Why nitrogen first?
In the world of human economy, politics and society, there are many different currencies, all of which are traded for goods. This could be oil, US dollars, Facebook Likes or Grandmas cookies. The natural world has similar currencies- trading in oxygen, carbon, and perhaps the most precious currency of all: nitrogen.
The whole world fights for it, tooth and nail, harvesting it honestly, cheating for it, and stealing it brutally. It is a powerful element, required in the highest quantities for good growth, defense and reproduction.
Almost all fertilizers have an NPK rating- telling you the relative concentrations of Nitrogen, Phosphorus and Potassium in that order. For vegetative growth (the growth of stems, leaves and roots), nitrogen is required more than any other mineral nutrient.
Nitrogen in aquaponics
Nitrogen typically occurs in several forms in the natural world. Nitrogen is used to make amino acids, which are used to make protein. All plants and animals contain proteins, and when they die, other organisms consume them and scavenge these proteins for energy and parts.
When land animals consume proteins they eventually break them down into amino acids and then to ammonia. Ammonia (NH3), is very nasty stuff- it’s very toxic, and the best way to get rid of excess ammonia is to excrete it. In order to make it safe until it can be excreted, in land animals, ammonia is converted to chemical called urea and excreted in urine.
In fish it is much simpler. Ammonia is usually present as Ammonium (NH4+) which moves across the cell membranes of the fish and eventually diffuses out into the water. Depending on the pH, ammonium may mostly stay ammonium or turn into ammonia, which can be very dangerous. Ammonia has no charge, so the fish have difficulty keeping it out of their bodies. Uncharged, the ammonia molecules can seep back into the fish’s cells and poison the fish.
Watch the video:
So, once ammonia is in the solution, it must be transformed or it will eventually kill the fish.
In almost all environments (except for anaerobic environments) ammonia is quickly transformed into nitrite (NO2-). Microbes in the soil or solution oxidize ammonia and in the process, get energy to fix carbon (break carbon off of carbon dioxide to use to build cells). This process has traditionally been attributed to a bacterium called Nitrosomonas, although research is showing that there are many hundreds, if not thousands of different species that do this work.
In the process, hydrogen ions (H+) are produced- the very ions that are measured in the pH test, and cause water to become acidic. Nitrite is very toxic, but fortunately for us, it represents a lot of stored energy to many other bacteria that again, oxidize nitrite and use the energy from the process to fix more carbon. The result is nitrate (NO3-), a relatively non-toxic form of nitrogen that plants can take up. The bacterium that has been most commonly accused of this reaction is called Nitrobacter, although research indicates that there are many bacteria that participate in this reaction besides Nitrobacter.
These reactions both produce Hydrogen (hydronium) ions (H+) which make the solution more acidic, causing the solution of older systems to trend towards more acidic pH values. Nitrogen is available at a broad number of pH values- so pH matters less when it comes to nitrogen availability.
However, pH can influence the form nitrogen available (see the video on ammonia/ammonium and pH) as well as the efficiency of nitrification. If nitrifying bacteria are not given time to adjust to changing pH levels (like almost any other system variable), nitrification will suffer.
In fact, nitrification proceeds just fine at low pH values so long as the nitrifying ecology is given time to adjust. Nitrifyers are renowned wimps when it comes to changing system variables. They often die off or go dormant when exposed to too much light, temperature fluctuations, fluctuations in salinity and pH as well as many other changes to their environment.
In any case, the end product is nitrate (NO3-). Some plants can take up ammonium and use it however, most prefer nitrate, and in systems where there is an excess of ammonium, the plants can tend to be more leggy and are often less salable. On the other hand, in systems with lots of nitrate, the problems with aphids and other pests can be more dramatic, requiring more intervention. . . But again I’m off topic.
Nitrate dissolves in the solution, but is instantaneously being competed for by bacteria, fungi, algae and other plants. All of these organisms are taking nitrate up and using it in their tissues. As the bacteria, fungi and algae die, that nitrogen (often in the form of protein) re-enters the system and the cycle begins again. Much of the nitrate however is delivered, safe and sound, to the root zone, where the plants in your system take it up and use it to grow.
While it is dangerous to have ammonia or nitrite levels much above 2 ppm and 1 ppm respectively, nitrate can often run well above 100 ppm (well off the chart for many nitrate tests) without posing a threat to your fish. Many hydroponic systems run nitrate in the range of 160 ppm when growing plants hydroponically, and plants can appreciate levels even higher than that, but the aquaponic grower must strike a balance between the needs of the fish, the system ecology (including pests) and the needs of the plants.
For this reason, I recommend that most aquaponic growers shoot to maintain their nitrate in the range of 40-80ppm for good, consistent plant growth.
Maintaining consistent nitrate ranges
Many systems have a hard time maintaining these levels, especially as the system matures, plants get large and the system ecology becomes more complex. This may require feeding to be increased to meet the increased demand. Many folks initially want to increase stocking density, but this is often a mistake. Instead increase feeding rates (but don’t overfeed!), and see if higher nitrate levels can be achieved with the same amount of fish.
In many ways, nitrogen is the most important plant nutrient, but also the most simple. The next one on the list is Phosphorus- a very important plant nutrient that doesn’t get much attention in aquaponic systems, but is incredibly important- especially if you’re trying to grow fruiting or seed crops.