What Kind of Lime Is Best for Ponds? (Pros & Cons)

Lime plays several roles in artificial freshwater systems. The application of lime, often referred to as “liming”, is a typical step in pond preparation and regular maintenance.

The primary role of lime is as a soil or water conditioner that promotes a well-balanced, alkaline state. It helps optimize pH levels to prevent drastic fluctuations throughout the day. Available in a variety of forms, lime is crucial for the maintenance of most artificial pond systems.

Lime is rich in calcium. Apart from enhancing alkalinity, it may also increase a pond’s productivity. It intensifies the rate of organic matter decomposition and frees up usable phosphates. Once nutrients are highly available, primary producers can rapidly assimilate them and convert them into natural food sources for pond inhabitants.

Lime can also prevent the spread of fish diseases, reduce the formation of toxic gases in pond water, increase dissolved oxygen levels, and help reduce turbidity by forming bonds between suspended sediments.

Of course, proper application, dosage, and timing are vital to ensuring the effectiveness of liming ponds. Additionally, there are several types of lime, each with its own unique chemical composition and mode of neutralization. Usage of the right type, with consideration for the initial alkalinity and total hardness of a pond, is vital to achieving optimal results at manageable price points.

Despite its remarkable potential to improve pond conditions, lime can also be harmful when it is haphazardly used at the wrong time, in excessively high doses, or without protective equipment! Make sure to familiarize yourself with all the types of lime and their modes of application before treating your pond.

Types of Pond Lime & Recommendations on Application

1) Agricultural lime

Agricultural lime is the safest type of lime for recreational fish ponds and aquaculture ponds. Also referred to as agricultural limestone, garden lime, or aglime, this type of lime is made by breaking up limestone to create a finely pulverized material. It is chiefly composed of calcium carbonate. Depending on its source, it may contain traces of calcium oxide and various minerals. This mass-produced pond additive is also used in power plants, crop fields, and leached land that has been converted into breeding or foraging grounds for livestock.

The quality and effectiveness of aglime are largely determined by its average particle size and chemical profile. Smaller particles tend to be more effective when used on soil as they can permeate through the layers, correcting alkalinity conditions up to a greater depth. It is often considered a harmless type of lime because it is less likely to cause drastic rises in pH levels even when introduced in high doses. It is also more cost-effective, available in small quantities, and relatively easy to find. Enumerated below are some of its primary uses in ponds:

• to increase the pH of pond soil or stabilize alkalinity levels
• as a water buffer to maintain pH levels at around 7 – 8.5 throughout the day, preventing potentially harmful swings in pH
• to provide calcium for primary producers and improve rates of nutrient uptake, especially for plants and algae
• to improve the effects of pond fertilization
• as a disinfectant for the substrate and pond water
• to increase phosphorus availability

Application: The best time to apply agricultural lime to a pond is during one of the early stages of pond preparation – before the pond is filled with water. This way, lime can be applied directly and evenly onto the soil. For this type of lime, the recommended dosage is based on both the neutralizing value (NV) and neutralizing efficiency (NE) of the product. The NV is affected by the chemical impurities in the powder. A product with a purer concentration of calcium carbonate would have a higher NV, approaching around 90 – 100%. NE, on the other hand, is higher when the particle size is reduced. The liming recommendation value should ideally be indicated on the product’s packaging, though corrections can be made for lower NE or NV values. The dosage can vary anywhere from 1 – 5+ tons per acre, depending on the NV, NE, and purpose of application.

If the pond has already been filled with water, agricultural lime can be applied as a powder or pre-mixed slurry. To spread the lime as evenly as possible, use a small motorboat or barge with a platform and a water pump. Slowly make your way around the pond while using the water to wash the lime off of the platform. The prop wash of the boat’s motor should help distribute the substance. Note that changes in pH may take place almost immediately, though they should not shoot up to levels that are lethal for fish.

2) Hydrated lime

Hydrated lime is also referred to as slaked lime, caustic lime, or builder’s lime. Its chemical formula is denoted as Ca(OH)₂ as it is predominantly composed of calcium hydroxide. This substance is actually produced when quicklime is mixed with water. Its applications extend to industries beyond farming and aquaculture as it is also used in food preparation. Despite the low solubility profile of this inorganic compound, it is widely used in commercial aquaculture (particularly for mussel and oyster production).

Unlike agricultural lime, hydrated lime can be quite harmful in ponds that are filled with fish. Its capacity to dramatically increase pH levels can endanger living organisms. This is why it is ideal for use several days to weeks prior to stocking. If you can spare about 1 – 3 weeks before having to stock a pond, it is recommended as an alternative to agricultural lime for the following purposes:

• to rapidly increase pH levels (up to 12) as a means to thoroughly neutralize pond water and the upper layers of soil
• to increase total hardness and total alkalinity in soft water ponds with a rapid water exchange rate
• as a pond sterilizer to eliminate potential pathogens and pests (e.g. fouling organisms and predators), especially when a previous stock has been affected
• as a flocculant to reduce turbidity rates in pond water (without stocked fish)
• to quickly reduce dissolved CO₂ levels during periods with low oxygen levels

Application: Before stocking the pond with fish, make sure to monitor pH levels throughout the day and ensure that they have stabilized to acceptable values. Smaller amounts can be used for the quick amendment of undesirable soil and water conditions. If using the substance to resolve toxicity issues, a dosage of about 50 pounds per acre may suffice. Higher dosages, applied over a period of 12 hours, are required to thoroughly sterilize the pond bottom.

Hydrated lime can be applied similarly to agricultural lime, but protective gear must be used as it is a strong, caustic base. If it comes into contact with the skin and eyes, it can cause rashes, chemical burns, severe irritations, and blindness. If inhaled or ingested, it can cause lung damage and tissue necrosis. Note that this type of lime is not generally recommended for issues that can be resolved with the use of agricultural lime.

3) Quicklime

Quicklime or burnt lime is an alkaline compound that is made up of calcium oxide. It is known for being highly reactive as it generates heat upon contact with water or vapor. Outside of aquaculture or recreational pond cultivation, it is heavily used in the steelmaking and cement production industries.

In ponds, the uses of quicklime are largely similar to those of hydrated lime. Its primary functions as a pond conditioner include disinfection and the neutralization of acidic substrates. Its NV tends to be highest among the types of lime, averaging at around 178%, but it is not recommended as a treatment for stocked ponds as it can drastically increase pH levels. Due to its potency, it is largely effective as a sterilizer in contaminated ponds.

Application: As it is caustic, quicklime must be handled with protective equipment. Its high NV allows for small dosages to be effective when treating pond substrates. This type of lime is best applied to the pond bottom prior to filling in the pond with water. It should also be used several weeks before the scheduled stocking date so that the pond’s pH can return to acceptable levels.

4) Dolomitic lime

Dolomitic lime or dolomite is a natural compound that contains higher levels of magnesium compared to other types of lime. Its chemical composition is denoted by the formula CaMg(CO₃)₂. Dolomite can be purchased as a fine powder, as coarse granules, or as pellets. It may contain trace amounts of other elements as it is rarely mined in its pure form.

Dolomitic lime can be just as effective at increasing pH levels as agricultural lime. For fish ponds, however, it is rarely chosen over agricultural lime unless there is a need for its magnesium component. If magnesium levels are initially quite high, its introduction can compromise the quality of soil and enhance the growth of weeds. Fortunately, it is considered the least reactive among the liming agents. Its application to stocked ponds should not be detrimental to fish survival if conservative doses are used. The primary functions of dolomite in fish ponds are listed below:

• to increase pH levels and total alkalinity in soft water
• to neutralize soil acidity
• to increase general hardness (GH) levels in the soil and water
• as a feedstock for beneficial algae
• to sterilize the top few inches of pond substrate
• to enhance molting rates and recovery of crustaceans

Application: To apply pulverized dolomite, sprinkle it directly onto the water’s surface. Try to spread the substance across the pond’s surface evenly, without dumping large concentrations into a single area. It can also be applied onto the substrate prior to filling in a pond with water.

Throughout the culture period in intensive ponds, dolomite can be applied daily or at intervals of 3+ days, with an input rate of about 150 – 250 kg/hectare. When using this liming agent for recreational or ornamental ponds, its application may be restricted to periods where pH and water hardness levels have decreased significantly.

What Is the Best Type of Lime for Ponds?

The effectiveness of any type of lime depends on the soil and water conditions. There is simply no single, best additive to resolve all pH, hardness, turbidity, and pathogen-related problems. Crushed agricultural limestone is widely regarded as the best lime to use in ponds, but its poor solubility in water with a pH above 8.3 or alkalinity levels above 50 mg/L may render it ineffective and insoluble. As a result, alternatives to lime, such as gypsum, may be required. A pond can strongly benefit from being limed when its alkalinity rate is less than 20 mg/L.

In between grow-out periods and prior to stocking, there’s much more room to use types of lime with a higher neutralizing value, such as hydrated lime or quicklime. While these are not recommended for use in stocked ponds, they can help sterilize contaminated substrates and water. Moreover, they tend to be more reactive with water and can quickly form alkaline suspensions that help reduce turbidity. Interestingly, the mechanism by which they reduce dissolved carbon dioxide levels is being explored as a means to counter ocean acidification.

Lastly, both aglime and dolomite as pond conditioners are ideal for use during the culture process as they are the least reactive types of lime. In stocked freshwater ponds with low general hardness, they can quickly enhance the water quality and encourage the growth of beneficial microbes and plankton. The magnesium content in dolomite may also increase the growth rate of plants.

As each type of lime has its merits and downsides, it’s crucial to take some time to determine the best option for your own pond. Your judgment should be based on the pond’s current conditions. Make sure to correctly sample your water and soil to obtain data on parameters such as pH, alkalinity, total hardness, temperature, and dissolved oxygen levels. Use calibrated test kits or take your sample to an accredited laboratory for more accurate results. Make sure to test the same parameters after the application of lime to ensure its effectiveness.