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.