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Use a Secchi disk to measure water transparency

Fertilization and Liming is part four of seven in the Management of Recreational Fish Ponds series.

Fertilization provides planktonic algae with nutrients for growth much the same as fertilizing pasture increases grass yields. Proper fertilization increases available food throughout the food chain, thus increasing the amount of fish the pond supports.

Fertilization, however, will not stimulate a good algae bloom if total alkalinity of the water is below 20 ppm. Check alkalinity first. If alkalinity is below 20 ppm, add agricultural limestone to neutralize acidity in the soils. Do not use quick or slaked lime; these will cause a rapid pH change that may kill fish. The amount of lime necessary depends on the characteristics of mud in the pond bottom. A mud sample should be analyzed to determine the amount of lime.

Take mud samples from many places in the pond. Combine these samples and spread them out to dry. After samples are dry, mix them together thoroughly and take one sample for analysis. Send this sample to the Auburn University Soil Testing Lab in a soil-test box (available from your county Extension office). Mark the sample “fish pond” so that the proper tests can be run. The analysis will recommend the proper liming rate.

Lime application using a pontoon barge

Lime application using a pontoon barge

Lime application using a pontoon boat and a water pump to evenly distribute the lime

Lime application using a pontoon boat and a water pump to evenly distribute the lime

Lime must be applied evenly over the entire pond so that it can react with the bottom mud. If the pond is thoroughly dry, a spreader truck could distribute the lime. If the pond is full, however, the lime will have to be shoveled or washed into the pond from a boat. Several pond management consultants in Alabama will lime ponds at a modest cost. Ask your county Extension office for a list of Alabama Fisheries Consultants.

Lime slowly dissolves into the pond water and is washed out with overflow water. This means that ponds usually need to be relimed every 2 to 4 years. Many pond managers find it practical to increase the liming rate by one and a half or two times the amount recommended. This increases the length of time between lime applications. Some managers reapply half the recommended lime every 2 years to maintain alkalinity. Adding more than the recommended lime (agricultural lime only) will not harm the pond. A typical liming rate in Alabama is around 2 tons per surface acre of pond. Remember: if a pond needs lime it will not respond well to fertilizer.

Fertilizing ponds will increase fish production two- to threefold. Infertile ponds will seldom produce more than 100 pounds of fish per acre. Well-managed fertile ponds can maintain 300 to 400 pounds of fish per acre. If, however, the pond is naturally fertile and is not going to receive much fishing pressure, it may not require fertilizer. If the pond receives only minor fishing (or harvest) pressure, do not fertilize or fertilize at only half the recommended rate.

Once fertilization is started it should be continued. If fertilization is stopped the fish will stunt because of the reduced food supply, and they become more susceptible to disease.

Not all fertilizers work well in ponds. Phosphorus is the nutrient most needed in ponds. Given time, the phosphorus will be absorbed and trapped in the mud of the pond through chemical processes. Once trapped, it is not available to planktonic algae but can promote the growth of weeds and filamentous algae. Nitrogen can also be limiting in ponds. Lack of available nitrogen can lead to blooms of bluegreen algae (cyanobacteria) some of which can convert nitrogen from the air (nitrogen fixation) to usable nitrate.  These bluegreen algae can form unsightly surface scums and toxins that can make pets, livestock, and even people sick.  To help encourage more acceptable forms of algae, we recommend using fertilizers with both phosphorus and nitrogen.

Fertilizers are labeled with N-P-K ratios or percents of nitrogen (N), phosphorus (P205), and potassium (K2O). The equivalent of 8 pounds of granular — 4 pounds of liquid — phosphorous per acre per application is the commonly recommended rate.  Granular fertilizer should applied on a platform a few inches beneath the surface where it can be held off the bottom and allowed to dissolve slowly. Liquid fertilizers can be easier to apply and may produce blooms quicker than granular fertilizers. With liquid fertilizer, always dilute the fertilizer with 2-3 parts water and then slosh it out or dribble in the prop wash of a boat.. Soluble powdered fertilizers are the easiest to apply.  To apply soluble fertilizer, make sure there are no lumps in the powder and sling it out over the surface. Table 2 lists recommended rates for commonly available fertilizers.

Table 2. Recommended Fertilization Rates for Ponds

Fertilizer Formulation
Soluble powderPounds/acre/application
10-54-54 - 6
10-48-54 - 6

A simple method of knowing when to fertilize is based on water clarity. The depth that light can penetrate into the pond is a measure of the algae density or bloom. Light penetration can be measured using a Secchi disk. A Secchi disk can be made from an 8-inch diameter disk of plywood, metal, or plastic. Mark the disk into quarters and paint the two opposite quarters white and black, respectively. Attach the disk to a yardstick or to a pole marked at 12, 18, and 24 inches from the disk.

The optimum algae bloom is one that allows light to penetrate to a depth between 18 and 24 inches. Submerge the Secchi disk into the pond until it just disappears and note that depth. Follow Table 3 as a guide to fertilization.

Table 3. Recommendations for Fertilization and Management Based on Secchi Disk Readings

Secchi Disk ReadingRecommended Management
18 to 24 inchesGood bloom - do nothing
12 to 18 inchesDense bloom - watch closely
12 inches or lessBloom too dense - determine source and be prepared to aerate at night
6 inches or lessOxygen depletion imminent
Greater than 24 inchesFertilize

If the Secchi disk disappears between 18 and 24 inches, there is no need to fertilize. It is time to fertilize again if the disk visibility is increasing rapidly toward 24 inches or if the disk is visible past 24 inches. If the disk disappears between 12 and 18 inches, the bloom is too dense: do not fertilize and watch the pond closely. If the disk disappears in less than 12 inches, the bloom is very dense and a severe oxygen depletion could occur. Remember: do not consider low Secchi readings that are the result of muddiness rather than algae.

A Secchi disk reading of 12 inches or less means the pond is too nutrient rich. At that point you need to determine where excess nutrients are coming from. Have you over-fertilized? Are livestock manures or crop fertilizers entering into the pond? If you are feeding the fish, are you overfeeding? Try to discover the source of the problem. Dense blooms can consume most of the pond’s oxygen at night. Be prepared to aerate at night if the visibility is low and there are consecutive days of cloudy weather.

Granular or liquid fertilizers can be used in ponds. Granular fertilizers must not be broadcast into the pond. Granules will sink to the bottom, and the phosphate will be absorbed directly into the mud and be lost. Granules should be placed on a platform that is submerged about 12 inches underwater. Usually one platform is needed for every 25 surface acres of pond. Place the platform in an area of the pond that has wave action. Granules placed on the platform dissolve slowly, spread throughout the pond by water currents, and stimulate a bloom.

Liquid fertilizers are dense and must be diluted with water before applying, or they will sink to the bottom and be absorbed into the mud. Dilute liquid fertilizers about 10 to 1 (water to fertilizer) and spray, splash, or mix into the pond. Apply fertilizer mixture as evenly as possible over the pond surface.

Lowering a Secchi disk to measure water transparency

Lowering a Secchi disk to measure water transparency

Fertilization should begin in late February or early March, depending on the pond’s location in Alabama. This first fertilizer application does not always stimulate a bloom. Continue to fertilize at 2- to 3-week intervals until the pond blooms green. Once a bloom is established, fertilize as necessary to maintain it. Use the Secchi disk guide in Table 3 to help make management decisions. Continue fertilizing until late October.

Some pond managers continue to fertilize through the winter. Although research suggests that winter fertilization does not increase the growth of fish, many managers feel that maintaining the bloom provides some extra food and reduces filamentous algae problems.

A Secchi disk can be used to determine when to fertilize.

A Secchi disk can be used to determine when to fertilize.

One important word of caution: do not fertilize ponds that are infested with aquatic weeds. The fertilizer will only stimulate growth of the weeds. Control weeds before fertilizing (see Weed Control). Establishing a good fertilization program before weeds appear is one of the best methods of weed prevention.

Ponds that are flushed by large volumes of water will lose fertilizer more rapidly and may not sustain a bloom. In this case fertilization is usually ineffective and should be discontinued unless the excess water can be diverted (see Pond Construction). Many ponds will flush repeatedly in winter and early spring but respond well to fertilization in late spring, summer, and fall.

Muddy ponds (12 inches or less visibility) usually will not respond to fertilization. Several methods have been used to clear muddy ponds; however, in most cases, the addition of lime to reduce acidity will settle a muddy pond. If liming does not settle the mud, contact your county Extension agent for advice.


Management of Recreational Fish Ponds Series


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