5. POND CONDITIONING THROUGH LIMING
5.0 Introduction
1. The aim of good pond management is to increase fish production through an improved supply of natural food such as phytoplankton and zooplankton. The supply is usually increased by fertilizing the pond water. You will learn more about this and the fertilizers to be used in Chapter 6. However, in this chapter you will first be told how to prepare and, if necessary, treat your ponds so that these fertilizers will work as well as possible. This preliminary step is called conditioning the ponds.
2. Earthen ponds are conditioned by liming, i.e. preparing the ponds and treating them with various types of lime, chemical substances rich in calcium (Ca)similar to those you have learned to use to control pests (see Section 4.6).
3. This process improves the structure of the pond soil, improves and stabilizes water quality and makes the fertilizing materials act more efficiently to increase food supply.
4. One of the most important effects, and one which you can measure and use to control liming, is the effect on the total alkalinity of the pond water.
Total alkalinity
5. The total alkalinity (TA) of water is a measure of its total concentration in carbonates* and bicarbonates* of substances, such as calcium (Ca) and magnesium (Mg), which are characteristically alkaline (see Section 2.2). In natural waters, calcium bicarbonates are usually predominant.
6. Total alkalinity has great importance in fish farming. It tells you how much the water pH can vary and how good the availability of carbon dioxide (C02) is, for example, for the production of microscopic algae (see Section 2.0). The toxicity of certain chemicals, such as copper sulphate, can vary according to the total alkalinity of the water (see Section 4.9).
7. Total alkalinity depends on the local characteristics of the soils and water, and on the way your farm is operated. It is affected directly by liming, which adds calcium material to the water and to pond soils.
8. Water with high alkalinity is also said to have a good buffering capacity. It is quite stable chemically, and its quality does not vary much through the day.
9. You may also have heard about total hardness of water, which is primarily a measure of the amount of calcium and magnesium present. In waters good for fish farming, total hardness does not greatly differ from total alkalinity. Thus, soft water (low hardness), with little calcium and/or magnesium, usually also has a low alkalinity, while hard water (high hardness) tends to have a high alkalinity.
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Measuring total alkalinity
10. To measure the total alkalinity of water, you need two chemicals:
- a 0.1 normal solution of hydrochloric acid (HCI);
- some methyl orange indicator solution.
11. Proceed as follows.
(a) Obtain 100 ml of the water to be tested.
(b) Add three drops of the methyl orange solution to colour the water yellow.
(c) Using a pipette graduated in millilitres and filled with HCI solution, slowly add this acid drop by drop while mixing the water sample well, until its yellow colour turns orange-yellow.
(d) Confirm this by adding one more drop: the orange-yellow colour should now turn orange-pink.
(e) Measure how many millilitres of HCI solution was used, for example A ml.
Expressing total alkalinity
12. Total alkalinity (TA) is usually expressed in one of two ways:
- as the acid-binding capacity, in SBV units, SBV referring to the original German name "Sa�rebindungsverm�gen" (when the above measurement is completed, determine TA = A SBV units); or
- in mg/l of equivalent calcium carbonate (CaC03) (when the above measurement is completed, determine TA = 50 A mg/l CaC03).
Example
You measure A = 2.5 ml; the total alkalinity of the water is equal to 2.5 (SBV) or equivalent to 2.5 x 50 = 125 mg/l CaC03.
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Using total alkalinity in fish farming
13. To be of some value for pond fish farming, water should have a total alkalinity greater than 25 mg/l CaCO3. Best fish production may be obtained in waters where the total alkalinity ranges from 75 to 175 mg/l CaC03 (see the chart below). The toxicity to fish of certain chemicals, such as copper sulphate, can vary according to the total alkalinity of the water (see Section 4.9).
Alkalinity and fish farming
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5.1 When to use lime to condition your ponds
Introduction
1. Liming fish ponds is not always necessary. In certain cases, it may not only be a waste of money but it can also be harmful to your fish. Before making any decision, you should carefully study your own ponds and their particular water and bottom soil characteristics. Check for the following:
(a) If the pH of the pond bottom soil is less than 6.5, liming of the bottom soil is justified.
(b) If the pond bottom is very muddy because it has not been regularly drained and dried, liming will improve soil conditions.
(c) If there is the danger of the spread of a contagious disease or if common pests should be controlled, liming can help, especially in drained ponds (see Section 4.6).
(d) If the amount of organic matter is too high, either in the bottom soil or in the water, liming may be advisable.
(e) If the total alkalinity of the water is less than 25 mg/l CaC03, liming could be justified.
(f) If the pH of the pond water at the end of the day is low (see Section 2.2), the chart below shows the need for liming.
Liming desirability
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2. Liming will have little effect and might be difficult to economically justify if:
- the bottom soil pH is above 7.5;
- the water exchange through the pond is too fast;
- the water pH at the end of the day is 7.5 or above;
- the water total alkalinity is above 50 mg/l CaC03.
3. Generally, ponds should not be limed if:
- fertilizers will not be used subsequently, unless the water is very acid;
- natural food is not important, the fish being fed a complete diet;
- the water pH reaches more than 8.5 by the end of the day.
The beneficial effects of liming
4. If the above criteria justify treating your ponds with lime, a series of beneficial effects both on the bottom soil and on the water should result in increased fish production.
5. The effects on bottom soil are:
- structure will be improved (see Soil, 6);
- decomposition of the organic matter will be accelerated; and
- pH will increase.
6. All of these factors will result in a faster and greater release of minerals and nutrients from the bottom soil back into the pond water, together with a reduced demand for dissolved oxygen.
7. The effects on pond water are:
- pH will increase and become more stable;
- total alkalinity will increase, providing more carbon dioxide for photosynthesis;
- calcium content will increase, to be used by plants;
- certain toxic substances such as iron compounds will be neutralized and precipitated as pH increases; and
- excess organic matter will precipitate, decreasing the demand for dissolved oxygen in the pond water.
5.2 Chemicals for pond liming
1. Three basic chemicals are commonly used for liming fish ponds:
- calcium carbonate, CaC03;
- calcium hydroxide, Ca(OH)2, or hydrated lime;
- calcium oxide, CaO, or quicklime.
2. Each of these produces a different type of lime; toxicity for fish, the effectiveness for pond liming and the cost will vary (see Table 12). Some of the other characteristics of various liming materials have been given in Table 9
3. The efficiency of liming materials increases as their individual particle size decreases. Before use, you should ensure that the lime is finely ground, preferably passing through a sieve with 0.25-mm mesh.
Note: quicklime in lumps or granules can only be used as a lime milk for the disinfection of drained ponds (see Section 4.6).
BEWARE: Quicklime, hydrated lime and concentrated lime/water mixes can cause serious chemical burns. Avoid contact with skin and eyes as explained earlier in Section 4.6. If you do accidentally come in contact, wash immediately with plenty of water.
Making lime yourself
4. Natural deposits rich in calcium carbonate, such as limestone, shell deposits, or coral, can often be located near your farm. The best materials are usually white to light brown in colour. You should enquire about availability, for example at the Agricultural Department or the nearest office responsible for agricultural development. If this carbonate material is available, you can easily make lime yourself by remembering that:
- CaC03 + heat produce CaO + C02 (gas in the air);
- CaO + water produce Ca(OH)2.
Note: if the material has too much clay, it will produce lime that sets and hardens when in contact with water and it is poorer for pond liming.
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5. To prepare quicklime from calcium carbonate material, proceed as follows.
(a) Mark out a circle 7m in diameter on the ground. At its centre, mark a smaller circle 1 to 1.5 m in diameter. Draw two perpendicular lines through the centre of these two circles.
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(b) Stack four piles of logs about 1m high in the outer circle leaving free spaces between the piles as shown in the illustration.
(c) Fill the free spaces separating the log piles with dried leaves, twigs and small branches, and cover these spaces with a layer of smaller logs.
(d) Break CaC03 material into pieces 10 to 15 cm in size. Place the broken pieces on top of the log pile to a depth of 50 to 90 cm.
(e) On a windy day, set the log pile on fire.
(f) When all the wood is burned and while the material is still hot, sprinkle some water on it.
(g) Cover the burned pile with wet sacks, banana leaves or similar. Let it cool for at least 24 hours.
(h) Remove any over-burned or under-burned pieces. From 1 m3 of raw material, about 0.4 to 0.5 m3 of lime can be produced.
Note: quicklime should be stored in dry, airtight conditions; it will keep for several months in enclosed heaps or sealed bags. If it is exposed to water and air it returns to CaC03
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6. To prepare hydrated or slaked lime from quicklime lumps or granules, proceed as follows.
(a) If necessary, break the lime lumps into 5- to 8-cm pieces.
(b) On clean, non-porous ground, build a mound 20 cm deep.
(c) Sprinkle it with water using a spray, at the ratio of about 12 l of water to 100 kg of lime. The reaction produces heat and some noise as the material breaks up.
(d) Turn over the heaped material while applying water, until the lime breaks up into a fine, white powder.
(e) The process is finished when no further heat is produced when adding more water.
(f) The slaked lime can then be screened for the required particle size.
7. Slaked lime can also be prepared in a tank or drum. Use about three parts of water to one part of quicklime, add the lime to the water and let it stand for 24 hours.
Note: use this slaked lime as fresh as possible; if necessary, store it dry, in closed bags or sacks.
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5.3 How to calculate the amount of lime needed
1. The lime requirement of ponds is defined as the amount of liming material needed:
- to neutralize the acidity of the pond bottom soil; and
- to increase the total alkalinity of the water above at least 25 mg/l CaC03.
2. The lime requirement therefore varies, depending on:
- the nature of the bottom soil, heavy clay requiring more lime than sandy soil;
- the soil pH, acidic soil requiring more lime than neutral soil;
- the water total alkalinity, soft water with a low TA requiring more lime than harder water with a higher TA.
3. It also varies with the thickness of the bottom mud. If the mud is 30 to 40 cm thick, the pond requires much more liming than if the mud is 5 to 10 cm thick.
Determining the lime requirement of a pond
4. Usually, new ponds require more liming than older ponds that have been regularly treated, for example once a year. There are therefore several different treatments at different doses, depending on the circumstances. Among these are:
- initial treatment of a new pond;
- routine treatment of a drained pond bottom;
- routine treatment of the pond water.
Treating a new pond
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5. Initial treatment of a new pond:
- according to the nature of the soil, spread from 2 000 kg/ha CaC03 (sandy soil) to 4000 kg/ha CaC03 (heavy clay soil) on the dry pond bottom or an equivalent amount of another type of lime (see note below);
- fill the pond with water;
- one to two months later, determine the total alkalinity of the water; if it is greater than 25 mg/l CaC03, no more liming is required;
- if the TA is less than 25 mg/l CaC03, apply another dose of 2000 kg/ha CaC03 to the water;
- one month later, determine the TA of the water again, and if it is greater than 25 mg/l CaC03, no more liming is required;
- if the TA is still less than 25 mg/l CaC03, apply a third dose of 2 000 kg/ha CaC03 to the water;
- check the TA one month later when it will normally be greater than 25 mg/l CaC03
Treating a drained pond
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6. Routine treatment of a drained pond bottom: about once a year, apply one-quarter of the total quantity of liming material you required for the complete new pond treatment, as described above.
Treating pond water
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7. Routine treatment of the pond water: about once a month, check the pH of the pond water at the end of the day, and:
- if the pH is less than 6.5, add 150 to 200 kg/ha CaC03, checking the pH one week later and repeating liming if the pH is still too low;
- If the pH is between 6.5 and 8.5, check the TA of the water and if it is less than 75 mg/l CaC03, you could use lime to increase its value by adding one or several doses of 150 to 200 kg/ha CaC03 at weekly intervals until the TA is improved;
- if the pH is greater than 8.5, no liming is required.
Note: do not forget to correct the above quantities according to the percentage of CaC03 present in the particular liming material you are using. For example, if you are using a limestone containing 90 percent CaC03, multiply the amount of CaC03 suggested above by 100 ÷ 90 = 1.11.
8. If you do not use a calcium carbonate (CaCO3) material for liming, for example if you prefer to apply quicklime or hydrated lime on the drained bottom to control pests more efficiently, convert the above quantities according to the following equivalences:
100 kg CaC03 = 70 kg Ca(OH)2 = 55 kg CaO
Example
To lime your ponds, you require 2000 kg/ha (or 20 x 100 kg/ha) CaC03. Instead, you could use either:
- Ca(OH)2 at the rate of 20 x 70 kg/ha = 1400 kg/ha; or
- CaO at the rate of 20 x 55 kg/ha = 1100 kg/ha.
Note: when applied directly in water, the effects of quicklime or hydrated lime are much more rapid and need to be carefully monitored. In such cases it may be useful to apply some of the lime as CaC03 to moderate the speed or effects of the other materials.
5.4 How to lime ponds in practice
Introduction
1. Usually liming materials and fertilizers are applied separately. Liming then should be done at least two weeks, preferably one month, before any application of fertilizers (see Section 6.1).
2. Annual liming therefore is carried out in different periods of the year according to the pond management schedule.
(a) In temperate climates, such as in Europe, fattening ponds may be limed in the autumn just after they have been drained and harvested. Quicklime or hydrated lime is spread on the wet pond bottom, and fertilizers are applied in spring only. On the other hand, nursery ponds are limed in the spring just before they are half-filled with water, fertilizers being distributed later.
(b) In tropical climates, ponds are best limed as soon as the fish have been harvested and at least two weeks before new batches of fish are stocked. Fertilizers are then applied 15 to 30 days after liming.
Liming drained ponds
3. Drained ponds can easily be limed by distributing the finely ground liming material equally over the whole of the bottom surface. If the objective of liming does not include pest control (see Section 4.6), the bottom surface should preferably be dry.
Applying dry lime to the bottom of a drained pond
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Liming filled ponds
4. It will be easier to distribute the liming material evenly if it is first diluted in some water before being thrown into the pond. Buckets, wheelbarrows or old steel drums can be used to dilute the lime.
Applying diluted lime to the water of a filled pond
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5. Lime diluted in water can be applied using a large wooden spoon. In small ponds this can be done directly from the banks, but if the pond is large it may be necessary to use a boat or a floating platform.
Note: be particularly careful if you use quicklime in the presence of fish. In any one day, do not apply more than 200 kg/ha CaO, and frequently check the water pH at the end of the day. Be sure that it always remains below 9.5, or your fish might die.
Apply diluted lime from the banks in a small pond
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Apply diluted lime from a boat in a larger pond
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