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phosphate, Jobe's sticks and the substrate

Phosphorus is a macro nutrient required in significant amounts by plants
for growth. Free phosphates in solution are captured by plants (or
algae) very rapidly. When you have a healthy fish population and you are
feeding several milligrams of dried food (say 200-300 mg in a good
pinch), then you are inputting a significant amount of phosphate and
other nutrients in there. If you have a large, well lit aquarium with a
plentiful supply of all nutrients except phosphorus, then your growth is
phosphate limited (by definition). The first job is to make sure nothing
else is limiting: CO2, nitrogen, potassium, magnesium, calcium, sulpher,
iron and so on. That's pretty much fundamental in all of the strategies.
Here are the ratios of elements in typical plant tissue from a table
given by George from adapted from from the Feb, 1988 "Today's Aquarium,
the International Magazine of the Optimum Aquarium", ("Aquarium Heute"
in German), published by Aquadocumenta Verlag GmbH. Units are mg/Kg. I
don't know if its dry weight or wet; George do you remember?

Oxygen         48,000
Carbon         36,000
Hydrogen        6,000
Potassium       3,600
Nitrogen        3,200
Sulphur           660
Calcium           650
Phosphorus        460
Magnesium         210
Iron               15
Trace elements     10

This illustrates the ratios of the nutrients so we can get an idea of
just how much phosphorus we'd need; about 1/7 by weight of nitrogen so
if we had 10 mg/L of nitrate that is 10*14/62= 2.3 mg/L of nitrogen.
That means we need 2.3/7 = .32 mg/L of phosphorus or .32*95/31 = .99
mg/L of phosphate! If you had this much phosphate in your water all at
once, you'd get rapid algae growth. The key is that the phosphate is not
released all at once. In food or fish feces, phosphorus is not soluble.
When those phosphate compounds are in an oxygen free environment (i.e.
in the substrate), they are acted upon by bacteria and become more
soluble. This is why in nature, most phosphates are drawn up by plants
through their roots and free phosphates in the water are *extremely* low

Different phosphate compounds and mixtures have varying phosphate
solubility. Most fertilizers have relatively soluble phosphate forms. In
Jobe's sticks or in clay balls with fertilizer pellets, the fertilizer
is contained in a dense mixture (as in clay) which slows down the rate
that phosphate ions can diffuse out. I know from experience with clay
balls and Jobe's sticks that you get a bunch of roots growing into the
fertilizer and a rather large plant so I know that the plant is getting
more phosphate than its buddies who are just getting free phosphate from
the water. The evidence is the growth rate of the plant but its not just
that plant which benefits; all the plants in the tank get a phosphate
boost and so long as there is a surplus of other nutrients and light,
the plants absorb the free phosphate and grow just as fast as its being
released. However, if you have too many fish or add too much phosphate
fertilizer, it can't be used up fast enough and suddenly, within the
space of a few days, you'll end up with an unicellular algae bloom:
green water. If you also have other algae forms, they also grow
explosively, just like the plants.

If you have a densely planted, well lit, well fertilized aquarium with a
low fish population with no phosphate sources other than the fish food,
and you are experiencing relatively slow growth rates, chances are that
you can benefit by a phosphate addition. Recently, folks have mentioned
growth spurts following addition of dissolved phosphates in a low dose.
In my experience, you can usually safely add 3 or 4 Jobe's sticks broken
into small pieces into the substrate of a well planted 50 gal tank
without any risk of an algae problem. I think that clay fertilizer balls
may be a little bit better at preventing the release of phosphates than
a Jobe's stick but I don't have any hard evidence to support that. At
some point in the near future, I'm hoping to propose a series of
experiments in The Aquatic Gardener periodical for the Aquatic Gardeners
Association which will measure the nutrient release ratios of a whole
range of materials and mixtures which should give us some quantitative
numbers to toss around.

I hope I have all my facts straight and my multiplications and divisions
ok but please correct any mistakes.

Steve Pushak
Aquatic Gardeners Association       Vancouver