Liming

 Q. Lime is a fertilizer- isn’t it?

Ans: Lime is a fertilizer no doubt but usually it is considered by most scientists as more than a fertilizer which is very important and sometimes essential in fish culture because lime promotes biological productivity since it enhances the breakdown of organic substances by bacteria creating a more favourable oxygen and carbon reserves.

In most cases lime is used not as fertilizer but as a chemical substance which improves the aquatic environment to be suitable for fish culture, because lime kills the fish enemies such as insect and their larvae. Lime also destroys the algae and submerged weeds.

Importance of liming

We need to lime the pond because; lime has many beneficial actions on the pond water and soil, such as:

1.                  Lime kills the fish enemies such as insect and their larvae and also kills most microorganisms especially parasites due to its caustic reaction.

2.                  It destroys the algae and submerged weeds.

3.                  It raises pH of acidic water to neutral or slightly alkaline value.

4.                  It makes the carbon-buffer system stronger to prevent abnormal fluctuations of pH.

5.                  It neutralizes the harmful action of certain substances like sulfides and acids.

6.                  It promotes biological productivity since it enhances the breakdown of organic substances by bacteria creating more favorable oxygen and carbon reserves.

7.                  It controls quickly outbreaks of diseases.

8.                  It increases the beneficial action of organic and inorganic fertilizer used in the pond.

9.                  It precipitates suspended or soluble organic materials, decreases biological oxygen demand (BOD), increases light penetration.

10.              It acts as an antidote.

11.              It enhances nitrification due to the requirement of calcium by nitrifying organisms.

12.              It indirectly improves fine-textured bottom soil in the presence of organic matter.

Disadvantages of excessive liming

Excessive liming can be damaging because it decreases phosphorus availability through precipitation of insoluble calcium or magnesium phosphate.

Liming substances

The chemicals used for liming of soils are the oxides, hydroxides and silicates of calcium or magnesium since these are the ones capable of reducing acidity. Typical liming substances are the following:

1. Quick lime (CaO): Calcium oxide is variously known as quicklime, unslaked lime or burned lime. This is manufactured by roasting calcitic and hydroscopic and is sold commercially in powder and granular forms.

This form is the most reactive and hence dangerous to use. It reacts vigorously with water to form calcium hydroxide, generating much heat is the process.

Quick lime in granular forms can only be used for the preparation of lime milk which is particularly effective for disinfecting and killing parasites in small ponds. Quick lime in powder is used just as much for liming as for the destruction of enemies of the fish or against diseases. It is also used for liming heavily mudded pond bottoms and to produce the precipitation of an excess of the organic matter in suspension in the water.  

It is used at 500-700 kg/ha in thick mud or 200-300 kg/ha in thin mud. It also has strong disinfectant properties at 1000-1500 kg/ha.     

2. Calcium hydroxide [Ca(OH)₂]: This also called slaked lime or powdered hydrated lime, is obtained by stirring water on quicklime or leaving it exposed to the air. It contains at least 65 percent CaO.

This form is sold in bags as ‘builders’ lime. It is quite reactive and thus should be handled with care, but is more freely available than quick lime.

It is used at 1.5 times the rate for quicklime and is used for the same functions.

3. Calcium carbonate, CaCO₃ and mixed calcium-magnesium carbonate, [CaMg(CO₃)₂]: The carbonates occur widely in nature. Among the common forms that can be utilized as liming substances are calcitic limestone which is a pure calcium carbonate and dolomitic limestone which is a calcium-magnesium carbonate with varying proportions of calcium and magnesium.  

Commercial calcium carbonate is known as agricultural lime. This form of lime is used to control acidity of the water (pH and SBV too low) or when the use of quicklime is a danger to the fish. It is used at 750-1000 kg/ha.

The carbonates are the least reactive liming substances than other.

Calcium cyanamide (CaCN₂): Calcium cyanamide comprises 60 percent calcium cyanamide and 17 percent CaO. The use of this product is advisable for the control of encysted and very resistant parasites such as those of whirling disease in Salmonids.

Ref.

Huet, M., (1979), Textbook of fish culture Breeding and cultivation of fish.

Laszlo Horvath, Gizella Tamas and Chris Seagrave. Carp and pond fish culture.