Soil type is an important influencing factor in determining the chemical composition in productivity of a pond. The ascending order of productivity of soil types are the following:
Soil type can be changed by following a plan of fertilization and water control. Organic fertilizer will increase the amount of organic particles in the pond bottom if, during the pond drying season, organic deposits on the surface of the pond bottom are incorporated with the bottom soils. The presence of more organic particles in the bottom will increase production.
Range of soil pH of 8.0 to 9.5 are optimum for algal growth. A buffer, such as CuCO3, Mn(OH)2 or Na2CO3, act to reduce fluctuation in pH. Pond bottom soil is buffered with sodium chloride (NaCl) and sodium sulfate (NaSO4) by alternately wetting and drying the pond. Nutrients are stored in the pond bottom unless the pH of the pond water remains above 7.0 during the production season.
The optimum chemical analysis for growth of algae are:
| (NH3-N | 25 ppm | Result of analysis depends on the form of nitrogen in the soil |
| (NO3-N | 15 ppm | |
| P | 45 ppm | |
| K | 400 ppm | |
| Ca | 700–1200 ppm | |
| Mg | 300–600 ppm |
Soil tests should be done from samples taken in the pond bottom following standard soil test technique. The Philippine Ministry of Agriculture and Natural Resources and Bureau of Soils maintains many soil test laboratory throughout the country.
Determination of salinity in pond is necessary to determine the best species to be cultured and for the growth of fish food. A practical means of determining the salinity is by the use of a corked, weighted bottle with a wooden stick.
Then float the bottle in sea water and notch the stick where the water level is. By adding proportionate amount of fresh water a notch can be for each increased freshness.
There are three main sources of naturally occuring dissolved oxygen in water:
Plant growth usually contributes more dissolved oxygen than the other two sources.
Dissolved oxygen can be measured chemically using 1) modified wrinkler analysis, 2) potassium hydroxide meter, and 3) relative oxidation of organic material (Dendy, 1960). Garcia Fishermans oxygen meter is a handy tool for oxygen analysis. Tang, 1969, personal communication, said that bangus ponds frequently have near zero dissolved oxygen in early morning hours and during periods of cloudy weather. Warmer water can carry less dissolved oxygen than cool water.
Production of food organisms is higher in ponds with higher dissolved oxygen. Mechanical agitation of pond water is practical if the energy costs are low or the species produced has high value. Occasional aeration may be necessary to prevent loss of stocks. Windmill aeration is a possible method of reducing energy costs but a standby engine should be available in case aeration is necessary when wind velocity is too low to do the work. (See Joint SCSP/SEAFDEC Workshop on Aquaculture Engineering, vol. 2, Technical Report).
Acid water is less productive than neutral or basic water. During early morning, noon and just before sunset, pH readings give a profile of pond productivity. Samples should be taken from a minimum of three locations in the pond and at two depths, if the pond depth is 50 cm or more. The pH range during a series of tests will give a better idea of food production potential than a single test. For instance, if a midday reading of 7.2 occurs, the extension worker does not know if that is a high or low reading for the pond, however, if the early morning pH was 6.5, the midday reading is 7.2 and the 6:00 PM reading is 9.0, then the extension worker can advise the management needed. Aeration has also an effect on pH. Aeration tends to maintain the daily fluctuation of pH during cloudy weather. Fluctuation of pH is a natural process of photosynthesis (plant growth).
When pH is above 9.5 the food and fish are in trouble. Collapse of the food chain and therefore the decrease of dissolved oxygen in the ponds will follow shortly. Again, a single sample in a shallow corner when bluegreen algae is present will often show a pH of 9.5 to 10.5. This is not a true indicator of the whole pond but may warn the manager of a coming problem (see vegetation control-bluegreen algae/ copper sulfate).
- Ammonium nitrate (NH4NO3) is the most soluble form of a commercial fertilizer available in solid form (see pond fertilization). If it is apparent that food levels are low and dissolved nitrogen is below 5 ppm, enough soluble nitrogen should be added to the pond to bring the level to 14 ppm.
- Phosporous us usually the limiting factor of fish food production. The best sources of soluble phosphate is triple super phosphate (45%). (The level is too low at 25 ppm when the fish are two or more weeks from harvest).
- Potassium levels below 1 ppm are too low for fish food production. If fish are two or more weeks from harvest, 5 pounds murate of potash or fifty pounds of ash should be applied in the pond. Ash will also raise the pH.
Most production ponds have a total hardness (CaCO3) in the range of 75 to 150 ppm. When lower readings occur, liming with hydrated lime is needed. This is a very inexpensive pond treatment. Hydrated lime can be broadcasted at the rate of 50 pounds (25 kg) per hectare if one or more weeks is needed before harvest.
