Aquaculture CRSP 21st Annual Technical Report
Effects of Pond Age on Bottom Soil Quality
Tenth Work Plan, Pond Dynamics Research 1 (10PDR1)
Claude E. Boyd and Taworn Thunjai
Department of Fisheries and Allied Aquacultures
Auburn University, Alabama, USA
Department of Fisheries
Bottom soil samples were collected from 35 ponds in the vicinity of Samutprakarn, Thailand. Ponds ranged in age from 3 to
39 years and had been used continuously for production of tilapia. Liming materials had been applied in large amounts, and bottom soils of all ponds had pH above 7, low exchangeable acidity, and free carbonates. Pond soils often contained between
1 and 2% total sulfur, suggesting that they were potential acid-sulfate soils. However, acidity from sulfide oxidation was not expressed because carbonates in the soil neutralized it. Although farmers apply liming materials to ponds during each crop, this practice is no longer necessary in many of the ponds because of residual effects from previous years. Cessation of liming would be acceptable in ponds with soil pH above 7, but pH should be checked annually and applications resumed when necessary. Where liming is practiced, Thai fish farmers should be careful about the selection of liming materials. Evaluation of available liming products revealed that only about 50% of them were correctly labeled and of good quality. Concentrations of total carbon in pond soils seldom exceeded 4%, and the average for organic carbon was 1.90%. The correlations between pond age and both total carbon and organic carbon were weak (r = 0.34 and 0.36, respectively). Concentrations of nitrogen in bottom soils did not differ with pond age and ranged from 0.1 to 0.3% with an average of 0.19%. The average carbon:nitrogen ratio was 11:1 at both sites. Acid-extractable phosphorus concentrations averaged 217 ppm, but the phosphorus adsorption capacity averaged 768 ppm, suggesting that the soils still have considerable reserve capacity to adsorb phosphorus. Results of this study revealed that ponds can be used annually for semi-intensive production of tilapia and presumably other species for many years without serious deterioration of bottom soil quality.
Water quality in aquaculture ponds is influenced by the exchange of substances between soil and water, but only a few studies have clearly demonstrated relationships between bottom soil quality and fish production in ponds (Boyd, 1995). Nevertheless, aquaculturists insist that soil quality deteriorates rapidly in semi-intensive and intensive aquaculture ponds and that older ponds tend to have lower pH and higher concentrations of organic matter in bottom soils than newer ones. There have been only a few studies of the relationship between pond age and bottom soil quality (Tucker, 1985; Munsiri et al., 1995, 1996; Tepe and Boyd, 2002), and the results of these studies do not support the opinion that pH declines markedly and organic matter accumulates to high concentrations in older ponds.
The most common practices used in pond soil management are liming, drying of pond bottoms between crops, and sed
iment removal (Boyd, 1995). It is well documented that liming increases bottom soil pH, concentrations of total alkalinity, and total hardness in pond water (Boyd et al., 2002). The necessity for annual or more frequent liming, as often done, is not supported by research findings. Moreover, fish farmers often do not have information on the properties and quality of liming materials applied to ponds. Drying pond bottoms between crops can accelerate the decomposition of fresh organic matter and oxidize reduced substances in soil. The benefit of sediment removal on sediment quality is not well established, and it likely is unnecessary unless sediment is so deep that it causes a loss of pond volume or interferes with pond management operations (Boyd et al., 2002). Concentrations of nitrogen and phosphorus increase in pond soils over time (Masuda and Boyd, 1994; Munsiri et al., 1995). It is not known if the carbon:nitrogen (C:N) ratio, which influences microbial activity, or the ability of the pond soil to adsorb or release phosphorus change enough with pond age to influence bottom soil-water interactions.