POND PRODUCTIVITY AND FERTILIZATION

1 INTRODUCTION

The quantum of natural food produced in a pond is the result of interplay of various factors viz., climate and fertility status of pond soil and water. Phytoplankton being the primary producers in aquatic environment depends upon all these factors. Increase in primary production means an increase in pond productivity. With the adoption of suitable management measures through fertilization, the fertility status of pond soil and water could be maintained which will help in an increased primary production. Satomi (1967) observed that the composition of C:N:P in phytoplankton is approximately 50:7:1 by weight. Seymour (1980) from his data indicated that 1:4 ratio of P:N was advantageous to phytoplankton production in pond. It is obvious that lack of these elements in the environment would affect the productivity to a great extent. Various workers have observed that addition of these elements in the form of fertilizers or manures resulted in an increase in natural food and also fish production (Rabanal, 1967, Saha & Chatterjee, 1975 and Dobie, 1967).

2 FACTORS AFFECTING POND PRODUCTIVITY

The importance of bottom soil in influencing the pond water for productivity has been recognized by various workers (Golterman, 1967, Matida, 1967 and Banerjee, 1967). In ponds, not influenced by external factors, the physico-chemical properties of water which govern the biological production are more or less a reflection of the bottom soil.

Pond soils are formed on primary soils where the pond has been constructed and consists of two biologically significant layer. The upper layer is loose aerated and is a mixture of colloidal mineral and organic matter formed by precipitation of decomposed minute plant and animal organisms and through sedimentation of suspensions carried into the pond by rain and from different profiles. The lower layer is anaerobic and contains mineral matter of varying composition, depending on the parent rock from which the soil has been formed. The proportion of these two layers regulate various physical, chemical and biological processes in soil like, release of nutrients, adsorption of ions on colloids and decomposition of organic matter by micro-organisms. Thus the productive potential of a particular soil is guided by its physical and chemical conditions. Soil texture is one of the important physical factor while soil reaction (pH) and nutrient status are the chemical factors which determine water holding capacity and productivity of a pond to a great extent.

The texture of a soil i.e., the mechanical composition of various components (sand, silt and clay) along with the organic matter, influence the chemical and biological properties to a great extent. Claye and silty soils owing to their large surface areas possesses high adsorptive and retention capacitios for moisture, gasses and nutrients, while sandy soils are well drained and aerated. Therefore, sandy bottom and very claye bottom soils are not desirable as in the former nutrients are lost due to heavy leaching while in the later, high adsorption capacity impoversh the water of all its nutrients. Both clay (inorganic) and humus (organic) are colloidal in nature and exhibit colloidal properties like absorption and cation exchange phenomenon in soil. Sandy and sandy loam soils are low in colloidal clay and are very likely to be deficient in humus while havier soils contain more clay and generally more organic matter. These factors are of significance for deciding the use of fertilizers and manures alone or in combination for pond productivity.

2.2 Soil Reaction (pH

pH of the soil is considered one of the single important factors affecting pond productivity. It not only influence the soil microbial activity but also affect the availability of nutrients to pond water - either native or when applied externally. The availability of native or added phosphorus greatly influenced by soil reaction. In aoid soil phosphorus fixed or rendered unavailable as iron and aluminium phosphate and in alkaline soil as calcium phsophate (Banerjee & Ghosh, 1970). Banerjee and Mondal (1965) have shown that the concentration of added phosphate decline rapidly in acidic, neutral and alkaline soils and the rate of decline in phosphate concentration had shown a significant relationship with the composition of the soil. Like phosphorus the response of different nitrogenous fertilizers depend on soil reaction. Saha (1969) observed that loss of added nitrogen is minimum with ammonium form in acid and neutral soils and with nitrate form in alkaline soils. Calcium ammonium nitrate, urea and ammonium sulphate were found to be more effective in enhancing primary production, survival and growth of carp fry for moderately acidic, slightly acidic to neutral and alkaline soils respectively (Saha et al., 1975). In general slightly acidic to slightly alkaline soil pH is considered favourable for productivity.

2.3 Nutrients

The various basic elements (carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, iron, sulphur etc.) needed for the existance and nutrition of biologically productive organisms in aquatic environment are obtained from the soil and atmosphere for the enrichment of pond water. Among these elements nitrogen, phosphorus carbon and potassium are of importance compared to calcium, magnesium, iron sulphur etc. Nitrogen, being a basic and primary constituent of protein and chlorophyle, is necessary for the formation of living matter in a large quantity. But though, phosphorus is required in smaller quantity compared to nitrogen, it is considered as the single critical element for maintaining aquatic productivity. Phosphorus takes part in all aspects of cellulose metabolism, synthesis of protein and growth. Potassium helps in the formation of protein, chlorophyll and also stimulates the growth of aquatic flora. The atmosphere is the main source of inorganic carbon required for photosynthesis, thus converting the inorganic carbon to organic, while the decomposition of soil organic matter by microbial activity converts organic carbon to inorganic carbon and acts as a source of energy for microbial action.

Nitrogen in soils is present mostly in organic forms which are broken down into simpler inorganic compounds through bacterial action. Due to unlimited supply of this element from the atmosphere through fixation by azotobacter, blue green algae and atmospheric electric discharge, the deficiency of this element is less acute compared to phosphorus defficiency, usually encountered in most soils. Potassium due to its easy availability in pond soil and water is not found to be limiting productivity.

Banerjee (1967) from the study of large number of fish ponds under different agro-climatic conditions in the country observed that pond productivity depends on soil qualities such as its reaction, status of available nutrients (nitrogen and phosphorus) and organic carbon and classified these ponds into low, medium and high productive on the basis of these nutrient status as given below:

Tang and Cheng (1967) also classified milk fish ponds according the level of organic matter, total nitrogen, available phosphorus, potassium calcium and magnesium.

3 FERTILIZATION

Fertilization as a means of increasing fish production is well accepted (Hickling, 1971, Lin & Chen, 1967, Wolney, 1967). The natural productivity of a pond can be greatly enhanced by the use of fertilizers. It stimulates the growth of natural food by providing some essential elements. which found to be deficient in pond. Elements added as fertilizers or manures are utilized by phytoplankton which are the primary producers in aquatic environment. Once the primary production is increased, the whole of the food chain is activated resulting finally in increased productivity. Nitrogen, phosphorus and carbon are the major important elements added to ponds either as inorganic fertilizers or organic manures. Fertilizing ponds with organic manures was an old practice, but with the introduction of inorganic fertilizers the practice, has been changed from purely organic to inorganic or a combination of both.

Besides the use of major elements like N,P,K and carbon, use of micronutrients in enhancing productivity have also gained importance in recent years. Srenivasan et al., (1975) indicated the significance of micronutrients like sinc, molybdenom, cobalt, born, venadium and manganese in plankton production. Banerjee and Benerjee (1967) recorded an increase in plankton weight by about 14 per cent due to manganese alone. Das (1967) showed significant increase in survival of carp fry with micro-nutrient elements. Sen and Chatterjee (1967) obtained significant response in survival and growth of fry of Indian major carps in ponds using cobalt chloride, bron, manganese and molybdenom along with the supplementary feed. Saha et al. (1979) also recorded enhanced survival and growth of Labeo rohita spawn in the laboratory and mrigal fry in yard experiments using zinc, cobalt, born and molybdenum in near neutral soil.

To obtain an economic return of added fertilizers or manures, suitable forms and their optimal dosages are decided on the basis of soil texture, reaction and nutrient status of individual elements present in the soil so as to have a balanced aquatic environment for production. In order to achieve better results of fertilization inorganic fertilizers are preferred for highly claye soils rich with organic matter, while a combination of inorganic and organic are suitable for loamy soils with moderate organic matter. Organic manures with a limited quantity of inorganic fertilizers are desirable for sandy or sandy loam soils.

The mode and frequency of application of fertilizers and manures are of significance for proper maintenance of nutrient levels for production of aquatic organisms. The total quantity of fertilizers or manures are usually applied in split doses at periodic intervals during the entire growing period. While fortnightly application of inorganic fertilizers are resorted, monthly application of organic manures are prefered and alternately when both are used, since the chemical, biological and economic factors restrict the dose of added fertilizers.

3.1 Liming

The first step in fertilization is the application of lime. Liming raise the soil pH to a desirable level (near noutral) for establishing a strong buffer system, stimulates microbial decomposition of organic matter which favours mineralization of nitrogen and other nutrients from organic matter. It reduces toxicity of harmful compounds including disinfecting the environment. Sills (1974) stated that treatment of the damp bottom of ponds that have been drained with CaO or Ca(OH)₂ will destroy parasites and other undesirable organisms. Concentration of calcium sometimes calcium and magnessium (dolomite application) increases following liming which favours the precipitation of collodial matter. Hasler et al. (1951) found that lime application reduced concentration of collodial organic matter and increased the depth of adequate light penetration for photosynthesis in bog lakes in Wisconsin. Removal of turbidity increases the depth to which there is adequate dissolved oxygen for habitance by fish. A wide fluctuation of pH which is undesirable for aquatic organisims, is prevented by liming which reacts with carbon dioxide and soil colloids for stabilizing both the soil and water pH. Bowling (1962) observed that liming enhanced the response of plankton to fertilization in Georgia ponds and it increased the production of benthic food organisms. Arce & Boyd, (1975) observed that limed ponds had higher rate of phytoplankton productivity and yielded 25% greater weight of fish than control ponds.

The pH, texture and organic content of the soil are considered in deciding the amount of lime to be applied. The lime requirement of ponds is the estimated amount of liming material needed to neutralize the acidity of bottom muds and increase the total hardness and total alkalinity of the water to atleast 2) mg/l (Boyd, 1974). Boyd (1974) modified a lime requirement procedure for agricultural soils (Adams and Evans, 1962) for use with pond muds. He found that the base saturation and pH of muds were highly and positively correlated with the total hardness of pond water and in increasing the mud pH.

Generally ground limestone (CaCo₃) is extensively used and applied either to the pond soil while dry or broadcast over the water surface in a single application or in several equal instalments. Acccording to the soil pH lime at the following rates are usually applied to ponds.

These may be modified depending on the mechanical composition of soil. The dose may be increased or reduced by 50% for claye and sandy soil respectively. Liming is unnecesary when the soil is either neutral or slightly alkaline. But when acidic fertilizers are used it necessiates a pretreatment with lime before fertilizers are applied.

3.2 Organic manures

A varicty of agricultural wastes, dung of various farm animals (Cow, pig, buffalo, sheep, horse, poultry and duck) and also domestic sewage are used as organic manures for fertilizing fish ponds. These manures serve as direct source. of food for invertebrate fish food organisms and fish. Hickling (1971) stated that organic manures are especially efficient in increasing the abundance of zooplankton and benthic organisms. It also improves the cation exchanges capacity and buffering action of soil. The nutrients contained in organic manures are generally low compared to inorganic fertilizers and remain mostly in complex forms. These are made available only after their breakdown to simple inorganic compounds through microbial action. Thus, the release of nutrients from organic manures are slow and their availability remain for a longer period depending on the rate of decomposition which in turn is dependent on the C/N ratio of the added organic manures. Plant materials in general have a wide C/N ratio compared to animal manures. When such materials are used on initial depression of mineral nitrogen occurs due to their appropriation by soil microbes, while with narrow C/N ratio mineralization of nitrogen occurs during the process of decomposition. So, to avoid such immobilization of mineral nitrogen, ammonium compounds are sometimes added to organic manures having high C/N ratio.

A wide variation in the quantity of organic manures applied to ponds are adapted according to the nature of organic manure, its availability and also the duration of culture period. In composite fish culture, the normal dose, of cowdung varies from 5000–10,000 kg/ha/yr. depending on the organic carbon content of the soil. In absence of supplementary feeding, this is raised to 10,000–20,000 kg/ha/yr depending the productivity of ponds.

Though organic manures are useful in enhancing fish production, it has some disadvantages such as low and variable nutrient contents, their availability due to other alternate uses and ecological problems. Being composite in nature the results of their use are often unpredictable. They offend cesthetic value of a pond and are liable to transmit parasites and diseases. To avoid the chances of pollution and depletion of oxygen, organic manures are applied in heaps under water in split doses at monthly intervals during the culture period.

3.3 Inorganic fertilizers

Inorganic fertilizers have a definite and constant chemical composition of nutrient elements. Fertilizers certain these elements in the form of soulble or readily available chemical composition. They are sometimes called ‘chemical’ or ‘artificial’ manures. The fertilizers are usually classified according to the particular nutrient element which forms their principal constituent. They may thus be grouped as nitrogenous fertilizers, phosphate fertilizers, potassic fertilizers and sonon. Some fertilizers with specific grades are made by mixing appropriate quantities of two or more groups of fertilizers. These are known as mixed fertilizers, while fertilizers containing more than one of the three principal elements in a single compound are called compound fertilizers. Inorganic fertilizers are usually contain a high and fixed percentage of one or the other element as compared to organic manures and are customarilly expressed as percentage of available nutrient elements like nitrogen (N), phosphosic acid (P₂O₅) potash (K₂O). Liquid inorganic fertilizers have also been used and offer several advantages over the conventional granular or powdered NPK fertilizers (Geiger, 1983).

Due to high solubility of these fertilizers, the nutrients are readily available soon after their application to the pond and are rapidly utilized thus necessiates their application frequently. The dose of nitrogen, phsophorus and potassium usually varies from 100–200 kg N, 50–100 kg P₂O₅ and 25–50 kg K₂O/ha/yr on the basis of low, medium or high productive ponds.

4 CONCLUSION

Increased production of either natural food or fish has been reported by various workers through fertilizing ponds using either nitrogen or phosphorus alone or in combination of two or three elements in varying ratios (Wolny, 1967) Gooch, 1967, Lin & Chen, 1967, Saha & Chatterjee, 1974, 1975, Saha et al., 1978, Murty et al., 1978 and Chakrabarty et al., 1975). The importance of micro-nutrients in increasing production have also been recognized by many workers (Srenivasan et al., 1975, Das 1967, Banerjee & Banerji, 1967, Sen and Chatterjee 1976, Saha et al., 1979) but their large scale use in pond as fertilizers has not been done as compared to the use of major nutrients. Inorganic fertilizers have found much wider application due to its many advantages over organic manures for fertilizing ponds. Variation in the fertilizer dose with their suitable forms depending on the chemical and biological factors of the pond will enhance the productivity. But the observations made on the balance sheet of nitrogen in some composite fish culture ponds by Sinha, et al., (1980) and the tentative nutrient balance of carbon, nitrogen and phosphorus in intensive fish culture with supplementary feeding by Avnimelech and Lacher (1978) are of great importance which need to be consulted for their rational use in fertilizing fish ponds and increasing pond productivity.

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