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Fertilizing Your Pond
maximizing your ponds potential
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March 2006
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Greetings!
You spent a lot of money building, stocking, and
maintaining your pond; why wouldn’t you want to
maximize the production in it? Fertilizing and feeding
are two ways to do this. This issue will cover
fertilization.
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About Fertilizer
Fertilizers are labeled according to their percentage
of the major fertilizer elements nitrogen (N),
phosphorus (P), and potassium (K). For instance, an
8-8-8 fertilizer will have 8 pounds of each major
element per 100 lbs. Nitrogen is very soluble, so
water, unlike land, generally has all the nitrogen it
needs. Adding more nitrogen just promotes the
growth of algae. Phosphorus is the element most
needed in aquatic systems. It is listed in fertilizers
as the orthophosphate equivalent, which is soluble in
water. Fertilizing your pond will promote the growth
of phytoplankton, which is the start of the food
chain and adds oxygen to the water as a by-product
of photosynthesis. This will also help control weeds
and algae by shading them out – no sunlight = no
photosynthesis = no weeds.
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Liquid Fertilizer or Granular?
Liquid fertilizers such as Dunn’s ‘Trophy Grower’ Liquid
Pond Fertilizer are usually 10-34-0, or 34%
phosphorus. These are easy to use and not as bulky
as granular fertilizers. The best granular fertilizers
are either super phosphate (0-20-0) or super triple
phosphate (0-45-0). Super phosphate is a mixture
of calcium dihydrogen phosphate and calcium
sulphate (gypsum), and made by treating rock
phosphate with sulphuric acid. Super triple
phosphate is made by decomposing rock phosphate
with phosphoric acid to produce calcium dihydrogen
phosphate. Granular fertilizers are high in
phosphorus, contain little or no nitrogen, and contain
calcium, which helps raise the water hardness.
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When Should I Fertilize?
You should begin fertilizing your pond when the water
temperatures reach the low 60’s. If not labeled
otherwise, use ½ to 1 gallon
of liquid fertilizer or about 10 pounds of granular per
acre. Note: It is always easier to put something in a
pond than it is to take it out. After the initial
treatment, the pond condition will dictate when you
should fertilize. Biologists use a Secchi disc, which is
a weighted 8” diameter disc with alternating black
and white quadrants. This is lowered until it cannot
be seen and that depth is recorded as the Secchi
depth. You can make your own by attaching a white
pickle jar lid, a pie pan, or a piece of plywood to a
wooden yardstick. You want the “Secchi depth” in
your pond to be between 18” and 24”. If the depth
is greater than 24”, it is time to fertilize. If not
labeled otherwise, use ½
gallon of liquid or 5 pounds of granular fertilizer per
acre, wait 5-7 days and check it again. You will
probably need to add fertilizer every 3-4 weeks until
the water temperatures drop below 60 degrees.
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When Should I NOT Fertilize?
There are times when you should not fertilize your
pond. First, if the “Secchi depth” is less than 18”,
quit fertilizing. If it drops below 12”, you should run
an aerator at night, if you have one. The reason for
this is that the phytoplankton has become so thick
that the stuff on top will shade out that below. The
shaded phytoplankton will then die, quit producing
oxygen, and decompose, taking oxygen out of your
pond. Another situation when you should not fertilize
is immediately after adding lime. Wait a week so that
the lime doesn’t precipitate the fertilizer out of the
water. Also, if your water hardness is less than 20
ppm, the phosphorus will be tied up in the bottom
sediments and not in the orthophosphate form that is
available. If your pond is muddy, adding fertilizer
won’t help, because sunlight will be unable to
penetrate enough to drive photosynthesis. Finally, if
you have more than 20-30% weed coverage,
fertilizing will just increase the problem.
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