Introduction
Management of ponds and lakes mirrors the same management measures that all good land managers use to improve habitats on their properties. That’s because the same principles that guide game management also guide the management of ponds and lakes. A little knowledge, coupled with some common sense, will turn your lake into a great place to fish. Fertilizing may be the most valuable tool at the pond manager’s disposal. It has a dramatic effect on fish abundance and takes little effort or skill to accomplish. However, it isn’t a magic pill and can’t solve other common problems that limit fishing quality.
While many lakes and ponds used for sport fishing can benefit significantly from fertilization, if you are satisfied with the fishing you have and the appearance of the pond, there is really no point in making changes to your current management. There are signs that indicate whether fertilization might be beneficial in increasing fish production. If your pond water is relatively clear, especially in the middle of the growing season, that often indicates limited nutrient concentration. If you find yourself restocking or adding fish regularly, that could be a sign that fertility is low. Also, if your pond water is extremely muddy or turbid with clay, it should not be fertilized until that problem is addressed.
However, fertilization is not appropriate for all ponds. If your pond already stays green and soupy during the growing season, with visibility severely restricted, it may already have more than enough nutrients. If the primary function of your pond is something other than sport fishing, fertilization may not be for you. If you use the pond for swimming—either by people or pets—fertilization may not be compatible.
Ponds in northern climates generally should not be fertilized because the short growing season and cooler water temperatures limit plankton growth, reducing the effectiveness of fertilization. Fertilized ponds also carry a higher risk of oxygen depletion during fall turnover and under winter ice, increasing the chance of fish kills. Additionally, many northern ponds already have adequate natural fertility, so added nutrients often create water-quality problems without improving fishing.
Benefits
Fertilization can increase the overall production in a pond, which can culminate in a higher carrying capacity for fish compared to a pond without fertilization. However, if the fish biomass in a pond exceeds its carrying capacity, additional fertilizer can cause severe problems.
The basis for fish production in most bodies of water is the fertility of the water, which is determined by the fertility of the soils in the watershed and in the sediment. In most waters, as on most land, the most limiting nutrient in ponds is phosphorus. How does fertilizer transform an ordinary pond into a bona fide fish factory? Pond fertilizer increases small, green, single-celled plants called phytoplankton. These microscopic plants typically give pond water its color. Tiny microscopic animals and invertebrates consume the phytoplankton and thrive. These small invertebrates are then eaten by larger insects, which are eaten by small forage fish. The forage fish provide food for larger fish, and voilà—you have a higher carrying capacity and the potential for a great fishing pond, assuming no other problems are present. Fertilizing can increase the abundance and size of fish, but it cannot, by itself, produce a balanced fish pond.
Applying proper pond fertilizer has the same effect as fertilizing your garden, lawn, or agricultural crops: it increases your potential for production. Research in ponds has shown that the biomass of fish produced can increase by 300 percent or more with proper fertilization, providing many more opportunities for catching quality-sized fish. It is crucial to continue fertilization once you have started. Fertilizing for a year or two and then stopping will result in more fish than the pond can support, leading to a population imbalance.
One of the initial steps in establishing a productive fish pond is to have your water tested. Typically, a laboratory that conducts soil tests for nutrient analysis can also provide similar analyses for water. Based on their experience with the soils and water in your region, the lab can likely suggest specific tests that may be beneficial. If your pond is filled with runoff from the watershed, test the water in the pond. However, if your pond is filled by a well, it is generally best to test the well water directly.
The parameters to be tested may vary depending on your location. If your pond is filled with well water or benefits from groundwater, these waters often lack dissolved oxygen. Such ponds may also contain high levels of dissolved gases, such as carbon dioxide and hydrogen sulfide. Additionally, these waters may contain elevated levels of certain metals, such as iron and manganese. While these chemicals may not directly affect fertilizer performance, they can certainly impact fish survival.
Analyzing water samples can significantly improve your ability to fertilize effectively. Total alkalinity is perhaps the most useful measure of a pond’s baseline productivity. It represents the concentration of bases (typically carbonate and bicarbonate) in the water, which help reduce large and rapid changes in pH. A total alkalinity of less than 20 mg/L limits primary productivity, and lime is necessary in such ponds to effectively utilize dissolved nutrients. Why not just measure phosphorus directly, since it is the nutrient of primary interest? Phosphorus interacts with so many other elements and minerals that measuring the amount of available phosphorus is often very difficult. Likewise, measuring pH directly can be problematic. Especially in soft water with low alkalinity, pH can change very rapidly and is not a good indicator of the actual average pH of the pond bottom or water.
While fertilization can be very successful in improving fish populations, it is not equally helpful across all climates. It has the greatest impact where the growing season is longest. As the growing season shortens in northern latitudes or at higher elevations, the period during which fertilization is useful decreases. For instance, in South Georgia, fertilization is beneficial over an almost 10-month growing season. However, in southern Michigan, the growing season may only be half as long, and the effects will be proportionally reduced. Fertilization can still increase fish production, just not to the same extent as in warmer climates.
Kinds of Fertilizer
Fertilizer used in ponds should generally contain high amounts of phosphorus. Phosphorus is a crucial metabolic nutrient that regulates the productivity of surface waters. However, the complexity of the various forms of phosphorus in water makes it impractical to quantify phosphorus levels in ponds with more than general accuracy. The other nutrients—nitrogen and potassium—are almost always naturally abundant enough that their addition has minimal impact on aquatic production.
There are several types of fertilizers suitable for pond use. Conventional granular fertilizers, developed in the early 1950s, are still widely used. The standard 40-lb bag of 20-20-5 (N-P-K) is a common choice. However, these granular fertilizers should never be broadcast or poured directly into the water from a boat. They contain clay as a carrier that can rapidly sink to the bottom, where the nutrients become chemically bound to the sediment and are prevented from dissolving. Instead, granular fertilizer should be placed on a platform about 18 inches beneath the water’s surface, allowing natural wave action to slowly dissolve it. Even under the best conditions, getting these conventional fertilizers fully dissolved and contributing to phytoplankton growth can take a week or more. This delay unnecessarily limits a pond’s potential.
Liquid fertilizers have gained popularity among some pond owners. Their analysis varies, but they typically consist of 10-34-0. Despite being liquid, these fertilizers are heavy and surprisingly resistant to dissolving in ponds. For optimal results, they should be premixed in a container with at least an equal volume of water before being added to the pond. Some innovative pond owners have even “plumbed” their ponds with PVC pipe, enabling them to apply fertilizer without entering the pond.
A new type of concentrated, highly soluble crystalline fertilizer has revolutionized pond fertilization. It requires no mixing, platforms, or handling of heavy bags to achieve desired results and has become a game-changer. Formulations vary, but the most popular and widely available is Biologic’s Perfect Pond Plus (12-48-8). It contains a micronutrient package beneficial to many crops, including phytoplankton. This fertilizer, which has a consistency similar to powdered sugar, dissolves quickly in water and can be broadcast from the bank or a pier without the need for even distribution across the pond.
With the advent of this soluble powdered fertilizer, even large ponds can be effectively fertilized from a single location. Its ultra-high solubility allows for uniform dissolution throughout the productive surface layer of pond water. Ponds with high water-exchange rates, previously difficult to fertilize with slow-dissolving conventional fertilizers, can now be treated efficiently.
There are other sources of nutrients that can contribute to a pond’s fertility. If livestock are present in the immediate watershed above the pond, they can contribute dramatically to nutrient input. One drawback is that you have little to no control over the amount or composition of these nutrients. More often than not, livestock do a poor job of delivering nutrients in the proper amount or balance.
If you feed the fish in your pond with prepared fish food, that practice will also contribute nutrients. As with livestock runoff, the manager has little control over nutrient input from fish food. I suggest limiting fish-food input to no more than about 50 pounds per acre of pond to avoid over-nutrification.
Application and Rates
If using traditional granular fertilizers, do not apply them in a manner that allows the granules to sink to the bottom. If that occurs, the nutrients will dissolve at a glacial pace. Instead, pour the granules onto an underwater platform and allow natural wave and water movement to dissolve them. Another proven method is to place a fertilizer bag full of fertilizer in shallow water and cut the bag in a way that exposes the contents while keeping them separated from the bottom. The standard application rate should translate to about 8 pounds of phosphorus per surface acre.
Liquid fertilizers should be mixed with a generous amount of water before being spread around the pond by boat. If the liquid is poured directly into the pond, it will sink to the bottom surprisingly quickly and bind to bottom sediments, where it does little good. Most pond managers have found that 1 to 2 gallons of liquid fertilizer per surface acre produces good results, which amounts to about 8 to 9 pounds of phosphorus.
Perfect Pond Plus dissolves almost immediately upon contact with water. The standard rate is about 5 pounds per surface acre, which equates to about 2.5 pounds of phosphorus per acre. It disperses very quickly, so there is little need to spread it around the pond.
Measuring the effectiveness of fertilization is relatively straightforward and requires no laboratory testing or expensive equipment. Water visibility has been used for decades as a reliable indicator of phytoplankton abundance. I use a white plastic disk attached to the end of a yardstick. Looking down from above, lower the disk until it disappears, then measure the depth at which it vanished. If that measurement is less than 20 inches, no additional fertilizer is warranted. However, if water clarity exceeds about 20 inches, it is time to add fertilizer. It is common to find ponds constructed in environments where nutrient abundance is already high, and such ponds will not benefit from additional fertilization.
When to Fertilize
Fertilization should begin early in the spring when water temperatures reach the low 60s. Applications should continue throughout the growing season until water temperatures fall below this mark in the fall. If you are confident fertilizer is needed but unsure about timing, it is generally better to fertilize than not. It is far preferable to fertilize slightly too often than too infrequently. If you are uncertain about timing based on water temperature, NOAA data showing average first fall freeze dates and average last spring freeze dates can be helpful. These data are easily accessible on the NOAA website or through your local Extension Service.
During the growing season, fertilization should continue to sustain its benefits. I recommend monitoring water clarity approximately every two weeks. If clarity exceeds about 20 inches, add fertilizer. This schedule should continue until water cools in the fall. The dosage rates provided are based on average conditions; if your pond responds differently, do not hesitate to adjust the dosage.
Generally, ponds with aquatic weeds present should not be fertilized, as additional nutrients may stimulate further plant growth. This is particularly true for floating plants such as duckweed, watermeal, water hyacinth, or Salvinia. These plants absorb nutrients directly from the water and respond quickly to fertilization without providing benefits. Rooted aquatic plants, however, are less affected by fertilizer dissolved in the water column.
Why Use Lime?
As with fertilizing gardens or agricultural crops, soil pH plays an important role in fertilizer effectiveness. In many soil types, agricultural limestone should be applied to ponds to neutralize acidic soils and make fertilizer nutrients available for phytoplankton production.
Amount of Lime Needed
Determining lime requirements involves measuring total alkalinity or total hardness. If either value is less than 20 mg/L, adding lime is advisable. Nutrient utilization peaks at approximately 20 mg/L. Experience has shown that each ton of lime added increases total alkalinity or total hardness by about 3–4 mg/L. For example, if total alkalinity is 12 mg/L, adding 2 tons per acre would be expected to raise it to approximately 18–20 mg/L. Because agricultural lime dissolves slowly, it should be applied 2 to 3 months before the growing season begins.
Lime Application
For best results, lime should be spread evenly across the pond rather than dumped in a few locations. If a spreader truck can access most of the shoreline, it can be adjusted to throw lime to one side, simplifying application. If a boat must be used, it should be decked with plywood, steel roofing, or plastic sheeting. Lime can then be placed in the center of the boat and washed off as the boat moves around the pond.
Often, the need for lime is first recognized when a recommended fertilization program fails to produce a plankton bloom.
Hydrated Lime
To temporarily raise total hardness and allow for a fertilizer response, hydrated lime can be applied. While hydrated lime dissolves very quickly, the resulting increase in total hardness and alkalinity usually lasts only 6–8 weeks. The recommended application rate is 50 to 100 pounds per surface acre, spread evenly over the entire pond. Caution is essential, as over application can rapidly raise pH and potentially kill fish. Hydrated lime is extremely caustic, and proper safety precautions must be followed during handling.
Many existing lakes and ponds, particularly those containing both largemouth bass and bluegill, can benefit significantly from a few basic and cost-effective management techniques. Fertilization is one such technique that can increase a pond’s capacity to support more fish. While fertilizer can help grow more fish, the relative size distribution of the population depends on other management tools that affect predator–prey balance. That topic, however, is best addressed in a separate article.
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