When to Fertilize Your Lake


Fertilization should begin in early spring for optimal lake management. This first fertilizer application does not always stimulate a bloom, but continue to fertilize at 2 to 3 week intervals until the pond blooms green. Once an algae bloom is established, fertilize the lake as necessary to maintain it. A pond manager can continue fertilizing until late September, but the growing season is depends upon latitude.

A simple method of knowing when to fertilize is based on water clarity. The depth that light can penetrate into the pond is a measure of the algal density or bloom. Light penetration can be measured using a Secchi disk. A Secchi disk can be made from an 8-inch diameter disk of plywood, metal or plastic. Mark the disk into quarters and paint the two opposite quarters white and black, respectively. Attach the disk to a yardstick or to a pole marked at 12, 18 and 24 inches from the disk. Use the Secchi disk guide below to help make fertilization decisions.

Lake Management Based on Secchi Disk Readings

  • Secchi Disk Reading — Recommended Management
  • Greater than 24 inches — Fertilize your lake or pond.
  • 18 to 24 inches — A good algae bloom with no maintenance necessary.
  • 12 to 18 inches — A dense bloom is present, so watch the lake closely.
  • 12 inches or less — Bloom is too dense, determine problem and aerate at night.
  • 6 inches or less — Oxygen depletion is immiment.

The optimum algae bloom is one that allows light to penetrate to a depth of 18 to 24 inches. Submerge the Secchi disk into the pond until it just disappears and note that depth. If the Secchi disk disappears between 18 and 24 inches there is no need to fertilize. It is time to fertilize again if the disk visibility is increasing rapidly toward 24 inches or if the disk is visible past 24 inches.

If the disk disappears between 12 and 18 inches, the bloom is too dense do not fertilize and watch the pond closely. If the disk disappears in less than 12 inches, the bloom is very dense and a severe oxygen depletion could occur. Remember, do not consider low Secchi readings that are the result of muddiness rather than algae.

A Secchi disk reading of 12 inches or less means the pondor lake is too rich in nutrients. At that point you need to determine where excess nutrients are coming from. Have you overfertilized? Are livestock manures or crop fertilizers entering the pond? If you are feeding the fish, are you overfeeding? Try to discover the source of the problem. Dense blooms can consume most of the pond’s oxygen at night, so be prepared to aerate at night if the visibility is low and there are consecutive days of cloudy weather.

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