PONDS -- ALKALINITY, HARDNESS, pH & CARBON DIOXIDE

Interactions of pH, Carbon Dioxide,

Alkalinity and Hardness in Fish Ponds

William A. Wurts and Robert M. Durborow*

Water quality in fish ponds is affected by the interactions of several chemical components.  Carbon dioxide, pH, alkalinity and hardness are interrelated and can have profound effects on pond productivity, the level of stress and fish health, oxygen availability and the toxicity of ammonia as well as that of certain metals.  Most features of water quality are not constant.  Carbon dioxide and pH concentrations fluctuate or cycle daily.  Alkalinity and hardness are relatively stable but can change over time, usually weeks to months, depending on the pH or mineral content of watershed and bottom soils.

pH and carbon dioxide

The measure which indicates whether water is acidic or basic is known as pH.  More precisely, pH indicates the hydrogen ion concentration in water and is defined as the negative logarithm of the molar hydrogen ion concentration (-log [H+]).  Water is considered acidic when pH is below 7 and basic when pH is above 7.  Most pH values encountered fall between 0 and 14.  The recommended pH range for aquaculture is 6.5 to 9.0 (Figure 1).

Fish and other vertebrates have an average blood pH of 7.4.  Fish blood comes into close contact with water (1- or 2-cell separation) as it passes through the blood vessels of the gills and skin.  A desirable range for pond water pH would be close to that of fish blood (i.e., 7.0 to 8.0).  Fish may become stressed and die if the pH drops below 5 (i.e., acidic runoff) or rises above 10 (e.g., low alkalinity combined with intense photosynthesis by dense algal blooms-phytoplankton or filamentous algae).

Pond pH varies throughout the day due to respiration and photosynthesis.  After sunset, dissolved oxygen (DO) concentrations decline as photosynthesis stops and all plants and animals in the pond consume oxygen (respiration).  In heavily stocked fish ponds, carbon dioxide (CO₂) concentrations can become high as a result of respiration.  The free CO₂, released during respiration reacts with water, producing carbonic acid (H₂CO₃) and pH is lowered.

H₂O + CO₂ = H₂CO₃ = H⁺ +HCO3

Table 1 summarizes the relative changes in dissolved oxygen, CO₂ and pH over 24 hours.

Table 1.  Relative concentration changes for dissolved oxygen, carbon dioxide and pH in ponds over 24 hours.
		Change
Time	Dissolved Oxygen	Carbon Dioxide	pH
Daylight	Increases	Decreases	Increases
Nighttime	Decreases	Increases	Decreases
Tucker (1984)

Carbon dioxide rarely causes direct toxicity to fish.  However, high concentrations lower pond pH and limit the capacity of fish blood to carry oxygen by lowering blood pH at the gills.  At a given dissolved oxygen concentration (e.g., 2 mg/L, milligrams per liter; same as parts per million, ppm), fish may suffocate when CO₂ levels are high and appear unaffected when CO₂ is low.  Catfish can tolerate 20 to 30 mg/L CO₂ if accumulation is slow and dissolved oxygen concentrations are above 5 mg/L. In a reservoir or natural pond, CO₂ , rarely exceeds 5 to 10 mg/L.

High CO₂ concentrations are almost always accompanied by low dissolved oxygen concentrations (high respiration); the aeration used to increase low dissolved oxygen will, to some extent, help reduce excess CO₂, by improving its diffusion back into the atmosphere.  Chronically high CO₂ levels can be treated chemically with hydrated lime, Ca(OH)₂.  Approximately 1 mg/L of hydrated lime will remove 1 mg/L of CO₂. This treatment should not be used in waters with poor buffering capacity (low alkalinity) because pH could rise to levels lethal to fish.  Also, fish could be endangered if hydrated lime is added to waters with high ammonia concentrations.  High pH increases the toxicity of ammonia.

Alkalinity

The quantity of base present in water defines what is known as total alkalinity.  Common bases found in fish ponds include carbonates, bicarbonates, hydroxides, phosphates and berates.  Carbonates and bicarbonates are the most common and most important components of alkalinity.  Alkalinity is measured by the amount of acid (hydrogen ion) water can absorb (buffer) before achieving a designated pH.  Total alkalinity is expressed as milligrams per liter or parts per million calcium carbonate (mg/L or ppm CaCO₃).  A total alkalinity of 20 mg/L or more is necessary for good pond productivity.  A desirable range of total alkalinity for fish culture is between 75 and 200 mg/L CaCO₃.

Carbonate-bicarbonate alkalinity (and hardness) in surface and well waters is produced primarily through the interactions of CO₂ water and limestone.  Rainwater is naturally acidic because of exposure to atmospheric carbon dioxide.  As rain falls to the earth, each droplet becomes saturated with CO₂ and pH is lowered.  Well water is pumped from large, natural underground reservoirs (aquifers) or small, localized pockets of underground water (groundwater).  Typically, underground water has high CO₂ concentrations, and low pH and oxygen concentrations.  Carbon dioxide is high in underground water because of bacterial processes in the soils and various underground, particulate mineral formations through which water moves.  As ground- or rainwaters flow over and percolate through soil and underground rock formations containing calcitic limestone (CaCO₃) or dolomitic limestone [CaMg(CO₃)], the acidity produced by CO₂ will dissolve limestone and form calcium and magnesium bicarbonate salts:

CaCO₃ + H₂ O + CO₂ = Ca⁺² + 2HCO₃

or

CaMg(CO₃)₂ + 2H₂O + 2CO₂ = Ca⁺² + Mg⁺² + 4HCO₃

The resultant water has increased alkalinity, pH and hardness.

Alkalinity, pH and carbon dioxide concentrations

In water with moderate to high alkalinity (good buffering capacity) and similar hardness levels, pH is neutral or slightly basic (7.0 to 8.3) and does not fluctuate widely.  Higher amounts of CO₂ (i.e., carbonic acid) or other acids are required to lower pH because there is more base available to neutralize or buffer the acid.  The relationship among alkalinity, pH and CO₂ can be determined from Table 2.  The number (factor) found in the table which corresponds to the measured pH and water temperature is multiplied by the measured alkalinity value (mg/L as CaCO₃).  The product of these numbers estimates CO₂ concentrations (mg/L).

For example, Mr. Jacobs measures a pH of 7.2, a temperature of 77°F (25°C) and total alkalinity of 103 mg/L in his catfish pond.  He determines the corresponding factor, 0.124, from Table 2 and multiplies this number by the measured alkalinity, 103 mg/L.  The resulting product gives him an estimate of the CO₂ concentrations in his pond:

0.1  24x 103 mg/L alkalinity = 12.8 mg/L CO₂

A prompt pH measurement within 30 minutes of water sampling is required to minimize error when using this method.  Due to several sources of error that can occur with this method, direct measurement of CO₂ using a chemical test procedure is preferred.

Table 2.  Factors for calculating carbon dioxide concentrations in water with known pH, temperature and alkalinity measurements.a
Temperatures (°C)
pH	5	10	15	20	25	30	35
6.0	2.915	2.539	2.315	2.112	1.970	1.882	1.839
6.2	1.839	1.602	1.460	1.333	1.244	1.187	1.160
6.4	1.160	1.010	0.921	0.841	0.784	0.749	0.732
6.6	0.732	0.637	0.582	0.531	0.495	0.473	0.462
6.8	0.462	0.402	0.367	0.335	0.313	0.298	0.291
7.0	0.291	0.254	0.232	0.211	0.197	0.188	0.184
7.2	0.184	0.160	0.146	0.133	0.124	0.119	0.116
7.4	0.116	0.101	0.092	0.084	0.078	0.075	0.073
7.6	0.073	0.064	0.058	0.053	0.050	0.047	0.046
7.8	0.046	0.040	0.037	0.034	0.031	0.030	0.030
8.0	0.029	0.025	0.023	0.021	0.020	0.019	0.018
8.2	0.018	0.016	0.015	0.013	0.012	0.012	0.011
8.4	0.012	0.010	0.009	0.008	0.008	0.008	0.007
Tucker (1984) aFactors should be multiplied by total alkalinity (mg/L) to get carbon dioxide (mg/L). For practical purposes, CO2 concentrations are negligible above pH = 8.4.

Alkalinity, pH and photosynthesis

The bases associated with alkalinity react with and neutralize acids.  Carbonates and bicarbonates can react with both acids and bases and buffer (minimize) pH changes.  The pH of well buffered water normally fluctuates between 6.5 and 9.  In waters with low alkalinity, pH can reach dangerously low (CO₂ and carbonic acid from high respiration) or dangerously high (rapid photosynthesis) levels (Figure 2).

Phytoplankton are microscopic or near microscopic, aquatic plants which are responsible for most of the oxygen (photosynthesis) and primary productivity in ponds.  By stabilizing pH at or above 6.5, alkalinity improves phytoplankton productivity (pond fertility) by increasing nutrient availability (soluble phosphate concentrations).  Alkalinities at or above 20 mg/L trap CO₂ and increase the concentrations available for photosynthesis.

 Because phytoplankton use CO₂ in photosynthesis, the pH of pond water increases as carbonic acid (i.e., CO₂) is removed.  Also, phytoplankton and other plants can combine bicarbonates (HCO₃) to form CO₂ for photosynthesis, and carbonate (CO₃^-2) is released:

2HCO₃ + phytoplankton = CO₂(photosynthesis) + CO₃^-2 + H₂O)

CO₃^-2 + H₂O = HCO₃ + OH-

(strong base)

High pH could also be viewed as a decrease in hydrogen ions (H+):

CO₃^-2 + H⁺ = HCO₃  or   HCO₃ + H⁺ = H₂O + CO₂

The release of carbonate converted from bicarbonate by plant life can cause pH to climb dramatically (above 9) during periods of rapid photosynthesis by dense phytoplankton (algal) blooms.  This rise in pH can occur in low alkalinity water (20 to 50 mg/L) or in water with moderate to high bicarbonate alkalinity (75 to 200 mg/L) that has less than 25 mg/L hardness.  High bicarbonate alkalinity in soft water is produced by sodium and potassium carbonates which are more soluble than the calcium and magnesium carbonates that cause hardness.  If calcium, magnesium and photosynthetically produced carbonate are present when pH is greater than 8.3, limestone is formed.  Ponds with alkalinities below 20 mg/L do not usually support good phytoplankton blooms and do not commonly experience dramatic pH increases because of intense photosynthesis.

Hardness

Water hardness is important to fish culture and is a commonly reported aspect of water quality.  It is a measure of the quantity of divalent ions (for this discussion salts with two positive charges) such as calcium, magnesium and/or iron in water.  Hardness can be a mixture of divalent salts; however, calcium and magnesium are the most common sources of water hardness.

Hardness is traditionally measured by chemical titration.  The hardness of a water sample is reported in milligrams per liter as calcium carbonate (mg/L CaCO₃).  Calcium carbonate hardness is a general term that indicates the total quantity of divalent salts present and does not specifically identify whether calcium, magnesium and/or some other divalent salt is causing water hardness.

Hardness is commonly confused with alkalinity (the total concentration of base).  The confusion relates to the term used to report both measures, mg/L CaCO₃.  If limestone is responsible for both hardness and alkalinity, the concentrations will be similar if not identical.  However, where sodium bicarbonate (NaHCO₃) is responsible for alkalinity it is possible to have low hardness and high alkalinity.  Acidic, ground or well water can have low or high hardness and has little or no alkalinity.

Calcium and magnesium are essential in the biological processes of fish (bone and scale formation, blood clotting and other metabolic reactions).  Fish can absorb calcium and magnesium directly from the water or from food.  However, calcium is the most important environmental, divalent sale in fish culture water.  The presence of free (ionic), calcium in culture water helps reduce the loss of other salts (e.g., sodium and potassium) from fish body fluids (i.e., blood).  Sodium and potassium are the most important salts in fish blood and are critical for normal heart, nerve and muscle function.  Research has shown that environmental calcium is also required to re-absorb these lost salts.  In low calcium water, fish can lose (leak) substantial quantities of sodium and potassium into the water.  Body energy is used to re-absorb the lost salts.  For some species (e.g., red drum and striped bass), relatively high concentrations of calcium hardness are required for survival.

A recommended range for free calcium in culture waters is 25 to 100 mg/L (63 to 250 mg/L CaCO₃ hardness).  Channel catfish can tolerate low calcium concentrations as long as their feed contains a minimum level of mineral calcium but may grow slowly under these conditions.  Similarly, rainbow trout can tolerate waters with free calcium concentrations as low as 10 mg/L if pH is above 6.5.  If freshwater culture of striped bass, red drum or crawfish is being considered, free calcium concentrations in the 40 to 100 mg/L range (100 to 250 mg/L as CaCO₃, hardness) are desirable; a value of 100 mg/L (250 mg/L calcium hardness) matches the calcium concentration of fish blood.  Tests specific for calcium hardness should be performed on samples of the water source being considered for these animals.

A low CaCO₃ hardness value is a reliable indication that the calcium concentration is low.  However, high hardness does not necessarily reflect a high calcium concentration.  But, since limestone is common in the soil and bedrock of the southern United States, it would be reasonably safe to assume that high hardness measurements reflect high calcium levels.

A CaCO₃ hardness value is a reliable indication that the calcium concentration is low.  However, high hardness does not necessarily reflect a high calcium concentration.  But, since limestone is common in the soil and bedrock of the southern United States, it would be reasonably safe to assume that high hardness measurements reflect high calcium levels.

A CaCO₃ hardness value of 100 mg/L represents a free calcium concentration of 40 mg/L (divide CaCO₃ value by 2.5) if hardness is caused by the presence of calcium only.  Similarly, a CaCO₃ value of 100 mg/L represents a free magnesium value of 24 mg/L (divide CaCO₃ value by 4.12) if hardness is caused by magnesium only.  These factors (2.5 and 4.12) are related to the molecular weight of CaCO₃ and the difference in weights between calcium and magnesium atoms.  Where hardness is caused by limestone, the CaCO₃ value usually reflects a mixture of free calcium and magnesium with calcium being the predominant divalent salt.

Agricultural limestone can be used to increase calcium concentrations (and carbonate-bicarbonate alkalinity) in areas with acid waters or soils.  However, at a pH of 8.3 or greater, agricultural limestone will not dissolve.  Agricultural gypsum (calcium sulfate) or food grade calcium chloride could be used to raise calcium levels in soft, alkaline waters.  Expense might be prohibitive if large volumes of water need treatment.  Identifying a suitable water source may be more practical.

Effects of pH, alkalinity and hardness on ammonia and metal toxicities

Ammonia becomes more toxic as pH increases.  Higher concentrations of the toxic form of ammonia (NH₃) are formed in basic waters; while the less toxic form, ammonium (NH₄⁺), is more prevalent in acidic waters.  Since alkalinity increases pH, ammonia will be more toxic in waters with high total alkalinity.  Hardness is not typically associated with ammonia toxicity.

Metals such as copper and zinc are commonly used around aquatic environments (tanks, plumbing and copper sulfate).  These metals become more soluble in acidic environments.  The soluble or free ionic forms of these metals are toxic to fish.  High total alkalinity increases pH and available bases which produce less toxic or insoluble forms of copper and zinc.  High concentrations of calcium and magnesium (hardness) block the effects of copper and zinc at their sites of toxic action.  Therefore, copper and zinc are more toxic to fish in soft, acidic waters with low total alkalinity.

Ideally, an aquaculture pond should have a pH between 6.5 and 9 are well as moderate to high total alkalinity (75 to 200, but not less than 10 mg/L) and a calcium hardness of 100 to 150 mg/L CaCO₃.  Many of the principles of chemistry are abstract (e.g., carbonate-bicarbonate buffering) and difficult to grasp.  However, a fundamental understanding of the concepts and chemistry underlying the interactions of pH, CO₂ , alkalinity and hardness is necessary for effective and profitable pond management.  There is no way to avoid it; water quality is water chemistry.

References

Boyd, Claude E. 1979. Water Quality in Warmwater Fish Ponds. Auburn University. Agricultural Experiment Station.

Tucker, C.S. 1984. Carbon dioxide, in T.L. Wellborn, Jr. and J. R. MacMillian (eds) For Fish Farmers 84-2. Mississippi Cooperative Extension Service.

LIMING PONDS -- ALKALINITY & HARDNESS

LIMING PONDS FOR AQUACULTURE

(view as Formatted PDF)

SRAC Publication No. 4100  (http://srac.tamu.edu)

Southern         

Regional

Aquaculture

Center

William A. Wurts

Kentucky State University Cooperative Extension

www.ca.uky.edu/wkrec/Wurtspage.htm

Michael P. Masser

Texas Cooperative Extension

The Texas A&M University System

The pH and mineral content of water are the result of interactions between the soil beneath a pond and the water used to fill it.  Clay soils are often acidic.  Because ponds are commonly constructed on these soils, especially in the southern and southeastern U.S., the effect on water quality can be significant.  Ponds with acidic bottom soils that are filled with poorly mineralized water characteristically have low alkalinity and hardness. When total alkalinity and hardness are below 20 mg/L (as CaCO₃) pH and productivity are usually reduced. Alkalinity concentrations below 20 mg/L often lead to large swings in daily pH values, which stress aquatic animals.  Acidic soils contain high concentrations of hydrogen ions and/or aluminum relative to the concentrations of calcium and magnesium, which are important minerals for good water quality.

The acidity of pond soils can be neutralized and the productivity of the pond improved by liming.  “Liming” refers to the application of various acid-neutralizing compounds of calcium, or calcium and magnesium.  Liming ponds has three important benefits. Liming may enhance the effect of fertilization. Liming helps prevent wide swings in pH.  Liming also adds calcium and magnesium, which are important in animal physiology.

The difference between alkalinity and hardness

It is important to understand the difference between alkalinity and hardness.  These two aspects of water chemistry are often confused.  The misunderstanding relates to the term used to report them – as ppm CaCO₃ (mg/L).  Total alkalinity indicates the entire quantity of titratable bases present in water, primarily bicarbonates, carbonates and hydroxides.  The most important components of alkalinity are bicarbonates and carbonates.  Hardness is the overall concentration of divalent salts (calcium, magnesium, iron, etc.) but does not identify which of these elements is/are the source of hardness.  Calcium and magnesium are the most common sources of water hardness.  Liming increases both alkalinity and hardness.

The effect of liming on fertilization

Both recreational and commercial ponds are often fertilized to improve fish production.  Fertilizers containing nitrogen, phosphorus and potassium (especially phosphorus) stimulate the growth of microscopic plants (phytoplankton) and animals (zooplankton), which, in turn, serve as food for animals in the aquatic food chain.  In recreational ponds, an abundance of plankton supports larger populations of species such as largemouth bass and bluegill.  In ponds used for commercial production of juvenile fish, plankton is the primary food source.  Healthy phytoplankton blooms also absorb toxic nitrogen wastes and raise daytime dissolved oxygen concentrations, so they are important to water quality.

Perhaps the most common reason to lime ponds is to improve the response to fertilization.  In ponds built on acidic soils and filled with fresh water of low mineral content, much of the phosphorus added in fertilizers becomes tightly bound in pond sediment where it is not available to support phytoplankton growth.  Proper liming can improve phosphorus availability and greatly enhance pond productivity.

Liming and pH swings

 

In ponds with acidic soils filled with poorly mineralized water with low total alkalinity, liming will increase total alkalinity.  This helps stabilize pH, which can swing widely from 6 to 10 during the day if total alkalinity is below 20 mg/L.  Fluctuations in pH are the result of the interplay of photosynthesis and respiration.  Nighttime respiration increases CO₂ concentrations, creating carbonic acid and causing pH to fall.  During the day phytoplankton absorb CO₂ for photosynthesis, causing pH to rise. Large, daily changes in pH can stress aquatic animals (Fig. 1).  Most aquaculture species can live in a broad range of alkalinity concentrations, but the desired alkalinity for many animals is 50 mg/L or higher.  Liming to raise total alkalinity to the required or preferred ranges buffers the water and reduces swings in pH (Fig. 2).

Liming and hardness

Hardness concentrations are important to aquatic animals also. Calcium and magnesium are essential for bone and scale formation in fish.  The most critical component of total hardness, however, is the calcium concentration or “calcium hardness.” Environmental calcium is crucial for osmoregulation, the biological process that maintains precise levels of internal salts for normal heart, nerve and muscle function.  In low-calcium environments, animals can lose (leak) substantial quantities of these salts into the water.  Calcium is also important in the molting process of shrimp and other crustaceans, and can affect the hardening of newly formed shells.  Most aquatic organisms can tolerate a broad range of calcium hardness concentrations, but a desirable range is 75 to 250 mg/L with a minimum concentration of 20 mg/L.  Adding liming materials or gypsum increases hardness.

Deciding whether to lime a pond

To determine whether a pond needs to be limed, first check total alkalinity.  Collect a water sample from the first several inches below the surface, making sure the sample contains no bottom sediment (mud).  Collect the sample in a clean quart container that has no chemical residues.  The sample can be tested for total alkalinity with a swimming pool test kit.  Or, the sample can be sent to a university laboratory or commercial testing company.   Check with your county Extension agent for information about water testing.

If the total alkalinity of the water sample is less than 20 mg/L, the pond may benefit from liming.  The amount of lime needed depends on the chemical characteristics of the bottom sediment.  Take samples of the pond bottom and have them analyzed to determine the soil pH and the amount of liming material to apply.  Collect the samples as you would for cropland.  Take samples to a soil depth of 6 inches from several locations in the pond (an S-shaped pattern is typically used).  In ponds less than 5 acres, collect at least ten samples per acre.  In a new pond, collect soil samples before filling.  In ponds with water, push a length of PVC pipe into the bottom and remove the mud plugs from the pipe.  Or, attach a can or small container to a long pole and scoop soil from the pond bottom.  Combine the samples, mix them evenly, and spread the blended sample out to dry.  After drying and crushing, mark the sample “pond mud” so the appropriate analysis can be made.  Approximately 1 pint of dried, blended soil sample is needed for lab analysis.  Contact your county Extension agent for information about soil testing services.

In some areas, specific tests for “pond mud” are not available.  However, there is a simple and reasonably accurate way to estimate the amount of liming material needed in a pond.  Submit the sample and request the recommendation for alfalfa production.  The amount of liming material needed to grow alfalfa will be very close to the minimum required for producing most aquatic animals.  Another method is to apply 1 ½ to 2 times the amount of liming material used to farm row crops in the surrounding area.

Choosing liming materials

Materials such as agricultural limestone, basic slag, slaked lime, quick lime and liquid lime have been used to lime ponds. While all these compounds neutralize soil acidity, some are more practical or effective than others.

It is not advisable to use quick lime (CaO) or slaked lime (Ca(OH)2). They are more expensive and can cause pH to rise rapidly to levels that can harm aquatic life.

  

Basic slag is a satisfactory liming material, but it is not commonly available and its effectiveness may vary signifi­cantly from load to load. A substance known as silicate slag is not an acceptable material and should not be used to lime recreational or commercial production ponds.

Liquid lime is popular among some farmers. This product is made by suspending finely powdered agricul­tural limestone in water. The small particles react more rapidly with the acid in soil and water and produce quick results. However, because this mixture is half water, it takes twice as much liquid lime as agricultural limestone to achieve the same results. Liquid lime can cost much more than agricultural limestone.

Finely crushed agricultural limestone is usually the best material to use. It is cost-effective and readily avail­able. Both pond alkalinity and hardness can be increased by adding either CaCO3 (calcitic) or CaMg(CO3)2 (dolo­mitic) limestone. It is difficult to add too much agri­cultural limestone to a pond. At a pH of 8.3 or greater, calcium combines with carbonate to form limestone and drops out of solution. Limestone does not dissolve well in ponds where soil acidity has been neutralized and water pH has stabilized at or above 8.3.

Pelletized lime has been available for farm and home use for many years, and there is some interest in using it for liming ponds. Typically, higher quality, finely ground agricultural limestone is used to make pelletized lime. To form pellets, the lime particles are bound together with lignosulfonates. Sufficient quantities of pelletized lime must be used to equal the neutralizing effectiveness of a recommended bulk agricultural lime application. Pel­letized lime may be easier to distribute than agricultural lime over small areas, especially if spread by hand in ponds already filled with water. However, pelletized lime is no more effective for reducing soil acidity than agricul­tural lime because the fine lime particles are bound with lignosulfonates into pellets. Pelletized lime is substantially more expensive to use than good quality agricultural lime with an equivalent neutralizing effectiveness.

Neutralizing value and efficiency

Commercial liming materials vary in their ability to neutralize soil acidity – their neutralizing value (NV).  Pure calcium carbonate is the standard used for assigning relative neutralizing values to each of the liming compounds.  Calcium carbonate is considered to have an acid neutralizing value of 100 percent.  Agricultural limestone may have NV values between 85 and 109 percent depending on its specific chemical composition.  Slaked lime has an NV of 136 percent. Neutralizing values for the liming materials previously discussed may fall between 55 and 179 percent (Table 1).

Table. 1.  Common names, chemical names and neutralizing values (NV) of several liming materials.
Common Name	Chemical Name	NV (%)
Basic Slag	--	55-79
Calicitic Limestone	Calcium Carbonate, CaCO3	85-100
Dolomitic Limestone	Calcium Magnesium Carbonate, CaMg(CO3)2	95-109
Slaked or Hydrated Lime*	Calcium Hydroxide, Ca(OH)2	136
Quick or Burnt Lime*	Calcium Oxide, CaO	179

*Use of these materials is not recommended because their effects on pH can be harmful to aquatic life.

Finely crushed agricultural limestone is composed of different sizes of particles.  Small particles react faster and dissolve more rapidly and completely than large particles.  Therefore, the neutralizing efficiency (NE) of agricultural limestone depends on the fineness of the mixture.  The particle fineness and associated neutralizing efficiency are determined by passing limestone through a series of sieves.  Particles that pass through a 20-mesh sieve but that are retained by a 60-mesh sieve have an NE of 52.2 percent.  Those passing through a 60-mesh sieve have an NE of 100 percent.  The various quantities of each particle size grouping and their associated NE values must be averaged to arrive at an overall NE rating.

If the liming requirement, neutralizing value (NV) and neutralizing efficiency (NE) are known, it is possible to calculate the precise amount of lime needed.  Divide the amount of liming material recommended (tons per acre) by the product of the neutralizing value and the neutralizing efficiency (NV x NE).

For example, a farmer submits a soil sample and the analysis indicates that 3 tons per acre of pure calcium carbonate are required to neutralize the pond soil acidity (or to produce alfalfa).  The agricultural limestone available at the local farm supply store has an NV of 85 percent and an NE of 71 percent.  The amount needed is determined as follows (percent must be converted to a decimal first).

Tons/acre CaCO₃ divided by (NV multiplied by NE) = tons of limestone needed.

3.0 ÷ (0.85 x 0.71) = 4.97 tons of limestone needed.

When only one value is available (NV or NE), divide the tons recommended by that value.  For example, if only the NV (85 percent) is known: 

3.0 ÷ 0.85 = 3.53 tons of limestone needed.

Timing and application of liming materials

To be effective, liming materials should be applied evenly over the bottom of the pond.  The best, and easiest, time to lime a pond is before it is filled with water.  A liming truck or tractor-pulled liming wagon can be driven around in the dry pond to spread the lime evenly over the entire bottom.  It is not necessary to disc the lime into the soil, but this will accelerate its neutralizing activity.

If the pond contains water, lime should be applied evenly over the entire pond surface.  Lime is loaded onto a boat or barge and then shoveled or washed uniformly into the pond (Fig. 3).  Often a sheet of plywood can be attached across the front of one or two small boats and the lime placed on the plywood.  Lime is heavy and shoveling it is tedious.  Therefore, some pond owners hire professional companies with liming barges to spread the lime.  For small ponds of less than 1 acre, liming trucks can be backed up to the edge of the pond and the lime distributed with the spreader on the truck.  This method works best if the truck can move around the entire pond and broadcast the lime evenly.

Fig. 3.  Agricultural lime being distributed evenly over a

pond from a pontoon barge.

Agricultural lime does not dissolve quickly in water and will sink to the bottom.  Liming a pond filled with water has an immediate effect on water quality.  It increases pH, reduces soluble phosphorus, and reduces free carbon dioxide.  Increasing the pH may cause the water to clear of suspended particles (mud), which can help pond productivity by increasing the light available to plants.  However, liming a pond shortly after fertilizing may remove phosphorus from the water, which could prevent a phytoplankton bloom from developing.  Recreational ponds are typically fertilized in the spring with compounds containing phosphorus.  So it is usually best to apply lime in fall or winter when productivity is unlikely to be affected.  The pond will equilibrate within several weeks and then fertilizer can be applied to adjust productivity.

Limestone dissolves slowly over time.  Alkalinity and hardness are washed out of the pond with overflow and drainage water.  Ponds that require lime usually need repeat treatments every 3 to 5 years.  Alternatively, annual lime applications can be made using one-fourth the original recommendation to maintain alkalinity, hardness and pH at acceptable levels.  If a pond needs lime, it will not respond well to fertilizer.

Managing calcium hardness

If the alkalinity concentration is below 50 mg/L, agricultural limestone can be used to increase alkalinity and hardness.  If total alkalinity is above 50 mg/L, adding agricultural limestone will not be effective.  Similarly, if pond pH is stable at 8.3 or greater, limestone will not dissolve.  For several aquaculture species (e.g., striped bass, red drum and crawfish), the preferred concentration of calcium hardness is above 50 mg/L.

Liming with agricultural limestone, using recommendations based on soil analysis, will usually increase alkalinity and hardness to the minimum required concentration of 20 mg/L.  A low total hardness value is a reliable indication that the calcium concentration is low.  However, a high hardness value does not necessarily mean that the calcium concentration is high. Where hardness is caused by CaMg(CO₃)₂ (dolomitic limestone), the total hardness value reflects a mixture of calcium and magnesium.  Magnesium can represent as much as 50 percent of the hardness produced by CaMg(CO₃)₂.  Other

magnesium-containing compounds, such as magnesium sulfate, may be the source of hardness in high alkalinity environments.  Therefore, agricultural limestone may not always raise calcium to the required or minimum desired concentrations. Agricultural gypsum (calcium sulfate) or food grade calcium chloride may be needed to raise calcium hardness in waters with alkalinities greater than 50 mg/L and low hardness.  Where alkalinity is high and hardness is caused by magnesium, adding agricultural gypsum or calcium chloride is also an effective way to raise the calcium concentration.

Alternative materials for raising calcium hardness

Agricultural gypsum (calcium sulfate).  Calcium hardness and total hardness can be increased about 1 mg/L by applying 5 pounds of agricultural gypsum per acre-foot.  Adding 125 pounds of agricultural gypsum per acre-foot would raise hardness approximately 25 ppm.

Calcium chloride.  Calcium hardness and total hardness can be increased about 1 mg/L by applying 4 pounds of calcium chloride per acre-foot.  Adding 100 pounds of calcium chloride per acre-foot would raise hardness roughly 25 ppm.

It is important to note that if phosphorus is added to ponds immediately before or shortly after applying gypsum or calcium chloride, the phosphorus may combine with calcium.  This may cause both elements to drop out of solution as calcium phosphate.  Phosphorus-based fertilizers should not be added for several weeks before or after the application of compounds that increase calcium hardness.

If high volumes of water regularly flush through a pond, the agricultural limestone, agricultural gypsum or calcium chloride that have been added can be washed out.  Often more than the recommended amount of limestone or gypsum is added so the materials will not have to be applied as often.  These chemicals will not cause problems in a pond if added at two or three times the calculated amount.

Culturists often overlook the importance of hardness and alkalinity.  The pond environment and aquatic animals benefit from water that has the desired levels of alkalinity and hardness.  The minimum concentration for both is 20 mg/L.  Managing these two components of pond water stabilizes or buffers pH fluctuations, improves the availability of phosphorus for phytoplankton, increases the natural food in ponds, and provides calcium for osmoregulation, egg hardening and other metabolic needs. Water should be tested periodically so that hardness and alkalinity can be managed properly.  Apply liming materials as needed and keep good records to improve water quality and overall pond productivity.

Suggested References

(Click on underlined topics below for full-text articles.)

Boyd, C.E. 1990. Water quality in ponds for aquaculture. Alabama Agricultural Experiment Station, Auburn University.

Boyd, C.E. and C. S. Tucker. 1998. Aquaculture Water Quality Management. Boston: Kluwer Academic Publishers.

(pdf) FERTILIZATION OF FISH PONDS
Brunson, J.S., N. Stone and J. Hargreaves. 1999. Southern Regional Aquaculture Center, Publication 471.

(pdf) INTERACTIONS OF PH, CARBON DIOXIDE, ALKALINITY AND HARDNESS IN FISH PONDS.
Wurts, W.A. and R. M. Durborow. 1992. Southern Regional Aquaculture Center, Publication 464.

DAILY PH CYCLE AND AMMONIA TOXICITY.
World Aquaculture, 34(2):20-21.

ORGANIC FERTILIZATION IN PRODUCTION PONDS
As, Organic fertilization in culture ponds. World Aquaculture, 35(2): 64-65 .

PRODUCING JUVENILE RED DRUM
Professional Internship (WFS) 684, Texas A&M University -- Final Report, July 1981.

RECREATIONAL FISH PRODUCTION.
As: Managing recreational fish ponds. World Aquaculture, 23(2): 41-47.

AQUACULTURE SITE SELECTION.
World Aquaculture, 23(3): 42-43.

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SRAC fact sheets are reviewed annually by the Publications, Videos and Computer Software Steering Committee.  Fact sheets are revised as new knowledge becomes available.  Fact sheets that have not been revised are considered to reflect the current state of knowledge.

The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 2002-38500-11805 from the United States Department of Agriculture, Cooperative State Research, Education, and Extension Service.