Low Nitrate

[Tom Dumas]

Can anyone tell me a good nitrate level for a system with 1000 gallons of fish water and 10 grow beds, with approximately another 500 gallons, for a total of 1500 gallons of water. It's a flood and drain system.  At present I have (60) 4 inch catfish and the water temp is 52 degrees.  The PH is 7.4  I have 20 small broccoli plants that have not grown much but look healthy.  Most of the beds are empty of plants. The fish seem healthy also.  The system is over 6 moths old. The nitrate level is always 0 as is the nitrate and ammonia levels. I use a API freshwater master test it. Any feedback would be appreciated.

[Harold Sukhbir]

Hi Tom,

At those low temps, getting zero readings for ammonia/nitrates is commendable! The nitrates depend on how much you feed, the temperature, how much nitrifying bacteria you have, and plant types and their density, among other things.

A zero nitrate reading means your plants are using up all available nitrates in the system, which is an ideal situation. To me this is "maximum" AP. When/if you temps get warmer and the bacteria start growing in population, you can increase feeding and thereby increase your nitrates, which will in turn increase plant growth.

I have run AP at 70F - 80F with zero nitrate readings for almost 1 year with amazing plant growth.

[Tom Dumas]

Thanks Harold I was getting worried the plants weren't getting enough nutrients and was ready to add more fish. This appears to be a tricking subject.  I only have a few plants and they appear to have used all the nitrates. As the weather gets warmer I assume the plants will use more nitrates. If I add 100 more plants it appears there won't be enough nitrates to go around. More fish?  I realize the fish will grow and at some point a portion must be removed.  There are several theories as to how many fish and what size they must be to produce the appropriate amount of nitrates for plants but what about when you start from the beginning, small fish. Or is this just a wait game? In my soil garden I just add the correct amount of fertilizer the plants need at the time they need it.  The soil garden always produces, well almost always, consistently whatever the vegetable/fruit. The soil nutrients are also measurable by a simple lab test at very little cost. How can you equate the soil stability to aquaponics?  My aquaponics system will be replacing the soil garden in the near future but I have to get this nutrient issue settled first. Trial and error gets frustrating and expensive.    Thanks Tom

[Harold Sukhbir]

Hi Dumas,

I've always started AP systems with 2 to 4 times the rule of thumb recommendation. Then, I'll feed the fish to capacity, usually 4-8 times per day. I also have to monitor ammonia/nitrite readings very closely during the fish growout. Periodically, I'll have to cull specific amounts of fish(which i cook!) to match the feeding regime in order to maintain a balanced reading. As you can see, many layers of work, but if its your goal to maximize your AP, there are several ways of doing so. Of course, other factors you can optimize are light, supplements, oxygen, high quality feed, etc.

[George]

Thus far, I've never had much of a nitrate level.  You have a lot of potential for nutrients as those 60 catfish grow.  I think you'll see a big difference as the air and water temp increases and the fish grow and feed more.  As that happens, the biofilter increases too and you have a nice amount of biofilter potential.  It appears to me that you're doing a lot of things right.  You may need a light plant load for your summer vegetables.  They're heavy feeders.  Next fall, try cabbage, if you like to eat it.  It is one plant that seemed to do better from the very beginning in my aquaponics system.  Now, everything grows better in aquaponics compared to soil.  It's really the perfect system for growing plants - they feed on a liquid diet and liquid is always available.  It's amazing how well plants can grow.

I think you'll find that fertility is easier to establish and maintain in aquaponics than in soil, but I'm comparing a purely organic soil garden to aquaponics.  An organic soil garden seems to me to require a good bit of input of water and compost.  It's so much easier to throw fish food in a tank and top it up on occasion.  

You'll be pleased, I believe, as time goes on.  

[Tom Dumas]

Thanks George but I like Harold's thinking about 2 to 4 times recommendations.  Today I added another 120 catfish to the system.  I am planting at high density and I want as many nitrates as possible for the plants.  In addition to the grow beds I have a wicking bed connected directly to my system.  As water is needed I just open a valve and filler her up. I also have 60 feet plus of 4 inch horizontal gardening.  I will be adding two 5 foot vertical towers this week.  Also I use the water from the fish tanks to water my soil garden via filtered micro-irrigation.  I plan on adding an additional 50 Talipa when the weather warms. It's going to be quite an experiment. Keeping the nitrates in balance will be interesting. Hoping to keep it between 40 and 100 ppm.

Once a garden is established it needs little added organic matter to prepare it for future seasons.  I just buy leaf compost from the town. Water is minimal because on the drip irrigation. Fertilizer is all fish based; I do supplement the fish water.  I can't get the density as in aquaponics but the produce is on par with aquaponic vegetables/fruit.

George said:

Thus far, I've never had much of a nitrate level.  You have a lot of potential for nutrients as those 60 catfish grow.  I think you'll see a big difference as the air and water temp increases and the fish grow and feed more.  As that happens, the biofilter increases too and you have a nice amount of biofilter potential.  It appears to me that you're doing a lot of things right.  You may need a light plant load for your summer vegetables.  They're heavy feeders.  Next fall, try cabbage, if you like to eat it.  It is one plant that seemed to do better from the very beginning in my aquaponics system.  Now, everything grows better in aquaponics compared to soil.  It's really the perfect system for growing plants - they feed on a liquid diet and liquid is always available.  It's amazing how well plants can grow.

I think you'll find that fertility is easier to establish and maintain in aquaponics than in soil, but I'm comparing a purely organic soil garden to aquaponics.  An organic soil garden seems to me to require a good bit of input of water and compost.  It's so much easier to throw fish food in a tank and top it up on occasion.  

You'll be pleased, I believe, as time goes on.  

[Harold Sukhbir]

Hi Tom,

How much volume of GB's do you have now? 250/300 fish as a maximum to growout to 1 lb each is OK for the water capacity you have now. The longer your fish waste stays in the media beds the higher your nitrates will rise. So you can use system water to irrigate the soil plants but as you strain off the waste returning it to the GB's is a good idea.

[Tom Dumas]

Harold I have 120 square feet of grow bed with a gravel volume of 168 cubic feet.  I think I did the math right. Then there is the wicking bed, floating raft bed and the vertical and horizontal gardening sections. When planted I assume they will consume nitrates, at what level?  My soil garden is 160 square feet down from my usual 1/2 acre.  I know exactly how much and when to feed these plants.  It appears to be a mystery in aquaponics and I can see why. I will not water the soil garden with fish water until I think it won't effect the aquaponics department. I will be spending approx. $155 in fish and ? on food. This is close to what I would spent on the soil garden in minerals,compost etc. I purchased fish of different sizes hoping not to diminish the nitrate output drastically when I remove larger fish.  This is all guess work for me.  Spring planting will begin in very shortly. Although I have what seems to be a lot of fish I can't see them providing an appropriate amount of anything until they grow.  Plants don't like to wait.

Harold Sukhbir said:

Hi Tom,

How much volume of GB's do you have now? 250/300 fish as a maximum to growout to 1 lb each is OK for the water capacity you have now. The longer your fish waste stays in the media beds the higher your nitrates will rise. So you can use system water to irrigate the soil plants but as you strain off the waste returning it to the GB's is a good idea.

[Harold Sukhbir]

Hi Tom,

Thanks for sharing that. Fruiting plants grow well at 80ppm + nitrates and leafy plants below 80ppm. So, if you are growing a combination of both try aiming at an average of 80ppm.

[George]

Tom, I like your style.  Low nitrate will not be a problem for you.  You've got a great plan.