The Unfortunate Reality of Lettuce Bolting(!); Cool Lessons in Reducing Nitrate Levels to Induce Fruit Setting

Wow, well it's been 3 weeks since I updated the blog. Unfortunately, there have been some set backs. My lettuce production streak came to a screaming halt, but at least I can say it does not have to do with the aquaponics system.

We basically had some horrible weather, which is to be expected at this time of year. From the time of the last update 3 weeks ago, it literally rained non-stop for the following two weeks, so there was very little to no sun. As a result, all the lettuce in the raft beds bolted. A plant "bolts" when it is under stress - i.e., under unfavorable conditions. In the case of lettuce, the result is the plant stretches up, becoming spindly, and develops an unpleasant taste. The leaves stay small, and become elongated. After that, the plants (at least in this case), do not grow very much, if at all.

As a result, last week I threw all the lettuce out (54 plants) and replaced it with new starters. Unfortunately, those starters were purchased an entire week before planting, and during that week, the starters were also in really bad weather. They were not showing signs of stress or bolting when I transplanted them to the raft beds, but this weekend, one week into planting, they do not look very good. I think they have just about all bolted. Not having any new starters to replace them with, I left them in the raft beds this weekend. I will take new starters next week and will restock if need be.

    Weekly aquaponics system update; showing lettuce that has bolted due to

prolonged bad weather, and needs to be thrown out.

In the meantime, the cucumber and tomato plants, all growing in gravel beds, are showing tremendous green leafy growth. Unfortunately, they are not setting fruit. The cukes set, but then fall off before they develop. The tomato plants don't even get that far - no fruit setting at all.

The good news is, I think I know the solution. The other good news is the plants look fantastic. If I do succeed in inducing fruit production, the plants should be able to hold plenty of weight.

I went over my UVI course notes, because I remembered there is a key part of their system they regulate in order to achieve better fruit set. Sure enough, in, Recirculating Aquaculture Tank Production Systems: Aquaponics—Integrating Fish and Plant Cultureby James E. Rakocy, Michael P. Masser and Thomas M. Losordo, Southern Regional Aquaculture Center Publication No. 454, we learn that:

The accumulation of too much nitrate in aquaponic systems is sometimes a concern as fruiting plants set less fruit and produce excess vegetative growth when nitrate levels are high. The filter tanks in the UVI commercial-scale system have a mechanism for controlling nitrate levels through denitrification, the reduction of nitrate ions to nitrogen gas by anaerobic bacteria. Large quantities of organic matter accumulate on the orchard netting between cleanings. Denitrification occurs in anaerobic pockets that develop in the sludge. Water moves through the accumulated sludge, which provides good contact between nitrate ions and denitrifying bacteria. The frequency of cleaning the netting regulates the degree of denitrification. When the netting is cleaned often (e.g., twice per week), sludge accumulation and denitrification are minimized, which leads to an increase in nitrate concentrations. When the netting is cleaned less often (e.g., once per week), sludge accumulation and denitrification are maximized, which leads to a decrease in nitrate levels. Nitrate nitrogen levels can be regulated within a range of 1 to 100 mg/L or more. High nitrate concentrations promote the growth of leafy green vegetables, while low nitrate concentrations promote fruit development in vegetables such as tomatoes.

I should have known those darn nitrate levels were the problem since I spent so much time discussing them and trying to get them down! Anyway, mystery solved (hopefully).

In order to incorporate a mineralization section in the system, I set up the old blue barrel again. Only this time, instead of functioning as a swirl filter, it is going to function as a mineralization area. The barrel receives fish tank water through a 3 inch pipe that enters the barrel from above and does not end until it is almost flush with the bottom of the barrel. Then, greenhouse saran is stuffed in the barrel, in a circular fashion all the way to the top, where the drain pipe is. The drain pipe empties into the system sump, and from there it is pumped to the grow beds and to the fish tank. There is no centrifugal action in this set up.  The netting prevents the water from forming any kind of a lateral current.  The water is just pushed across the multiple layers of saran so that as many deposits as possible are left on the saran.

The saran will catch all sorts of solids which will serve as growing areas for anaerobic bacteria. These will denitrify nitrate to produce nitrogen gas, as described by Rakocy above. I have no idea how long this should take. I am guessing 1-2 weeks, and am very excited to see if I get positive results.

I wanted to know what concentration of nitrate one should aim for, exactly, and so I contacted one of the professors at UVI. The answer is 20-40ppm, which is about a tenth of where my nitrate levels were a week ago, and about half of what they are now. However, I think the reason for the sudden drop has to do with an inadvertent partial water change that took place upon incorporating the mineralization tank (a story for another day).

So that's the update for today. That covers the last three weeks. Hopefully there is better news coming soon with some good tomato and cuke production (as well as lettuce)! Actually, I will be very interested to see how lettuce does with the reduced nitrate levels, as what's good for fruiting plants is not optimal for leafy greens. That makes me want to have two systems...

Video showing set up of mineralization tank designed to host anaerobic bacteria. 
The anaerobic bacteria reduce nitrate to nitrogen gas, and therebye reduce nitrate
levels in aquaponics system.  Lower nitrate levels are desired for fruiting plants to
set fruit.  Skip to 1:50 for the mineralization tank discussion.

Lessons in Water Chemistry Part III; Phone a Friend; and Resumption of Fish Feeding

Since last week marked a full month since I last fed the fish, I decided to seek some expert advice on the nitrate levels (and fish feeding). Back in March I attended the University of the Virgin Islands International Aquaponics and Aquaculture Course. The course is taught by Dr. James Rakocy, who is responsible for most of the work leading to aquaponics as we know it today, and his staff at UVI. Although Jim has retired from UVI, he came back this year to teach most of the week-long course, along with his staff members, Charlie Schultz and Don Bailey. Dr. Wilson Lennard, whose work I sited in a previous blog, also participated by giving a lecture via a remote link-up to Australia.  

In any case, I wrote a quick note to Don Bailey, to see what he thought of the high nitrate levels. Don and the rest of the UVI team are professional scientists, growing tens of thousands of pounds of fish and vegetables annually, using aquaponics, year in and year out, so I value their opinions, to say the least. After he scolded me for starving the fish (sorry Don!), he said nitrate levels are safe up to 500 mg/liter. That's the same as 500 ppm (parts per million). He also said I definitely have a secondary source of ammonia, given that the fish had not eaten for an entire month and that nitrate was still present.

The only obvious source for that ammonia which later gets converted to nitrate, is fish waste and other waste (plant roots, leaves, and other material) trapped in the gravel grow beds. It is likely that the red composting worms are responsible for turning that waste into ammonia, which then gets converted by bacteria into nitrate. The ammonia measurements I took of red composting worm leach ate, which came in very high, support that thesis.

As a result of this interaction, we started feeding the fish again last Tuesday. We started them on a very light diet (1/4 of what they normally got for 2 days, and then 1/2 as much as what they were getting previously), as I was afraid of giving them too much after so much time without food. Also, because of the secondary source of ammonia, I want to make sure I do not add so much food that nitrate levels shoot up beyond the 500ppm level. As a matter of fact, I may leave the feed level at around this rate, or even reduce it, depending on nitrate levels.

Again, my goal is not to grow tons of fish. I am more interested in plant production. Therefore, my goal is to maintain nitrate levels that will result in the maximum amount of plant growth, without maxing out on fish feed and fish growth. I believe that if there is a way to accomplish this, it is by saving fish waste and letting it decompose over time in the gravel grow beds, assisted by red composting worms, rather than by removing it from the system (as is done at UVI and most other places).

In my measurements today, nitrate clocked in at between 100-200ppm. The particular kit I have maxes out at 160ppm, so I took a second reading of a 1:10 dilution for more accuracy, and came up with the 100-200ppm reading that way.

Here's the video for the week.  Besides the water chemistry discussed above, there is also an update on the plants, including cucumbers, tomatos, and lettuce.

Click here if the video does not play.

PS:  I harvested 6 fully-grown lettuce heads today!  Next week I should have a near raft-full (18) of lettuce ready to take out.  It's getting to be time to upgrade the system.  I'd like to be cranking out 100-200 lettuce heads a week, and have room for other stuff.

Oct. 2, 2011 Review of Plant Growth a Month Into Starving Fish; More Lettuce Harvesting

There is not a whole lot to report this week. I harvested 24 fully grown lettuce heads this week plus 3-4 not-so-fully grown ones. This is becoming a regular occurrence now, which is oddly disappointing. I guess the novelty is gone!

As far as water chemistry goes, nitrate levels continue to hover near the high end of the scale, clocking in at between 40-80ppm. Because of that, I am continuing to starve the fish. They have now gone an entire month without any fish feed being thrown in.

I am getting a little worried about approaching the limits here, but I am told Tilapia can go a "winter" without food. I don't know where I read that or what that even means. Tilapia are tropical fish, so what winter would they even be talking about?

Anyway, I'll have to do some more digging around, but I am not too worried. Fish are cold blooded, do not have to maintain any kind of internal temperature, and since they float in water, there isn't much energy used in keeping themselves "up" as it were. They use food for growth only, not for maintenance. Based on that set of facts, I suppose everything should be OK.

The plants continue to grow like gangbusters, and already one of the cuke plants is flowering. It's been in three weeks since planting. Truly amazing, in my book!

Here's the video update for the week:

click here if video does not play