Major concerns in pond bottom soil management are low soil pH, high soil organic matter, lost of the oxidized layer, and accumulation of soft sediment. Although, the procedures outlines bellow can be used to correct soil quality problems.
The reason for liming aquaculture ponds is to neutralize soil acidity and increase alkalinity and total hardness in water. This can enhance conditions for productivity of natural foods and increase aquaculture production. Sample of bottom soil may be analyzed for lime requirement, but if this not possible, the general guidelines given bellow are suggested:
Lime Requirement (Kg/ha)
Agricultural limestone should be spread uniformly over bottom of empty ponds, or alternatively, it may be spread uniformly over water surface. Agricultural limestone should be applies at the beginning of the crop, and it should at least one week before fertilization is initiated. Agricultural limestone will not react with dry soil, so when applying over the bottom of empty pond, it should be applied while soil are still visibly moist but dry enough to walk on.
The purpose of drying pond bottoms between crops is to reduce the moisture content of soil so that air can enter pore spaces among soil particles. Better aeration will improve the supply of oxygen and enhance aerobic decomposition of organic matter. By drying for three weeks, most of the labile organic matter remaining in the bottom soil from the previous crop will decompose and reduced inorganic compounds will be oxidizer. The main benefit of this practice is to reduce oxygen demand of bottom soil as much as possible before beginning the next crop.
The time required to dry a bottom soil depends upon soil texture, air temperature, rainfall, and infiltration of water from adjacent ponds or shallow water tables. Light textured soils dry faster than heavy textured soils. Warm, dry weather conditions hasten drying, while rainy weather or infiltration of water into pond retards dying. The decomposition rate in soil will increase up to the optimum moisture content and then decline if soils are dried more.
Tilling bottom soils can enhance drying to increase aeration and accelerate organic matter decomposition and oxidation of reduce compounds. Accumulations of organic matter of other substances in the surface layer of soil also can be mixed with deeper soils to reduce concentrations of the substances in the surface layer. Pond bottom should not be tilled when they are too wet to support tillage machinery. Ruts caused by machinery will fill with soft sediment and be likely sites for anaerobic conditions. Ruts also interfere with draining and increase the difficulty of drying pond bottom.
Tilling can be counterproductive in ponds where heavy mechanical aeration is used. Tilling will loosen the soil particle and aerator inducted water current will cause severe erosion of the pond bottom. Thus, if bottoms of heavily aerated ponds are tilled, they should be compacted with a heavy roller before refilling.
Sediment accumulated in ponds for several reasons. There may be a large external sediment load from turbid water supplies. Erosion of embankments can result in large amount of sediment in deep water areas event where there is not a large external sediment input. Mechanical aeration can caused aeration in front of aerators where water current are strong, and deposition of eroded particles will occur in areas of the pond with weaker water currents.
Accumulation of soft sediment in pond is undesirable for several reasons. It fills deeper areas and can cause pond to lose volume. Soft sediment can trap feed pellets and fertilizer granules. Anaerobic zones often occur in soft sediment, and soft sediment is not good habit for benthic organisms. Soft sediment should be removed periodically before it reaches a troublesome thickness.
There are ponds constructed on soil with high concentrations of fibrous organic matter. Decomposition in organic soil is slow because pH usually is low and the amount of carbon relative to nitrogen is high. Nevertheless, because of high organic matter content such soil often becomes anaerobic during shrimp culture. Application of agricultural limestone to increase pH and inorganic nitrogen fertilizers to supply nitrogen will increase soil organic matter degradation during fallow period between crops. Nitrate also can be used to oxidize wet soil that can’t be dried.
Urea can be spread over pond bottoms at 200 to 400 kg/ha at the beginning of the fallow period to accelerate decomposition of organic soil. Agricultural limestone should not be applied until a few days after urea if applied to prevent a high pH. Urea hydrolyzes to ammonia, and if pH is above 8, much of the ammonia will diffuse into the air. Bottom may be tilled to incorporate lime and urea into soil to avoid ammonia volatilization. Tilling also provides better aeration of the soil mass to encourage bacterial activity.
Sodium, potassium, and sodium nitrate can be applied to wet soil to encourage organic matter decomposition by denitrifying bacteria and to oxidize ferrous iron, manganous manganese, and hydrogen sulfide. The usually application rate is 20-40 gr/m2 over wet areas. Productivity of benthic organisms may be low in ponds with concentrations of organic carbon below 0.5 to 1.0%. Organic fertilizer can be applied to such soils at 1000-2000 kg/ha to enhance organic matter concentration.
Bottom soil can harbor aquatic animal pathogens or their vectors between crops and cause diseases in the succeeding crop. Its common practice to attempt to disinfect pond bottoms following diseases outbreaks. Drying can eliminate most diseases organism, but the combination of drying and application of chemical disinfectant is more effective. Chlorination with calcium hypochlorite is recommended to kill organism in pond bottom treatment, before next culture cycle.