Let me summarize where we left off:
1) The expanded shale in the grow bed had enough of a bacterial population that we could add fish to the system and be assured that any ammonia produced by the fish would be converted to nitrate.
2) The bacterial species that perform this conversion include the Nitrosomonas that converts ammonia into nitrite, and then Nitrobacter that converts nitrite to nitrate. The bacteria does this as a source of energy, using oxygen from the atmosphere to oxidize ammonia to nitrite and nitrate. Since these bacteria use inorganic substances as a source of energy, they are called autotrophes. This is referred to as chemosynthesis as opposed to autotrophic plants and algae which perform photosyntesis.
However, we are now posed with a dilemma: How do we remove nitrate from the water? Do we even
need to remove nitrate from the water? The answer lies in trying to recreate as natural an environment as we can for the fish. In nature, nitrate levels are typically very low, usually less than 20mg/Liter, and more often less than 10mg/L. My well water which is the source of the water for the aquaponics system doesn't contain enough nitrate to register on my testing kit.
While nitrate is far less toxic than ammonia or nitrite, it can reach toxic levels if there is nothing removing it from your aquaponics system. The two methods that are used to remove it are:
1) Water changes. This is why aquarium hobbyists do partial water changes in their aquariums, maybe 20% of the water per week. Obviously, the amount of water change can vary depending on how heavily the fish are fed, and how many fish/weight of fish there is. The more feeding, the more ammonia is produced from the metabolism of protein in the feed, the more ammonia is released by the fish into the water, and the more nitrate is produced from the bacteria converting the ammonia to nitrate. While many aquarium hobbyists keep a certain amount of plants in their aquarium, it usually grossly insufficient to keep the ammonia levels down unless you were to keep few small fish and a large amount of plants. It would also be critical to have enough light for the plants to grow and stay healthy. While there is a role for water changes in an aquaponics system, it is much more ideal to keep enough plants in your system to absorb enough nitrate from the water to keep the level low. Ideally, this is below 40mg/L but this can be difficult to maintain.
2) The second method to remove nitrate from the water is using plants growing in the grow bed. Plants need nitrogen to manufacture proteins and other substances. Plants cannot capture nitrogen from the atmosphere and must rely on naturally occurring nitrogen such as from lightening (nitrogen oxides formed when oxygen and nitrogen in the atmosphere react at high heat), bacteria that can capture or "fix" atmospheric nitrogen gas to ammonia, etc. The process of turning atmospheric nitrogen into ammonia is called "fixing nitrogen" because you are taking the relatively inert nitrogen gas and converting it into ammonia or the ammonium ion that can then be used by the plant. Most plants actually prefer to absorb their nitrogen in the form of nitrate, but they also have ammonia transporters that can absorb ammonia also directly from the water.
The following picture illustrates the nitrogen cycle whereby nitrogen gas is fixed by bacteria, then incorporated by plants, which are in turn eaten by animals, which then excrete ammonia in the form of actual ammonia as fish do, or as ammonia combined with carbon dioxide as mammals and many animals do, or as uric acid which is the preferred method of birds and reptiles. There are bacteria that can actually break down nitrate back into it's nitrogen and oxygen gas components and I will discuss this again at a later date.
The point here is that nitrate must be removed from the water in the aquaponics system. The basis of aquaponics is to use plants to remove nitrate, thereby creating a closed-loop system. Theoretically, the only thing that is added to the system is food for the fish and enough water to replace that lost by evaporation. The end product is fish meat and either a fruit or vegetable, or the plant leaves themselves are consumed. Since some fish like tilapia will eat plants, you can even take the system one step further and feed, say, spinach grown in the system back to the fish.
Now for the plants!
This is the grow bed after cycling with bacteria. You can add plants before the cycling is finished as they can absorb ammonia, nitrite and nitrate, but I didn't do so. I did run into some problems with the horseradish as I grew them indoors and then didn't acclimatize the plants slowly enough. This led to burned leaves and the plants had to regrow outside again. Not a big deal but I probably would have planted them earlier. I chose horseradish mainly because they have done well in my system in the past two years. They have several attractive features: they grow upright and therefore save space. They produce both an edible root and also edible leaves (yes, they're edible!). They grow at a wide range of temperatures, tolerating both extremes of cold and heat, although they are known for doing well in cooler weather. They also like a lot of water. The one thing I've found with the roots is that they may develop in two ways: Sometimes they will form a nice, thick root, and other times they form a very fine, large 'hair ball'. I'm not sure what the factors are that lead to one or the other. The hair ball definitely has a much larger surface area for collecting nitrate, but the root is edible. So either way I'm happy.
This is the horseradish growing in peat after they regrew their leaves (I left them outdoors). Horseradish is extremely easy to grow from even a tiny piece of root planted in the ground.
I washed all the peat off the roots. You could certainly take the mesh off the peat and plant it directly like that in the expanded shale grow bed, but I'd rather not have any soil/peat in the grow bed.
I used steamer clam shells to mark the areas in the grow bed where I would plant the horseradish. The total number of plants was 36 if I recall correctly. I did also plant some cucumber at one end of the grow bed.
Planting completed
I didn't want the water splashing up onto the plants and the inlet for the water into the grow bed from the fish tank worked out very nicely.
I didn't want the water splashing up onto the plants and the inlet for the water into the grow bed from the fish tank worked out very nicely.
The plants are in expanded shale only with no soil.
After a few weeks of growth:
View from the fish tank end:
This is a picture of the horseradish plants yesterday, 08/09/2014:
Long view:
Side view:
Close-up. Here you can see the cucumber plants:
These are close-ups of the cucumber plants from a while ago. I planted them in the grow bed after the first two leaves emerged, so all the growth you are seeing was mostly in the grow bed:
I put netting over the sump tank so the cucumber plants could climb along it:
Other side:
I started off with a total of 6 cucumber plants but one of them died. The following are pictures of the cucumbers from yesterday (08/09/14).
I'll eat this one today:
Small ones growing:
This is where the grown plants enter the grow bed. Some of the lower leaves can turn brown. I do have what I believe is Verticillium Wilt fungus throughout my garden and they can infect cucumbers, but I don't think that is what is happening here. The cucumbers are not wilting in the sun at all.
Now for the fish! These went in back in June. I only have pictures of them going
into the system, and only of the Channel catfish which all went into the sump tank. It's too stressful on them to remove them from the water and weigh them mid-season. I will of course weigh them when they come out of the system. I have goldfish, Yellow Perch, White Perch and Brown Bullhead catfish and those were all weighed going in.
First guy to go in:
I let the water acclimatize:
The Channel catfish all weighed about 5 grams when I bought them last fall. Going into the system, they weighed between 40 grams and 300 grams. Quite a spread!
You should handle the fish by hand whenever possible as netting can often tear the pectoral fin. Also, make sure to wet your hands before grabbing them.
Weighing can be a challenge as they don't like to sit still! The scale is in 20 gram increments but you can usually measure down to within 10 grams. That's enough for my purposes.
Channel catfish can be distinguised from other catfish by the deeply forked tail fin and the characteristic spots on the juvenile fish:
That's it for now. If you have any questions, please let me know.