by Claude E. Boyd, C. Wesley Wood, and Taworn Thunjai
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| Cutting a pond sediment core into segments. |
he PD/A
CRSP has supported our studies on pond bottom soils since thebeginning of
the current USAID grant in 1996. We have collected a large amount of information
on physical and chemical characteristics of pond soils and are using these
data to make a soil classification system for aquaculture ponds.
Moreover, some of the findings of the research have practical significance
towards understanding the dynamics of pond soil and for improving pond soil
management. The purpose of this article is to provide some highlights of our
findings that will be of interest to fish and shrimp farmers.
The main sources of sediment in ponds are suspended soil particles that enter
via the water supply or originate from erosion of the wet sides of pond embankments.
Suspended soil particles settle and accumulate in deeper areas of aquaculture
ponds. For ponds in this study, sediment accumulated in deeper water areas
at rates of 0.5 to 1 cm per year. Pond sediment is rather fluid in comparison
to the original pond bottom soil beneath it. Sediment in ponds can cause several
problems. Ponds ecome more shallow and decrease in volume as sediment accumulates.
Soft sediment is not good habitat for benthic fish food organisms. The effectiveness
of feeding and fertilization is negatively impacted when feed pellets and
fertilizer granules are “lost” by sinking into soft sediment. Deep,
soft sediment interferes with harvesting operations by occluding seines and
hampering the movement of workers. Fish and shrimp also may become covered
with soft sediment, and this can impair their market quality. During pond
draining, soft sediment can be resuspended and exit ponds as suspended solids
in effluents. This increases the water pollution potential of pond effluents.
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| Pond sediment cores ready for laboratory analysis. |
Organic matter in ponds is derived mainly from uneaten feed, undecomposed manure, dead plankton, and aquatic animal feces. Sediment does not have as high an organic matter concentration as most fish and shrimp farmers commonly believe. Original bottom soil in most ponds in our study had less than 2% organic matter. Sediment usually contained 3 to 4% organic matter, and even in 50-year-old ponds, sediment contained only 5 to 6% organic matter. The highest concentrations of organic matter are found in the upper 5 cm layer of sediment. Much of the organic matter in this layer is of recent origin and susceptible to rapid decomposition by microorganisms. The organic matter in deeper sediment layers and in the original pond bottom soil is older than the organic matter in the surface layer, and it has already partially decomposed. Thus, it decomposes slower than fresh organic matter near the sediment surface. Anaerobic conditions that sometimes develop in surface layers of pond sediment usually are caused by fresh organic matter deposited during the current crop rather than older organic matter accumulations from previous crops. Nevertheless, when ponds are drained for harvest, procedures to lessen organic matter concentrations should be applied so that fresh organic matter concentrations in pond bottoms will be low as possible at the beginning of the next crop. This will provide a degree of protection against anaerobic zones in the bottom during the next culture period.
Nitrogen and phosphorus concentrations in pond sediment are higher than in original pond soil. The primary sources of these two nutrients are feed and fertilizer inputs. Phosphorus is tightly bound to soil particles and its solubility is low. There was no correlation between soil phosphorus concentration and the amount of phosphorus that could be extracted with water. However, soils with high clay contents tended to retain phosphorus more strongly than those of lesser clay contents. Incubation studies of pond soil samples revealed that ammonia mineralization usually did not occur during decomposition. The sediment does not appear to be an important source of nitrogen or phosphorus for the water of aquaculture ponds. Thus, aside from the possible need for greater phosphorus input in ponds with heavy clay soils, soil composition probably has a smaller influence on fertilizer requirements than often thought.
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| Sediment cores from an aquaculture pond. |
A number of methods were used for measuring pH on a series of soil samples.
For the same soil sample, each method usually provided a different pH reading.
Differences were particularly great when pH values measured by inserting the
pH electrode directly in wet soil were compared to those obtained for mixtures
when the electrode was placed in a 1:1 mixture of dry soil and distilled water.
Moreover, hand-held soil acidity testers, which often are used by fish and
shrimp farmers, gave unreliable pH readings. Based on this research, the following
method for measuring pH of pond soils is suggested: dry soil at 60oC in a
forced-draft oven; pulverize soil to pass a 2-mm sieve; mix soil and distilled
water in a 1:1 ratio (weight:volume); stir intermittently with glass rod for
30 min; insert dual electrodes or a combination electrode into the mixture;
measure pH while stirring. There were three reasons for selecting this procedure:
1) it measures the pH of the soil under aerobic condition, and aerobic conditions
usually exist at the soil-water interface; 2) precision of this method is
high; and 3) most pond soil management recommendations in the literature are
based on pH measured in 1:1 mixture of dry soil and distilled water.
Most farmers will not have the equipment to measure soil pH as suggested
above. Nevertheless, it does not seem prudent to suggest some simpler method,
because most recommendations assume that pH measurements are made in 1:1 dry
soil to distilled water mixtures. Soil samples can be sent to laboratories
for pH analysis by the procedure recommended above, or groups of small farmers
might cooperate to purchase a standard, laboratory-style pH meter.
Practical Application
When ponds are drained for harvest, bottoms should be dried for at least
two or three weeks before refilling. This will assure better contact between
soil and the air, and aerobic microbial degradation of organic matter will
be accelerated. Acidic soils should be treated with agricultural limestone
to increase pH and enhance microbial activity. Further improvement in organic
matter decomposition may be achieved by tilling ponds to 10-cm depth with
a disk harrow to improve aeration.
A disk harrow is the best device for normal tilling of pond soils, for it
is an energy efficient device and it pulverizes the surface soil. However,
if there is a high concentration of organic matter or nutrients in the surface
layer of the pond bottom, a mold-board plow (turning plow) could be used to
bury the surface layer and expose deeper soil of lower organic matter and
nutrient concentration.
During the culture period, the oxidation of bottom soils can be improved
by scarifying the sediment surface. This may be accomplished by raking or
by dragging a heavy chain over the bottom. Sodium nitrate is a soil oxidant,
and it is widely claimed that low concentrations of nitrate in the water will
prevent anaerobic conditions at the soil-water interface. Application of sodium
nitrate to maintain 5 to 10 mg l-1 nitrate in the pond water did not increase
the redox potential of sediment in aquaculture ponds at Auburn University.
Sediment thicker than 20 to 25 cm is detrimental to several pond management
objectives. Thus, sediment should be removed from ponds before it becomes
too deep. Excavated sediment should be placed back on insides of embankment
from which it eroded. If sediment must be disposed of outside ponds, it should
be used as landfill or spread in a thin layer and stabilized by compaction
and grass cover to prevent erosion. 
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