nitrification & denitrification
Impact of Nitrogen on the System
Nitrogen is added to the system in the form of fish food.
The pond fish eat the food and convert it into more fish,
fish excrement, and ammonia. The fish excrement is
caught in the lava rocks and allowed to decompose in
a process called mineralization, which converts it into
plant-available nutrients.
If the pH isn’t too high and the water is cool, the
ammonia will capture a hydrogen atom from the water
then form ammonium. Since concentrations of ammonia
are more toxic to both fish and plants than ammonium is,
it is important to keep the water from becoming too basic.
Nitrifying bacteria convert the ammonium first into
nitrite and then into nitrate. Plants growing in the barrels
take up the nitrates and convert them into more plant
material. The lava stones collect any solid materials in
the water, which decompose in place for additional plant
nutrition. The water goes back into the pond cleaner than
when it was pumped to the highest barrel. Aquaponic
systems tend to work best under a mild load and the
backyard system described above was designed more
for ease of maintenance and aesthetic appeal than
maximized productivity.
Don’t Overcrowd the Pond
The nitrogen concentration in the water must be high
enough to feed the plants while being low enough
for fish to tolerate. That’s why leafy plants that do
well with mild fertilization tend to be simpler to
grow aquaponically than more nutrient-demanding
plants. Nutrient levels can be elevated by increasing
the number of fish the same size pond. But the more
crowded the pond is, the more likely nutrient levels,
pH, fish health, or other factors can spin out of control.
Crowded pond conditions can lead to an excess of
nitrogen in the water.
Use Denitrification to Lower Nitrogen Levels
If the plants don’t get enough nitrates to maintain
desired levels (and