Gypsum (CaSO4) is a water soluble mineral that can be found in fertilizers and mixed in many other products. It is pH neutral, therefore, its application does not have the risk associated with alum treatments. Gypsum works for water clarification by attracting clay particles together to form clumps or what we call floccules. As the clay particles continue to clump they eventually increase in weight and settle to the bottom of the pond of lake.
To determine the amount of gypsum, needed, obtain six one gallon glass jars. Fill 5 jars with the cloudy/muddy pond water. Reserve one jar of pond water as a control for comparison. The sixth jar is used to prepare the test solution. This jar should be filled will clear water that is not affected (collected rainwater is best, or obtain water from a nearby clear ponds or stream). The remaining four jars are treated at various rates of test solution to determine the rate that provides the best clearing of the water. Use the following procedure:
Using a standard measuring spoon, add two level teaspoons of gypsum to the gallon of clear water. Stir until the gypsum is in a slurry. Add different amounts of slurry to individual test jars. Allow the test jars to set undisturbed for 12 hours. The minimum amount of slurry that clears the water will be used to determine the amount of amendment to apply based on the following table
Source: Texas A&M
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