Knowledge of the relationships between pond age and bottom soil quality and of the influence of soil management procedures on soil quality is important. This information can indicate whether or not ponds can be expected to decline in productivity over time. It also can reveal if common pond management procedures can be beneficial in maintaining good bottom soil quality. Data on changes in bottom soil quality over time also might allow improvements in bottom soil management techniques.
There is an area near Samutprakarn, Thailand, in the central region of the country where tilapia has been produced continuously in the same ponds for up to 40 years. This study was conducted to obtain data on bottom soil quality in ponds of different ages in this area. Samples of liming materials available to farmers also were analyzed to determine their quality.
Methods and Materials
Ponds and Management
The Thailand Department of Fisheries (DOF) located farmers willing to allow bottom soil samples to be collected from their ponds and to provide information about ponds and management. Bottom soil samples were collected from 35 ponds in the vicinity of Samutprakarn, Thailand in February 2002 (Figure 1).
Bottom soil samples were taken with a 5-cm diameter, clear plastic core liner tube (Wildlife Supply Company, Buffalo, New York). Workers waded into ponds and inserted the tubes into the bottoms by hand at five places in the deep end of each pond where water was 1 to 1.5 m deep. Tubes were hammered with a wooden block to force them into the original pond bottom or P-horizon as defined by Munsiri et al. (1995). Upper ends of tubes were beneath the water, so by closing them with a plastic cap, the tubes could be withdrawn from the bottom with soil core and overlaying water intact. Caps were placed on the bottom ends of tubes to prevent cores from slipping out, and tubes were held vertically to avoid disturbing the surface of the core. Water was siphoned by aid of flexible plastic tubing from the liner tube leaving only 1 or 2 cm above the soil surface. The thickness of the S-horizon and the total sediment thickness (S- and M-horizons) were measured as described by Munsiri et al. (1995). Soil cores were pressed upward in tubes with a core removal tool. A core segment ring made from a piece of core liner tube (Munsiri et al., 1995) was placed on top of the liner tube, and the part of the soil core representing the S-horizon was pressed into the core segment ring. The S-horizon was separated by inserting a thin, wide spatula between the bottom of the core segment ring and the top of the core liner tube. The S-horizon from one core tube in each pond was placed in a tarred soil moisture canister. The S-horizons from the other four cores were cut and combined in a single plastic container. Soil samples were held on ice in an insulated chest for no more than 12 h before they arrived at the DOF laboratory in Bangkok, Thailand.
A sample of surface water was dipped from each pond and stored in a tightly sealed 500 ml plastic bottle.
Soil Analyses
At the DOF laboratory, samples in tarred canisters were dried to constant weight at 102&Mac215;C and the dry bulk density was calculated. Composite samples were dried at 60&Mac215;C for 72 h in a mechanical convection oven. Dry samples were transported to Auburn University (AU) where they were pulverized with
a mechanical soil crusher (Custom Laboratory Equipment, Inc., Orange City, Florida) to pass through a 40-mesh (0.425 mm) screen and stored in plastic containers.
Soil pH was measured with a glass electrode inserted into a
1:1 mixture of dry, pulverized soil and distilled water. Exchangeable acidity was measured from the pH change in a buffer solution caused by adding 20 g soil to 40 ml buffer. The buffer was made by dissolving 10 g p-nitrophenol, 7.5 g boric acid, 37 g potassium chloride, and 5.25 g potassium