Fisheries :: Fish Management

Soil

Bottom soil managemet

The role of bottom soil in determining productivity of a pond is well documented. The production of various primary food organisms depends largely on the availability of different nutrients. Dynamics of availability of most of these nutrients, in turn, is determined by the condition prevailing in the bottom soil. Considering this significance, bottom soil is designated as the chemical laboratory of the pond. However, suitable soil quality problems are common in aquaculture ponds, and therefore, many methods are used for the purpose of improving pond soils.

Texture

The nature and the properties of the parent material forming the soil determine the soil texture. Many important physico-chemical properties influencing the fertility of fishponds are affected to a great extent by the relative proportion of different size fraction of the soil. An ideal pond soil should not be too sandy to allow leaching of the nutrients or should not be too clayey to keep all the nutrients adsorbed on to it. If the pond is constructed on sandy soils, heavy doses of organic manure are essential to control seepage loss of water. In general, the dose of raw or composted farmyard manure varies from 10,000 to 15,000 kglha/year.

Soil acidity

Soil may be acidic, alkaline or neutral. The ideal range for soil pH is 6-8. Water passing over acid soil tends to be acidic with low alkalinity and hardness. High concentration of metal ions particularly aluminium (AI) and iron (Fe) also may be present. Acid ponds do not respond well to fertilization. Liming is the only way to improve water quality in ponds with acid soils and it is the pH of the soil that must be corrected for lasting effect, rather than the pH of the water.

Acid sulphate soils

Acid sulphate soils from mine spills and coastal mangroves contain high levels of pyrite (FeS2 1-6%). As long as sediments containing pyrites are submerged and anaerobic, they remain reduced and change little. However, as they are drained and exposed to the air, oxidation results and sulphuric acid is formed. Sulphuric acid reduces the pH of the water when pond is filled. In ponds the problems with acid sulphate soils usually originate in pond dykes. Pond bottoms are usually flooded and anaerobic, so sulphuric acid does not form. However, sulphuric acid is formed when dykes dry and enters pond with run-off water after rains. Acidity on dykes can be controlled by liming (0.5-1.0 kg/m2)and establishing good cover with an acid resistant grass species.

A procedure for rapid reclamation of ponds with acid sulphate soils involves drying and filling of the soil to oxidize pyrite, filling the pond with water and holding till water pH drops to below 4, and then draining the pond, repeating the procedure, until the pH stabilize at a pH above 5, and then liming the pond with 500 kg of calcium carbonate (CaC03)/ha.

Bottom soil oxidation

Dissolved oxygen cannot move rapidly into water-saturated soil, and pond soils become anaerobic below a depth of few millimetres. Aeration and water circulation are beneficial in improving bottom soil oxygenation, but the surface layer of soil may still become anaerobic in intensive fish culture ponds. When the redox potential is low at the soil surface (anaerobic conditions), hydrogen sulphide and other toxic microbial metabolites diffuse into the pond water. Sodium nitrate (NaNO) can serve as a source of oxygen for microbes in poorly oxygenated environments in which the
redox-potential will not drop low enough for the formation of hydrogen sulphide and other toxic metabolites.

Drying pond bottoms

When pond bottom dries up between crops, evaporation of waters from soil pores and cracking of the soil enhances aeration and favours microbial decomposition of soil organic matter. Excessive drying makes soil too dry for microbial activity, so a drying period of 2-3 weeks is usually is adequate. Tilling of dry soil with a disk harrow also can improve aeration, but tilled bottoms of aerated ponds should be compacted before refilling to reduce the tendency for erosion.

Fertilization of fish ponds

The natural productivity of a fish culture system depends largely on the availability of natural food organisms and on favourable environmental conditions for the fish, and optimum doses of fertlizers into the fish ponds.

(Source:Hand book of Fisheries and Aquaculture. 2006. Indian Council of Agricultural Research. New Delhi).

 

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