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                                                                                   INTRODUCTION

                 Water quality management has been considered one of themost important aspects of pond aquaculture for many years,but less attention has been given to the management of pondbottom soil quality. There is increasing evidence that thecondition of pond bottoms and the exchange of substances between soil and water strongly influence water quality(Boyd, 1995). More attention is being devoted to the study of pond soils, and practical aquaculturists are beginning to seek information on pond bottom management.
               The purpose of this small manual is to provide a practicalguide to the management of aquaculture pond soils.

                                                   POND SOILS
Development
              During pond construction, surface soil in the area to become the pond bottom usually is scraped off and used as earth fill for embankments. The newly finished pond bottom normally is subsoil low in concentrations of organic matter and nutrients. In tropical and subtropical areas with highly leached soils, pond bottoms often are high in clay content and of low pH.After filling with water, various processes begin to transform the bottom of a new pond into a pond soil. Erosion of the watershed results in suspended particles of mineral soil and organic matter entering a pond in runoff. Wave action, rainfall, and water currents from mechanical aeration erode embankments and shallow edges to suspend soil particles. In addition,nutrients added to ponds in fertilizers, manures, and feeds cause phytoplankton blooms that increase the concentration of suspended organic particles. Suspended particles settle in ponds with large sand particles settling first, followed by

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    silt-sized particles, and finally clay particles and fine-divided organic matter. The names and sizes of soil particle classes are provided in Table 1. Water currents and activity of fish and other organisms continually resuspend particles from the bottom, and these particles settle again. Boyd (1976, 1977) reported that continuous input, deposition, resuspension, and redeposition of particles in a pond result in a sorting of particles with the fine clay and organic matter particles settling in the deeper water and the coarser particles settling in shallower water (Figure 1). Deeper areas gradually fill in, and ponds may decrease in volume. Sediment thickness in deep areas of aquaculture ponds usually increases at 0.5 to 1 cm yr-1(Munsiri et al., 1995; Tepe and Boyd, unpublished data).
             Organic matter settles to the bottom and is decomposed by microorganisms. Easily decomposable organic matter, such as simple carbohydrates, protein, and other cellular constituents, is quickly degraded. More resistant material, such as complex
    carbohydrates and other cell wall components, accumulates because it is degraded slowly. There is a continuous input of organic matter to the bottom, so microorganisms are continuallydecomposing both fresh, easily degradable (labile)
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              Because there is a more or less continuous resuspension and redeposition of particles and stirring of the surface sediment by fish and other organisms, the organic matter becomes rather uniformly mixed in the upper layer of sediment. Nevertheless,there usually is a layer of fresh organic matter at the sediment surface that has not been completely mixed into the sediment. Organic matter concentration usually is greatest near the sediment surface (Munsiri et al., 1995). The ratio of labile organic matter to refractory organic matter also is greatest near the sediment surface (Sonnenholzner and Boyd, 2000).Organic matter concentrations in pond soils do not continue to increase indefinitely. If aquaculture practicese.g., species; stocking, fertilization, and feeding rates; water exchange;amount of mechanical aeration; and treatment of fallowbottom between cropsremain about the same, the annual input of organic matter and rate of organic matter decomposition also will remain about the same (Avnimelech et al., 1984;Boyd, 1995).

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              Because there is a more or less continuous resuspension and re ...

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           New ponds usually have little organic matter in bottom soil, and the labile organic matter added each year will largely decompose, while a considerable proportion of refractory organic matter will accumulate (Boyd, 1995). Over a fairly short period, often only four or five crops, organic matter in the soil will reach a high enough concentration that the annualrate of decomposition of organic matter will equal the an nual input of organic matter. At this time, equilibrium will occur and soil organic matter concentrations will remain about the same from year to year.
            Dissolved oxygen cannot enter rapidly in bottom soil because it must diffuse through the tiny, water-filled pore spacesamong soil particles. At a depth of only a few millimeters below the soil surface, the demand for dissolved oxygen by microorganisms will exceed the rate that dissolved oxygen can move to that particular depth, and anaerobic conditions will develop. The oxidized (aerobic) layer of surface sediment will have a lighter color than the deeper, reduced (anaerobic) sediment. The anaerobic sediment usually will be gray or black, and this color results from the presence of ferrous iron.
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    New ponds usually have little organic matter in bottom soil, and the labile organic matter added each year will largely decompose, while a considerable proportion of refractory organic matter will accumulate (Boyd, 1995). Over a fairly short period,

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    often only four or five crops, organic matter in the soil will reach a high enough concentration that the annualrate of decomposition of organic matter will equal the an nual input of organic matter. At this time, equilibrium will occur and soil organic matter. At this time, equilibrium will occur and soil organic matter concentrations will remain about the same from year to year.



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