By Greg Lutz*
Ponds with waters of less than 20 mg/l of total alkalinity normally need lime. Swimming pool and aquarium supply stores generally have inexpensive test kits available to measure alkalinity. If liming is required, applying agricultural limestone will increase water hardness and alkalinity while decreasing overall acidity. This will make fertilizer treatments more effective, and improve the natural productivity of the pond. Other benefits from liming a pond include increased levels of calcium and magnesium in the pond environment, and improved buffering capacity (which reduces daily fluctuations in pH), and both of these factors improve the general health of the fish population.
Liming improves fertilization response by allowing more of the phosphorus added to the pond to find its way into the water column, rather than being bound up in acidic soils in the pond bottom. This in turn allows for growth of phytoplankton, which fuels the natural food chain.
Agricultural limestone (calcium carbonate or dolomite), hydrated lime (calcium hydroxide) and quick lime (calcium hydroxide) are the most common liming materials for ponds. Agricultural limestone is not harmful to humans and will not cause high pH in water like the other forms of lime. It is always the best and safest liming material to use in farm ponds. Hydrated and/or quick lime should only be used under special conditions, and only by individuals with experience in their use.
A pond soil sample is needed to determine how much lime will be required. Collect samples from several locations evenly spaced across each pond, including both deep and shallow areas. Three to six samples per acre should be taken in ponds larger than 5 acres and at least 10 samples altogether from smaller ponds. Samples can be collected easily in full ponds with a can attached to a pole, or with a hollow piece of pipe. All individual samples should be mixed together and spread thin to dry. Dried soil samples should be pulverized, then placed in a soil testing box to be sent for analysis. Only about 2 cups of mixed soil are usually necessary.
Chemical analysis of pond soils can be conducted at most Land Grant university soil testing laboratories for a small fee. Soil samples should be processed through the Cooperative Extension Service office in your County. Procedures will vary from state to state, so check to see what your state requires. The Extension office nearest you can be located at http://www.csrees.usda.gov/Extension/ . Be sure to indicate “Fish pond” on the soil information sheet, since many soil labs are equipped to make liming recommendations specifically for ponds. If special handling is not available for pond mud samples, a good rule of thumb is to apply about 2 times the lime recommended for row crops in the local area. Test results and liming recommendations will be delivered directly to you.
New ponds can be limed before they are filled. Spread the liming material evenly over the dry pond bottom. A disk harrow can be used to mix the lime into the soil, if sufficient access is available. In ponds already filled with water, limestone should be applied evenly across the water surface. In small ponds, this may be done by spreading bagged limestone from a boat. In larger ponds, where several tons may be required, a platform can be built on the front of a large boat or between two boats tied together. Bulk limestone can be loaded (do not overload!) on the platform and distributed across the pond surface with a shovel. Even distribution across the entire bottom is essential for good results.
Do not apply limestone while a pond is being fertilized. Limestone settles phosphorus out of the water, making it unavailable to phytoplankton. Apply lime during late fall and winter. This will give it a chance to react with the acidic bottom mud before the spring application of fertilizer.
Frequency of Liming
A liming treatment may last almost indefinitely in ponds with no outflow. Most ponds have some water discharge or are drained and refilled periodically. Most ponds with acid soils and moderate water outflow will probably need lime every three to five years. A method frequently used with good results is to apply the amount of lime recommended by a soil test, then apply one-fourth of that amount each succeeding year to keep lime requirements satisfied.
More information on liming recreational and commercial ponds can be found at https://srac.tamu.edu/index.cfm/getFactSheet/whichfactsheet/264/
C. Greg Lutz, has a PhD in Wildlife and Fisheries Science from the Louisiana State University. His interests include recirculating system technology and population dynamics, quantitative genetics and multivariate analyses and the use of web based technology for result-demonstration methods. email@example.com