The role of sediment in a culture pond is crucial for maintaining the chemical balance of the water. This is because there is a constant exchange of molecules between the water and sediment. These exchanges are a result of bio-geo-chemical cycles, such as carbon, nitrogen, sulphur, iron, and manganese. The cycles are regulated by pH, Eh, organic carbon, organic nitrogen, sediment oxygen demand, and hydrogen sulphide production potential.
pH is a measure of whether sediment/water is acidic or basic. Fish have an average blood pH of 7.4, so pond sediment /water with a pH close to this is optimum. An acceptable range would be 6.5 to 8.0. pH of water is very much influenced by the underlying sediment as there is continuous exchange of irons between water and sediment. pH of water can be directly measured by pH meter of using pH solutions. Meanwhile to measure the pH of sediment the sample must be prepared. Weigh 20 gm fresh soil or sediment from which stones, twigs and larger materials have been removed and place in a beaker or wide neck bottles. Add 40 ml of distilled water and stir vigorously on a magnetic stirrer if one is available or manually. Allow stand for 30 min. Decant the clear supernatant and determine the pH using a pH meter or pH solution
A pH ranges from 6.5 to 8.0 is desirable not in relation to fish health, but in relation to the environment health as well because this range is most favorable to the microbial processes in soil.
pH of soil can be regulated by applying lime materials such as Calcium Oxide, Calcium carbonate, Calcium bicarbonate and calcium Magnesium Carbonate as per the requirement.
Redox potential (Eh) is the measurement of the tendency of an environment to oxidize or reduce substrates. An aerobic soil, which is an oxidizing environment, has an Eh of+800 mV; an anaerobic soil, which is a reducing environment, has a negative Eh which can reach −300 mV. Oxygen is found in soils at a redox potential of about+800 mV.
Aquaculture pond sediment it is not desirable to have Eh below -150 mVolts as the soil has the potential to generate hydrogen sulphide, a toxic gas to fishes. This is an indication of high concentration (above 2% W/W) of organic matter in the sediment, and by bringing down organic matter through bioremediation the Eh also can be brough to above -150 mV. This has been successfully achieved through the application of the bioaugmentor ‘Detrodigest’ to the pond bottom.
Total organic carbon in soil is an indication of the health of an aquaculture system in the sense of its productivity and oxygen availability. The ideal range is 1-3%, and if it is below 1% productivity will be too low for which organic fertilizers will have to be added, and if it is above 3% bioremediation protocols must be adopted to bring it down. In case the organic carbon is above 10% top soil must be removed. Total organic carbon is linked with Eh in the sense that soil having high organic content will be having low Eh suggesting reducing conditions unsuitable for aquaculture.
Organic nitrogen is an indication of productivity of soil. The ideal concentration is 5-75mg/100g soil. If below this value there will not be any negative impact other than low microbial activity. If it is above 75mg/100g soil there is the possibility of the production of ammonia from soil under anaerobic conditions. Bioremediation with ‘Detrodigest’ and Nitrifying bacterial consortium is the best option prior to the commencement of culture and during the culture operations.
Phosphorus in the form of phosphate is an important nutrient required for phytoplankton production. This phosphate gets liberated from the soil beneath and therefore it is recommended to test its concentration in the form of total phosphorus. Ideal concentration is 3-6mg/100g soil. If it is less than 3mg poor productivity is expected. Supplementation with rock phosphate is recommended. If it is above 6mg/100g High productivity in terms of excess bloom can be expected and hence addition of manure can be avoided
High content of sulphur in soil is not ideal as it will pave the way to the production of hydrogen sulphide under anaerobic conditions. If the sulphur content is <0.3% it falls under Low-risk category in the sense that hydrogen sulphide production potential is low. If it is 0.3 to 0.5 moderate risk 0.5 to 0.8 moderate to high risk and if above 0.8 very high risk with respect to hydrogen sulphide formation. The culture operations must be regulated accordingly by incorporating bioremediation protocols for organic matter and hydrogen sulphide such as application of Detrodigest and Photosynthetic sulphur bacteria (PSB).
As sediment consists of dissolved and particulate organic matter along with an active bacterial population it exhibits oxygen demand which must be satisfied from the overlying water. This is inversely related to Eh and directly related to total organic carbon and nitrogen. Therefore, it is possible to develop a relationship between Eh vs Sediment oxygen demand and Total organic carbon vs sediment oxygen demand. Such an understanding will help one to forecast oxygen deficiency in the system and take precautionary measures to avoid any such eventualities.
In sediment with low Eh, high organic carbon and nitrogen and high sediment oxygen demand, hydrogen sulphide production potential will be high. Hydrogen sulphide up to 6mg/L/gm sediment has been noted with soil having Eh -250mV, which is considerably higher in an aquaculture system. Therefore, it would be ideal to investigate it so that precautionary measures can be taken in advance to avoid motility due to hydrogen sulphide toxicity. Bioremediation of organic matter with Detrodigest and hydrogen sulphide with PSB are the recommended methods for preventing any eventuality in such ponds.
Whitish sandy: Having more sand content and less organic matter. Productivity is likely to be less. Less iron content and thereby lesser iron sulphide. In case hydrogen sulphide is formed likely to be released in to water.
Brown loamy: Comparatively less sand and more organic matter, with moderate productivity. Less iron content and thereby lesser iron sulphide. In case hydrogen sulphide is formed likely to be released in to water.
Light black: Indicates formation of iron sulphide due to the trapping of hydrogen sulphide formed. Indicates high organic matter content.
Dark black: High organic matter, higher iron content and hydrogen sulphide production potential.
Sandy: Not ideal for culture as it does not hold nutrients and leads to low productivity.
Loamy: Consists of nearly equal parts of sand, silt and clay, robust combination nutrient rich.
Clayey: Stabilized the bottom and binds large quantity of nutrients and releases slowly
Unlock the secret to a thriving culture in your earthen pond! The key lies in understanding the chemistry of the sediment, which plays a vital role in regulating biogeochemical reactions to support your cultured species.
By delving into the intricacies of this chemistry, you gain the power to optimize your aquaculture through effective bioaugmentation and bio stimulations. These techniques allow you to take control of essential factors such as dissolved oxygen content, pH levels, ammonia levels, nitrite levels, and hydrogen sulphide levels. These molecules hold the key to influencing the success of your culture.
Don't leave your aquaculture to chance - equip yourself with the knowledge and tools to create a thriving environment for your cultured species. Discover how a deep understanding of sediment chemistry can revolutionize your pond today!
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