Acidity and Alkalinity in Fish Culture Ecosystem

Acidity and Alkalinity in Fish Culture Ecosystem

• The most important characteristic features of any fish culture ecosystem that should be kept in mind, is the acidity and alkalinity.

• Microorganisms and aquatic macrophytes are responsible for controlling so much of the chemical environment in an aquatic ecosystem.

• Acidity occurs where appreciable quantities of exchangeable base-forming cations (Na, K, Ca2, and Mg2) are leached from pond mud.

• In many situations, this condition is so widespread and the effects on aquatic life is so acute that acidity has become and important feature of any fish culture systems.

CONT…• Acidic ponds are not desirable for fish culture.

When a high degree of saturation with base-forming cations

takes place, soil alkalinity occurs. Concentrations of calcium,

magnesium, and sodium carbonates can result in a

preponderance of hydroxy ions over hydrogen ions.

• Under this condition, the soil is alkaline.

• Ponds having alkaline soils are recommended for fish culture.

Because acidity and alkalinity significantly influences soil

chemical properties and organisms, it is necessary to give a

brief idea of how these conditions limit fish production.

Source of Acidity and Alkalinity

• Two adsorbed cations such as hydrogen and

aluminium are primarily responsible for soil

acidity. The mechanisms by which hydrogen

and aluminium exert their influence depends

on the degree of soil acidity and on the nature

of the soil colloids.

Strongly Acid Soils • In cases where soil pH is less than 5.0, excessive aluminum

becomes soluble and is either present in the form of aluminum

hydrogen cations or is bound by organic matter. These

exchangeable ions are adsorbed by the negative charges of soil

colloids, which at low pH values are dominantly permanent

charges associated with silicate clay soils.

• The adsorbed aluminum is in equilibrium with aluminum ions

in the soil.

• Aluminum ions have the tendency to hydrolyze and thus

contribute to soil acidity.

Moderately Acid Soils• In cases where, soil pH values vary between 5.0

and 6.5, hydrogen and aluminum compounds

contribute to soil acidity but by different

mechanisms.

• Moderately acid soils have higher percentage

base saturations than the strongly acid soils.

Alkaline Soils • In this type of soils, the permanent charge exchange

sites are occupied by base-forming cations such as

Ca2, Mg, Na4 and K.

• Both hydrogen and aluminum hydroxy ions are

replaced by these cations.

• Most of the aluminum hydroxy ions are converted

into insoluble gibbsite by reactions.

Association of Cations • The distribution of cations around soil colloids is changed along

with the change in soil pH. Studies have shown that in some soils (such as silicate-clay and organic soils), the exchange capacity of base-forming rations declines as the pH is lowered.

• Hydrogen and aluminum ions are strongly held by the covalent bonding is termed as bound hydrogen and aluminum.

• On the other hand, these ions are associated with permanent negative charges on the soil colloids and termed as exchangeable.

• Exchangeable ions have an immediate effect on soil pH; Bound and exchangeable forms are very important for

determining how much lime is required to change so pH.

• Two dominant groups of element such as hydrogen and aluminum ions, are responsible for soil acidity. While these two ions generate acidity, base-forming cations resist it.

Source of Hydroxide Ions • When liming substances such as calcium hydroxide are added to an

acidic water bodies, H and Al3 ions are replaced by Ca2 ions.

• Consequently, concentrations of H and Al3 ions will decrease

gradually following the increase in the concentration of OH ions

because there is an inverse relationship between OH- and H ions in

water.

Therefore, cations can become indirect sources of OH- ions as they

are adsorbed on soil colloids.

• Alkaline reaction takes place through the hydrolysis

ions of soil coiloids saturated with base-forming cations and the

following reversible reaction occur.

Types of Soil Acidity • Generally three types of acidity occur in the mud

soil:

• (a) exchangeable acidity

• (b) active acidity

• (c) residual acidity.

• These three types of acidity together constitute the

total acidity of a mud.

Exchangeable Acidity • This type of acidity is represented by the presence of H

and Al3 ions that are exchangeable (salt-replaceable) by other cations present in some salts such as potassium chloride.

• It is important to note that in contrast to highly acid soils, H and A13 ion concentrations, in moderately acid soils is very low.

• But in spite of the presence of quite limited quantity of exchangeable hydrogen and aluminum, the limestone required to neutralize the acidity is about 130 times that required for very high acid soils.

Active Acidity

• This type of soil acidity is principally due to the H ions

in the soil mud.

• Though the concentration of H’ ions in such soil is

very small, it is important because in this environment

microbes are exposed.

Residual Acidity

• This acidity is associated with aluminum hydroxy

ions and with aluminum and hydrogen atoms that are

bound in non-exchangeable forms by silicate clays

and organic matter.

• However, addition of lime to such soils increases the

pH and the aluminum hydroxy ions are changed to

uncharged gibbsite.

Changes of pH in Pond SoilAcid-Forming Factors

• When fish culture ponds are fertilized with organic manures, they undergo decomposition and consequently, both organic and inorganic acids are formed.

• Perhaps the most widely acid found is carbonic acid. The persistent solvent action of this acid on the mineral constituents of the soil is responsible for the removal of base-forming cations by leaching and dissolution.

• Generally, inorganic acids (such as sulphuric acid and nitric acid) along with organic acids encourage the development of acidic conditions should be helpful in illustrating this explanation.

• Inorganic acids are formed by two processes: (a) from the microbial action on some nitrogen-containing materials such as ammonium sulfate and calcium ammonium nitrate, and (b) variation by the organic decay processes.

CONT…• The precipitation of inorganic acids, oxides of sulfur and

nitrogen from the atmosphere around the industrial complexes

is termed as “acid rain” since it has a pH value of 4.0-4.5

whereas in case of “normal rainfall”, the pH value varies

between 5.0 and 5.6.

• These substances also accumulate in aquatic ecosystem

through acid drainage and consequently, H ion concentration

considerably account for increased.

• Although addition of H ions are not adequate to bring about

significant pH changes at once, over a long period of time

their accumulation may have a significant acidifying effect.

Base-Forming Factors• High values of the base-forming cations invariably contribute

towards a reduction in acidity affects and an increase in

alkalinity.

• The addition of agricultural limestone and other nutrient carriers

and fish make them available for adsorption by soil colloids and

hence generate acidity.

• Therefore, liming and fertilization programs will permit the base-

forming cations to remain in the mud that will encourage high pH

values.

• This situation is favorable for ecosystem productivity.

Variability in Hydrogen Ions• Considerable variation in the pH of the mud always exists in

different areas of any aquatic ecosystem.

• However, this condition is not so prominent in small ecosystems, but in larger ones the variation is extreme.

• Such variation may result from microbial action due to the uneven distribution of organic matter.

• To avoid this condition, uniform distribution of organic manures is necessary.

• Variability in hydrogen ions is very significant in some respects. For example, microorganisms unfavorably influenced by a given H ion concentration in one place may find a different environment to another place that is highly favorable.

• Thus the variety of environments may account for the great diversity in the population of micro-organisms present in the mud.

Role of pH in the Mud Soil • The pH of mud soil significantly influences soil micro-

organisms and chemical properties.

• The pH of soil dramatically affects/influence the availability

of most of the chemical elements of importance to micro-

organisms and fish food organisms.

• For examples, bacteria functions best at intermediate and high

pH values. The availability of nitrogen is restricted at low pH

value, whereas that of phosphorus is best at intermediate pH

levels. The tendency for toxic elements (such as iron, cobalt,

zinc etc.) is well pronounced at low pH values.

Measurement of pH of Soil and Water

• For effective management of any fish culture

ecosystem the pH of soil and water should be

tested.

• The test is very easy and can be made rapidly.

• The pH is measured directly in field, or the samples

are brought to the laboratory for accurate

determination of pH.

Dye Methods• These methods take advantage of the fact that some organic

compounds change color as the pH is decreased or increased.

• Mixtures of dyes contribute to color changes over a wide pH

range (4-9).

• Different types of dyes such as bromo-thymol blue and chloro-

phenol red generally used for effective determination of soil

pH.

• A few amounts (2-4 g) of dry powdered soil is kept on a white

porcelain plate. Then a few drops of the dye solutions are

placed in contact with the soil sample.

• After a few minutes, the color of the dye is compared to a

color chart that indicates the approximate pH.

Litmus Papers For determination of soil pH, a suspension of soil in

water (a ratio of 1:2 or 1:1) is made where a strip of litmus paper is inserted into the suspension (Figure 13.3). By changing the color of the paper indicates the pH.

• For determination of water pH, pond water is taken in a beaker and a strip of litmus paper is immersed into the water. After standing a few seconds, the paper absorbs water and by comparing the color change of the paper to a color chart, the pH value is obtained.

Electrometric Method • It is the most accurate and simple method of determining

soil and water pH.

• In this method a sensing glass electrode is immersed into

the sample (either water-soil mixture for testing soil pH

or water only for testing water pH) that stimulate the

solution.

• The difference between the H ion exchangeable

activities in the sample and in the glass electrode gives

rise to an electrometric potential difference over that is

related to the sample pH.

Increasing Acidity • Although acidic ecosystems are not encouraged to farmers for

fish cultivation, some natural organisms which are used by fishes as food, grow best on water and soil pH of 6.0 and below.

• Therefore, it is sometimes necessary to generate the acidity of ecosystem.

• For this purpose, acid forming organic and inorgnaic materials are added.

• Partial or complete decomposition of organic matter generate inorganic and organic acids that can reduce the soil pH.

When the addition of organic matter is not possible, certain chemicals (such as ferrous sulfate) may be used.

Decreasing Acidity • Soil acidity is usually decreased by adding carbonates or

hydroxides of calcium and magnesium compounds.

• Oxide Forms - Oxide of lime is referred to as quicklime, burned lime, or oxide. Oxide of lime is more costly and caustic than limestone and hence difficult to handle. It reacts more rapidly with the soil than limestone.

Hydroxide Form It is commonly referred to as hydrated lime. It appears on the market as a white powder and is more caustic than burned lime. It is quite expensive compared to limestone, and used where a rapid rate of reaction and a high soil pH is necessary.

CONT…• Carbonate Forms

Although the main sources of carbonate forms include oyster shells, marls, and basic slag, ground limestone is the most common and extensively used of all liming materials.

• The two important materials included in limestone are dolomite, which is calcium-magnesium carbonate Ca.Mg (CO3)2. and calcite, which is calcium carbonate (CaCO3).

• If the ground limestone is entirely composed of calcium-magnesium carbonate and impurities, it is referred to as dolomite.

• When little or no dolomite is present, it is termed as calcite. The average total carbonate level of the crushed

limestone is about 96 per cent.

Decreasing Alkalinity• In many oligotrophic ponds and lakes where pH of water

drastically increases (more than 9.0), fish culture strategy has created several problems.

• High pH causes some of the calcium in the water to precipitate and thus, rises the sodium absorption ratio and the hazard of increased exchangeable sodium percentage.

To counteract these problems, sulphuric acid is sometimes sprayed over water to reduce pH of the water.

• This practice, may, however, well recommend where there are economical sources of this acid and where the fish farmers using it have been trained and alerted to hazards of using the acid.

Depletion of Calcium and Magnesium

• Since soluble calcium and magnesium compounds are moved

from soil to water, algae and (more than macrophytes are

absorbed these elements from water.

• Consequently, the pH and percentage base saturation are

reduced and therefore, application of lime is necessary.

• These elements, however, are depleted from acidic soils.

• Hence the liming of soil must be repeated with regularity.

• To keep the nutrient of pond water and soil in balanced

condition, application of these elements through liming should

not be ignored.

An Example • If a fish farmer wants to know how much of limestone

(calcium carbonate equivalent = 90) ) would be required to neutralize the same acidity as 1 metric tonne of a burned lime with a calcium conditions. oxide equivalent of 95 per cent, then the following steps should be worth remembering: (i) The burned lime has the neutralizing ability of 1000 x 0.95 = 950 kg of pure CaO. (ii) This amount of pure CaO is equivalent to 950 x 1.79 (calcium cabonate equivalent of In an calcium oxide) = 1700.50 kg of pure CaCO3. (iii) Since CaCO3 equivalent is 90, the quantity of limestone required is 1700.50 = 1,889 kg

0.90

Fineness of Limestone and their Reaction

• It is obvious that if the liming material is fine, it reacts more quickly with the soil particles.

• Generally calcium hydroxide and calcium carbonate are available in powder forms and therefore, there is no doubt about their fineness.

• Since different forms of limestone are used in fish culture their particle size and hardness may vary considerably.

• To use these coarse and harder limestone, their fineness should be considered.

Measurement of Fineness• The fineness of different forms of lime is

measured by passing the liming substances through a series of screens openings of designated size.

• A number 200 mesh screen has 200 wires per inch s, the ratio and opening sizes of 0.075 mm.

• Similarly, a 100 mesh screen opening is 0.0375 mm and a 10 mesh screen opening sizes of 2 mm.

• Generally the size opening of a given screen indicates the maximum diameter of. lime particles that can pass through the screen.

Reactions with Soil and Fish Production

• The effect of calcium oxide and calcium carbonate on the rate of reaction with soil was determined. It has been found that at the end of 2 months, about 80 per cent of calcium carbonate by using in the 100 mesh size had reacted with the soil, but less than 30 per cent of the 30 mesh size particles had reacted.

• On the other hand, at different size particles the reaction rate of calcium oxide with the soil was less than that of calcium carbonate. This is significant to ascertain liming rates.

• Fish production in ponds is greatly influenced by different types of liming materials and their fineness .

• To obtain about 53 per cent of fish production, 2,000 kg/hectare/year of calcium carbonate would be required if 40 per cent of the particles passed through a 60 mesh screen.

CONT…• On the other hand, to achieve 90 per cent of the

production about 500 kg/hectare/year of calcium carbonate would be needed if 95 per cent of calcium carbonate particles could pass through a 60 mesh screen.

• Those data point to a conclusion that liming substances with about 95 per cent passing through a 60 mesh screen are satisfactory for higher production of fish in lateritic ponds.

• Although finer particles of liming substances may give higher fish production, the additional cost involved in grinding liming materials should not be overlooked.

Practical Considerations • The characteristic features of soils and liming materials along

with cost factors determine the type and quantity of lime to be used for pond productivity.

• Generally 1,000 kg/hectare/year of ground limestone is applied for pond management if the pH of soil is about 7.5. But even higher rate of ground limestone (about 2,000 kg/hectare/year) may be appropriate on acid lateritic pond (where pH drops below 6.0).

• Among different forms of lime, however, ground limestone (calcium carbonate) should be favored to maintain adequate nutritional balance and to increase the activities of soil micro-organisms.

CONT…• To achieve most satisfactory response of living substances to

pond productivity as a whole, their uniform application over

water surface through different methods should be

recommended to fish farmers.

• In ordinary practice, calcium oxide or calcium hydroxide is bulk

applied for fish culture. But in many situations, sparse

application of liming materials is recommended to fish farmers.

• This prevents uniform rise in soil pH, which might occur if

liming substances were not uniformly applied.

Application of lime in excess than requirement results drastic

increase in pH value which is very detrimental to any fish

culture ecosystem.

• .

Lime as Pond Fertility • Application of lime to fish ponds maintains the levels

of exchangeable magnesium and calcium. • Moreover, liming also provides a physico-chemical

environment of an aquatic ecosystem that initiates the growth and production of fish and natural food.

• Liming materials also counteract the acid-forming tendency of nitrogen carriers whose use has increased significantly in the past several decades.

• Application of liming materials in fish culture ponds is a foundation for tropical and sub-tropical zones.

• However, the maintenance of fish pond fertility levels largely depends on the judicious use of liming substances

Conclusion • Among different chemical characteristics of soil and water, pH

is more important in determining the chemical environment of ponds, and its effect on growth of the fish and natural food.

• In contrast to acidic ponds, alkaline ones are usually considered as favorable for fish culture.

• This condition must be recognized in any fish culture management strategy.

• The pH of pond mud is controlled by soil coiloids and their associated exchangeable cations.

• Several base-forming cations encourage soil alkalinity, whereas hydrogen and aluminium increase soil acidity.

• The influence of pH on the availability of nutrients to plankton, its effect on fish survival and growth, and the control of acidity is extremely significant.