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Liquid fertilizer can be purchased in three forms. One is clear green and is made from new acids. The second form is gray because clay has been added to suspend phosphate in the liquid. Either of these forms is suitable for use in ponds; however, the clear green form (Poly N) is preferable. The third form is brown-black, formulated from used industrial acids, and may contain undesirable metals or chemicals. The brown-black type of liquid fertilizer is not recommended. If the fertilizer has less than 30 percent phosphorus (the second number on the label), apply one gallon per surface acre per application. If it has 30 percent or more phosphate apply 3 quarts per surface acre per application.
If the pond is less than two acres, the fertilizer may be applied with a garden sprayer or diluted 10 to 1 with water and broadcast from the bank. It should be sprayed or broadcast along at least 1/4 of the shoreline. If the pond is larger, the fertilizer may be applied with a power or tractor sprayer if 1/4 of the surface area can be covered. If necessary, the fertilizer can be applied by boat by siphoning from a container into the propwash of a small outboard or electric motor. Liquid fertilizer should not be poured directly into the pond. It is heavier than water and will sink to the bottom before it can go into solution.
Liquid fertilizer can be obtained more economically by supplying your own containers and by purchasing a year’s supply. Poly N should be stored in plastic drums since it can be corrosive to certain metals. The drum should be mounted horizontally and a plastic valve inserted so that each application can be easily withdrawn. A hole should be cut in the upper side of the drum so that the fertilizer can be stirred before each application.
In the summer, if the pond clears within two weeks following fertilization, apply more frequently (at two-week intervals) with one-half the recommended rate. Liquid fertilizer is so soluble that the planktonic algae can quickly consume it. Smaller, more frequent applications should solve the problem.