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pH Levels & Liming Your Pond

Liming Your Pond

It will soon be time to begin your pond fertilization program for the year. However, you should first lime your pond to optimize the benefits of adding fertilizer. Unless you live someplace like west Texas or Missouri, which have limestone aquifers with hard water, your pond probably needs one to three tons of agricultural lime per acre. In this issue, we will look at how to test your pond, what the results mean, and how to add it.


To understand why a pond needs lime, you must first be familiar with pH. In technical terms, pH is defined as the negative log of the molar concentration of hydronium ions. In practical terms, pH is a measure of acidity. The pH scale ranges from 1-14, with a pH of 7 being neutral. Readings less than 7 are considered acid; readings above 7 are basic. To make matters more complicated, the pH of a pond's water increases during the day, as photosynthesis takes place, removing carbon dioxide. So a single measurement really does you no good.

Why Lime?

Several factors contribute to lowering the pH in ponds. Rain is acidic, usually with a pH of 5.2 to 5.6, and industrial pollution can lower it to 2.5. In areas with coniferous forests, rain percolates through the pine needles, making it even more acidic. Over time, this leaches all the minerals out of the soil. Also, the clay bottom necessary to keep a pond from leaking is acidic, and decaying plants can release additional acids. Agricultural lime is crushed limestone (calcium carbonate), which will neutralize these acids and act as a buffer to keep the pH from changing rapidly.

Fish can live in water with a wide range of pH, from about 4 to 10. However, rapid changes in pH can kill fish, even within this range. While fish can adjust their body chemistry to different environmental pH values, this takes energy which could otherwise be used for growth and reproduction. Maintaining a constant internal pH in an extreme environment causes fish stress, making them susceptible to disease and parasites. In a limed pond, the fertilizer element phosphorus is in the soluble, orthophosphate form that is available to plankton; otherwise, it will be mostly tied up in bottom sediments. Finally, liming can increase the amount of carbon dioxide in water, which is used in photosynthesis. For these reasons, liming ponds has been shown to double bluegill production in ponds, without adding any fertilizer.


The proper way to collect a sample is to gather bottom mud from several locations around the pond. It is best to lay out a grid to be sure all areas are represented. Attach a soup can to a long pole to get samples from a boat. Put all the samples together and allow the mud to dry out. Mix thoroughly and take a sample from this.

Testing your pond is a simple laboratory procedure. Check with your county extension agent or nearest agricultural and mechanical (A & M) college. Your local farmers' co-op may also be able to send samples to a commercial lab. Be sure to indicate that the sample is from a pond. Test results can take two forms: how much lime is required to raise the pH to 7 (or neutral), or what the pH will be if you add one, two, or three tons of lime.

Adding Lime

The best way to add lime to your pond is to scatter it evenly over the bottom. This is usually done by loading lime on a barge and either shoveling it off or washing it off with a hose. There are very few people who are equipped to provide this service. Check with your state game and fish department or local farmers' co-op for leads.

Other Methods

If you can drive around the pond, another option is to get a special lime spreader to distribute the lime. The problem is that half the lime will be spread on the bank, but some of this will wash down into the pond later. Some co-ops will loan you a truck if you buy the lime from them. There is usually an 8-12 ton minimum, but lime is cheap. Ag lime usually sells for $15-$20 per ton. Delivered and spread it costs around $40 per ton, depending on your location.

Another option is to build one or more platforms at the edge of the pond and 4-6 inches below the surface. This is naturally easier to do when the pond isn't full. Place lime on the platforms and allow wave action to wash it into the pond gradually. This has the added advantage of providing cover for your fish.

Finally, the least preferred method is to scatter lime around the edge of the pond. If you do it this way, pelletized lime formed from ag lime dust may be easier to handle, and is twice as strong as regular ag lime. The drawback to pelletized lime is the cost - $120 per ton in bulk, or even more in bags


In order to get ready for the growing season, you should have your pond tested and add any lime needed to neutralize the pond bottom sediments. Unfortunately, this has to be done every 3-5 years, as the lime dissolves and is washed out with the overflow. However, lime is cheap, and the cost and effort are greatly rewarded in healthier, faster-growing fish.



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