Pond fertilization with N-P-K has been used in the Southeast for the past 20 years to increase fish production and to control aquatic weeds and mosquitoes. In ponds which had been fertilized previously for a IS-year period with N-P-K, no significant decrease in production resulted from omitting both nitrogen and potassium from the fertilizer mixture during a four-year ‘experimental period. It appeared that adequate nitrogen for plankton production became available from nitrogen fixation by bacteria or algae and from the organic matter and ammonium stored in the bottom muds. Omission of phosphate, however, caused a decrease in production during all four years of the experiments. With no fertilization, production decreased during a two-year period; in the third and fourth years, production was approximately the same as in ponds that had never been fertilized. Phosphate fertilization only was tried in 14 ponds which had been fertilized for three to five years or more with N-P-K, and in which submersed pond weeds were absent. Satisfactory results were in general obtained in plankton production and in weed control. Eight ponds were subsequently drained and found to be supporting crops of fish at approximately the same levels as in previous years with 8-8-2 fertilization. However, in several ponds response to fertilization was delayed for 30 to 60 days, which allowed growth of the branched filamentous alga Pithophora. Also in ponds where Microcystis scums were a problem with 8-8-2 fertilization, they continued to be a problem with phosphate fertilization.
