The timing of inorganic fertilization of sunfish ponds

Fertilization with diammonium phosphate (20 kg/ha per application) was initiated at different dates in each of four treatments replicated three times in 0.04-ha ponds. Although there were some differences in averages of total and filtrable orthophosphate concentrations and phytoplankton abundance among treatments (P < 0.05), sunfish production was not influenced by the timing of fertilization (P > 0.05). Phytoplankton blooms did not develop in any of the treatments until late spring or early summer. The failure of phytoplankton to respond to fertilization in late winter and early spring was apparently related to low water temperature and competition by macrophytes. In larger, deeper ponds (> 0.5 ha), inorganic fertilization will cause phytoplankton blooms in early spring even when water temperatures are low.