Pond Fertilization and Liming

We respond to many of pond calls each week. The majority of these calls involve weed management in their ponds, but a few are related to liming and when to fertilize their ponds. Proper fertilization and liming can increase three to four times the pounds of fish a pond will support. Unfertilized ponds stocked with bream, bass and catfish usually have about 100 pounds of fish per surface acre. Ponds receiving adequate amounts of fertilizer typically contain 300 to 400 pounds of fish per surface acre. Fertilization increases fish production by increasing the amount of microscopic green plants (phytoplankton) in the water. Phytoplankton is the base of the pond food chain. The green color characteristic of fertilized ponds, called a bloom, is due to the abundance of phytoplankton. Phytoplankton is consumed by zooplankton (microscopic animals), which is eaten by aquatic insects and small fish. The small fish serve as food for the larger predators, such as largemouth bass.

Should Every Pond Be Fertilized?

Although fertilization can increase fish production significantly, it is not the best management practice for every pond.

1. Fertilization alone will not necessarily increase the size of individual fish in the pond. It will increase the total pounds of fish in the pond. If a one acre pond had 100 pounds of two to three inch bluegills, with few or no largemouth bass, fertilization would probably result in 300 to 400 pounds of two to three inch bluegills.
2. Ponds which have excessive water flow cannot be efficiently fertilized. If the total volume of water flowing out of a pond in 30 days exceeds the volume of the pond, it usually is not practical to fertilize. The added nutrients will not be in the pond long enough to promote the desired phytoplankton bloom.  All ponds may have excessive water flow in the wet seasons (spring and fall).  Ponds which exchange their water volume in 30 days or less in the dry season (summer) should not be fertilized.
3. Do not fertilize ponds with extensive areas less than two feet deep. The added nutrients will promote the growth of undesirable rooted aquatic vegetation in areas where sunlight penetrates to the bottom.
4. Do not fertilize ponds with no history of weed problems and very light fish harvests. It serves no purpose to increase pounds of fish in the pond if very few are going to be harvested.
5. Do not fertilize commercially fed ponds. Added nutrients promote excessive phytoplankton blooms that could lead to dissolved oxygen depletions, resulting in fish kills.

When and How Often Should You Fertilize?

Begin fertilization in February or March when surface water temperatures stabilize above 60 degrees F. Establish a bloom as early as possible to prevent aquatic weed growth. For initial applications (until a bloom develops) use 80 pounds of granular (20-20-5) or its equivalent, or two gallons of liquid (9-30-0 or 10-342) or its equivalent per surface acre per application. Once a bloom is established, use 40 pounds of 20-20-5 or one gallon of 9-30-0 per surface acre per application.

Each pond differs in the number of fertilizer applications required to maintain a satisfactory bloom. In a properly fertilized pond, visibility should be between 12 and 18 inches. If a shiny object attached to a stick can be seen deeper than 18 inches apply fertilizer. If the shiny object disappears from view before it reaches 12 inches, the phytoplankton bloom is too heavy and fertilizer should not be added. This visibility method is based on the green color due to phytoplankton. It cannot be used when the pond is muddy or in ponds that have a dark stain because they are fed by blackwater streams.

Liming:

The addition of lime increases fish production in soft (low total hardness) waters. Many ponds in the southeastern United States have very soft water and will not develop satisfactory plankton blooms unless lime is periodically added. Agricultural (dolomitic) lime is the best choice for sportfishing ponds. Check ponds for lime deficiency every three to five years. Many ponds in Georgia have very soft water, with a total hardness less than 10ppm (parts per million). Often, these ponds must be limed before a satisfactory bloom will develop. In ponds with a total hardness over 20 ppm, adding lime may have little effect on fish production. In ponds with a total hardness of 15 to 20 ppm, the increase in phytoplankton and fish production may be minimal.

Some Common Mistakes

1. Failure to Maintain Bloom Throughout the Season and From Year to Year
   A haphazard fertilization program is worse than no fertilization at all. Fertilizing a pond once or twice a year results in sudden increases in the food supply, increases in the weight of fish in the pond, then a rapid decline in available food. This can lead to an unbalanced fish population. Failure to maintain the bloom can promote aquatic weed growth as nutrients are added and the water clears.
2. Beginning Fertilization Too Late in the Spring
   If undesirable rooted aquatic vegetation becomes established prior to the initial fertilizer applications, the added nutrients may promote even more weed growth. Rooted aquatic vegetation that has reached the surface cannot be controlled with fertilizer. Treat exposed rooted aquatic vegetation with a herbicide prior to beginning fertilization.
3. Failure to Check Total Hardness (Lime Requirements)
   Check total hardness of fertilized ponds every three to five years. If you have eliminated other possible reasons for lack of bloom development, your pond probably needs liming.