Liming of the ponds in Aquaculture

The favorable influence of liming on fish/shrimp/prawn production in aquaculture ponds of soft and acid waters has been attributed to several of the effects on water quality. Liming increases the pH of bottom mud and thereby increases the availability of phosphorus added in fertilizer. Liming increases benthic production in fertilized ponds, apparently through increased nutrient availability and also liming increases microbial activity in mud through a favorable increase in pH.

The advantages of liming are:

• To kill most microorganisms, especially parasites, due to its caustic reaction.
• To raise the pH of acidic water to a neutral or slightly alkaline value.
• To increase the alkaline reserve in the water and mud which prevent extreme change in pH.
• To promote biological productivity, since it enhances the breakdown of organic substances by bacteria, creating increased oxygen and carbon reserves. 
• To precipitate suspended or soluble organic materials.
• To decrease biological oxygen demand (BOD).
• To increase light penetration.
• To enhance nitrification due to the requirement of calcium by nitrifying organisms.
• To neutralize the harmful action of certain substances like sulfide and acid.
• To indirectly improve fine textured bottom soil in the presence of organic matter.

Liming increases the alkalinity of water thereby increasing the availability of carbon dioxide for photosynthesis. Greater alkalinity after liming also buffers water against drastic pH changes common in eutrophic ponds with soft water. 

Early morning pH will be higher after liming, but, because of buffering by bicarbonate, afternoon pH values will not be as high as before lime application. Liming raises the total hardness by adding alkaline earths (calcium and magnesium – PearlSpar-Aqua). By lime treatment, waters may be cleared of humic stains of vegetative origin, which restrict light penetration. The net effect of changes in water quality following liming is to increase phytoplankton productivity, which in turn, leads to increased fish/shrimp/prawn production.

Actually, total alkalinity is a more reliable indicator of the need for liming than total hardness because some ponds may have a low total hardness and a high alkalinity or vice versa. Total hardness is easier to measure, particularly in the field, than alkalinity. 

Many times, the need for lime is first suggested when inorganic fertilization fails to produce an adequate plankton bloom. Nevertheless, total hardness or alkalinity analyses should be made and other possible reasons for the failure of the fertilizer to produce a plankton bloom determined before using lime.

Types of Liming Material

A number of different substances are used as liming materials, the chemical used for the liming of soil and water are the oxides, hydroxides and silicates of calcium or magnesium, since these are the ones capable of reducing acidity. Typical liming substances include the following:

Calcium (CaCO3) and dolomite (calcium-magnesium carbonate) [CaMg(CO3)2]

The carbonates occur widely in nature. Among the common forms that can be utilize as liming substances are calcitic limestone, which is a pure calcium carbonate and dolomite limestone, which is a calcium-magnesium carbonate with varying proportions of calcium and magnesium. Commercial calcium carbonate is known as agricultural lime. The carbonates are the least reactive of the three liming substances. It is especially recommended to use dolomite [CaMg(CO3)2] during the culture period.

Calcium oxide (CaO)

This is the only compound to which the term lime may be correctly applied. Calcium oxide is variously known as unsulated lime, burnt lime and quick lime. It is manufactured by roasting calcitic limestone in a furnace. Calcium oxide is caustic and hygroscopic and it is often recommended to apply this lime to acidic soils only. 

Calcium hydroxide (Ca(OH)2) 

Calcium hydroxide is known as flaked lime, hydrated lime or builders lime. It is prepared by hydrating calcium oxide. All are grayish-white powders. Liming materials differ in their ability to neutralize acid. Pure CaCO3 is the standard against which other liming materials are measured. The neutralizing value of CaCO3 is 100 percent and those for pure samples of other materials are as follows: CaMg(CO3)2, 109 percent; Ca(OH)2, 136 percent; and CaO, 179 percent.

Carbon dioxide in water reacts with dolomite as follows:

CaMg(CO3)2 + H2O + CO2 « Ca2+ + Mg2+ + 2HCO3 - + CO3 2-

This reaction suggests that dolomite will compete with phytoplankton for CO2 and possibly reduce photosynthetic rates. In addition to removing all of the free CO2 initially in the water, the CaCO3 reacts with CO2 released from the decomposition of organic matter and with CO2, which diffuses into the water. The net result is that a few days after liming, equilibrium concentrations of CO2 are higher than before. This occurs because the dolomite traps CO2, which would have otherwise been lost to the atmosphere. Dolomite will contribute equivalent quantities of cations and anions so increases in total hardness and total alkalinity following liming will be equal.

One might conclude that the amount of dolomite necessary to raise the total hardness of a pond to a specified level could be directly calculated. Using such logic, the amount of dolomite needed to raise the total hardness of a 1-hectare x 1 –meter deep pond from 5 to 20 mg/liter would be 15 mg for each liter of water or 15 grams for each cubic meter. Since the pond contains 10,000 m3, a total of 150 kg of dolomite would be required.

Lime serves two purposes – pH correction of water and pH correction of the soil bottom. When pH correction of water is the objective, lime can be made into slurry and either added to the incoming water or applied in front of aerators. If pH correction of pond bottom is the objective lime will need to be scattered like feed. Caution needs to be exercised while selecting lime. Most agricultural lime available in our country is granular not powder and has excessive quantities of moisture. It is strongly recommended that lime should be capable of passing 100 percent through a 60 mesh.

Since calcium is a major part of the bones and exuvia of fish and shrimp/prawn respectively requires high levels of calcium intake, especially after molting in shrimps/prawns. This requirement is met mainly by absorbing Ca available in seawater. The calcium content of the cuticle during the inter-moult stage is between 12% and 19% in shrimps and it loses about 23% of total body calcium by molting. However, the amount of minerals lost in the process of molting is higher than this because the exuvia includes other minerals in the form of calcium and magnesium salts.

Lime treatment:

1. To improve pond bottom condition during the preparation of grow-out pond. After each batch of cultivation, the bottom soil may become heavily polluted and acidic due to the accumulation of humus organic substances. Liming materials can be used to neutralize the organic acids released from humus substance and raise the pH value of bottom soil and to increase the degradation of organic substances, so that humus organic substances can be re-utilized as fertilizer during next cultivation.

2. Liming materials also posses disinfecting property and therefore serve as disinfectants when applied in the preparation of grow-out pond.

3. During cultivation period, when pH of pond water drops below the normal range for culturing shrimp (below pH 7.2), liming materials can be used to raise the pH value to an optimal level. The dosage is based on the pH of bottom soil and the kind of liming material being used.

The amount of lime used vs soil pH

pH of the soil in pond  at the bottom of the pond	Lime (tons/ha)
4.5 to 5.0	1.5-2.5
5.1 to 6.0	1.0-1.5
6.1 to 6.5	0.5-1.0

Source: www.neospark.com