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Pond Fertilization

Fertilization can increase fish poundage by as much as three fold in some ponds.  A properly managed fertilization program should increase both numbers and average size of fish caught.  However, applying fertilizer to a pond already rich in nutrients can be harmful and may even cause a fish kill.  Nutrients provided by the addition of fertilizer increases the quantity of phytoplankton (microscopic plants) which form the base of the food pyramid in all aquatic systems.  Fertilization can also increase the “catchability” of fish by reducing water clarity, thereby making the fish less “spooky”.

The quantity of phytoplankton produced must be closely monitored for a successful fertilization program.  Over fertilization can cause an undesirable buildup of dense phytoplankton.  If this occurs, decomposition or respiration of the phytoplankton at night can consume all of the available oxygen in the water and cause fish kills.  This problem is made more acute if an unusual number of cloudy, windless days occur in the summer.  Once you observe fish “piping” or gasping for air at the surface, it is usually too late to save many fish.  Fertilization can cause a pond to turn green and may reduce its aesthetic value.  Do not fertilize ponds used for swimming.

Most ponds located in properly managed watersheds have enough natural fertility to produce a well-balanced fish community.  Because of the potential hazards associated with an improperly managed fertilization program, you should consider fertilization as an option only if you are interested in intensive management or specifically interested in quality bluegill or trophy largemouth bass management.  In general, don’t fertilize a pond that is being fed, since the by products of feeding will supply plenty of nutrients.

Pond Conditions for Fertilization           

Let’s proceed from here assuming that fertilization is a chosen part of your pond management program.  Below are the physical and chemical requirements that should be met prior to initiating a fertilization program.

Water visibility, determined by taking a Secchi disk reading, should be greater than 20 inches.  A Secchi disk is an eight-inch round disk divided into alternating black and white quadrants and attached to a pole or line marked in inches.  This disk is lowered into the unshaded water and the depth at which the disk is no longer visible to the naked eye is called the Secchi disk reading.  This simple tool is important to a fertilization program.  As a temporary substitute for a Secchi disk, use any light-colored or shiny object (pie plate, white cap or ring on finger) lowered into the water.

Never fertilize a muddy pond!  If the pond is muddy, steps to remedy the causes of turbidity must be taken before fertilization.  First, establish good plant cover in the watershed and along the shoreline to reduce the amount of muddy runoff into the pond.  Some ponds may still be muddy, the result of small clay particles remaining suspended in the water through wave action, by aquatic organisms, or by the repelling action of negatively charged colloidal clay particles.  To get these clay particles out of suspension and settled to the bottom, add either gypsum or hay to the water.

Apply gypsum at the rate of 500 pounds per acre foot of water.  Scatter it evenly across the pond’s surface.  Your pond should clear in one to four weeks.  If it fails to clear, add more gypsum at a quarter of the original application.  After it clears, the pond may need an additional bag or two per year to keep it clear.  Since gypsum changes the pond’s water chemistry by increasing hardness, add no more than absolutely necessary.  Gypsum tends to clear water faster than hay, but is more expensive.

In addition to being cheaper, hay offers the beneficial side effect of adding organic fertilizer.  Hay should be applied at the rate of three to ten bales, or 200 to 500 pounds, per surface acre.  Use less hay in late summer because of the increased chance for oxygen depletion as the hay decays.  At 10- to 14-day intervals add more hay until the pond is clear, but use only four applications per year at most.  Break up the bales and scatter hay loosely on the pond surface.  Do not think that if a little is good, a lot is better,  because too much hay or any type of organic matter (manure, cottonseed meal, or other fertilizers) can lead to an oxygen depletion fish kill.

Rooted vegetation should not cover more than 20 percent of the surface area.  Vegetation must be reduced to this desired level prior to fertilization because the added nutrients will promote the growth of the vegetation rather than phytoplankton.  See Aquatic Vegetation.

Application and Monitoring Schedule

Initial fertilizer application should be made in the spring when the water temperature reaches 60 to 65°F (usually April).  Ponds differ in the amount of fertilizer needed to produce the desired response.  However, a good starting point is to fertilize with a blend high in phosphorus such as liquid 10:34:0 at the rate of one gallon per surface acre or a granular blend of 118:46:0 or other equivalent at the rate of eight pounds of phosphorous per acre.

The rate should be adjusted accordingly to the phytoplankton response.  The amount of liquid 10:34:0 needed varies from 0.4 to 2.7 gallons per acre.  Liquid fertilizer is more cost effective than granular fertilizer, but availability may be a problem in some locals, and the granular type is easier to apply.  Application of liquid fertilizer can be made with a commercial sprayer in large ponds or a small garden sprayer in smaller ponds.  Best results are obtained if the fertilizer is sprayed evenly over the entire pond surface,  although pouring the fertilizer in the prop wash of a small boat driven slowly around the pond also can yield good results.  Liquid fertilizer is considerably heavier than water.  Therefore, it should be mixed with water before applying to the pond; otherwise, it will sink to the pond bottom.

Granular fertilizer is best applied by pouring fertilizer directly onto a floating or permanent platform and allowing wave action to slowly dissolve and distribute the fertilizer.  The platform works best if placed on the leeward (upwind) side of the pond and the prevailing winds are allowed to distribute the fertilizer throughout the pond.

Fertilize at the above rate every two weeks until a plankton bloom results.  The density of the bloom is measured using the Secchi disk.  Optimum bloom density is obtained when the Secchi disk reading is between 20 and 25 inches.  Secchi disk readings should be made at no less than weekly intervals.  If the Secchi disk reading is 20 inches or less, discontinue fertilization until the reading exceeds 25 inches.  Once the bloom is achieved, reduce the amount of fertilizer applied.  Fertilize with the liquid formulation at half a gallon per acre.  The granular blend can be changed to a 0:46:0 formulation and applied at the rate of four pounds per acre.  Never add fertilizer more frequently than every two weeks and discontinue fertilization when water temperatures fall below 65°F (usually October).

Creation of a bloom in a pond never before fertilized can take months.  Be patient.  The worst thing that can be done is to over fertilize.  It will take much less fertilizer to create a bloom in subsequent years.

Avoiding Low Oxygen Fish Kills

The first indication of upcoming oxygen problems is the water turning a brown color.  If this begins to happen, immediate steps must be taken to prevent a fish kill.  Pumping large amounts of fresh aerated water into the pond, spraying or splashing water across the pond surface, or using commercial aerators may help avoid a fish kill.  Prevention through careful monitoring of water visibility (caution zone is less than 20 inches) and oxygen levels is much preferred to attempting cures.  Oxygen meters and test kits are valuable for avoiding potential problems.  Oxygen readings should be made at dawn during the summer months when oxygen levels are at their lowest.  If at any time the levels fall below two ppm, discontinue fertilization and aerate!