosphorus in the fertilizer becomes trapped in the mud and unavailable to the algae. A fertilizer platform positioned one foot under water can be used to hold granular fertilizer off the bottom. Pour fertilizer or place open bags on the platform. The fertilizer will slowly dissolve and be distributed by water currents. A platform measuring 45 square feet is adequate for a 5- to 10-acre pond. Smaller platforms can be used in smaller ponds. More than one platform should be used in ponds larger than 15 acres.

Fertilizer granules can be broadcast in shallow water, but this method is not as good as the platform method. Never broadcast fertilizer granules in deep water.

Excess Water Flow

A large continuous flow of water through the emergency spillway or standpipe will flush fertilizer from the pond. If the volume of water flowing out of the pond during any month of spring or summer is more than the total pond volume, fertilization is usually ineffective. Some ponds have heavy outflow only during winter and early spring, and respond well to fertilizer during drier weather. Constructing diversion ditches, enlarging the pond and/or constructing another pond above the existing one are techniques which can help to reduce excessive water outflow.

Muddy Water

When muddy water reduces visibility to less than 12 inches, algae respond poorly to fertilizer because the sunlight they need for growth is blocked. Therefore, fertilizing muddy ponds is usually ineffective. The following methods can be used to clear muddy water, but are only temporary until erosion of the watershed is stopped.

1. Apply barnyard manure at the rate of 1 ton per acre at 3-week intervals. Use extreme caution, however, as an oxygen depletion may result.
2. Apply 75 pounds of cottonseed meal with 10 pounds of triple superphosphate per acre at 2- to 3-week intervals.
3. Apply filter alum as recommended by your county Extension agent.

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Pond Weeds

Do not fertilize weedy ponds--the fertilizer will only make the weeds grow faster. Weeds can be controlled by using chemicals or the grass carp (white amur) fish. However, fertilizing in early spring before weeds are established may prevent their growth. Ask your county Extension agent for publication ANR-48, "Chemical Weed Control in Lakes and Farm Ponds", for methods to control pond weeds.

Liming

Ponds with soft, acid water may not respond to fertilizer. If the water does not turn green after 6 to 8 weeks of fertilization, liming may be necessary. Agricultural limestone will increase water hardness and alkalinity and decrease acidity, thereby increasing the effectiveness of fertilizer. A pond soil sample is needed to determine the lime requirement. Ask your county Extension agent for publication ANR-232, "Liming Fish Ponds", for soil sampling instructions and liming information.

Measuring Algae Density

Intervals between fertilizer applications can be determined by measuring algae density. This method may be more economical than strictly following the fixed intervals of the standard fertilization schedule.

Measure the algae density using a solid white disk or one painted with alternate black and white quadrants. The depth at which the disk disappears from view is the water visibility. When the visibility is more than 18 inches it is time to fertilize. Do not fertilize when visibility is less than 12 inches. A visibility of less than 6 inches is a warning that an oxygen depletion may occur. Do not use the visibility method when the water becomes temporarily muddy.

Reminders

1. Do not practice haphazard fertilization. Fertilizing once or twice a year is worse than not fertilizing at all. It's like alternately feeding and starving the fish.
2. Do not attempt to kill weeds by broadcasting fertilizer over them during the spring and summer. This can have the same effect as adding gasoline to a fire.
3. If algae will not grow after fertilizing, check for:
   • Lime requirement
   • Excess water outflow
   • Weeds
   • Muddy water
4. Sometimes algae "blooms" are difficult to initiate or reestablish even after potential problems have been eliminated. Temperature, clouds and possibly other weather-related factors may be the cause. Continue fertilizing every two weeks until the desired "bloom" occurs.

Appreciation is expressed to John Flynn, Graduate Extension Aide, for assistance in preparing this publication.