Backyard Aquaponics
A place to discuss aquaponics

My system has been up and running about 6 months now, but I continue to struggle with low levels of iron for the plants. I have added about a liter and a half of chelated iron over the past couple months (in a 150gal/570liter system), and every time I test the iron levels, it comes out at zero. Some plants are OK with this, but the cucumber and squash are particularly unhappy, showing the typical green veins/yellow background on newer leaves. So where is the iron going?

I started thinking (a dangerous thing), and have come up with a theory. Chemistry is not my strong point, though. I need second opinions.

Our water definitely has chloramines, not chlorine, so I treat water that I add generously with Kent Chlorinex (sodium thiosulfate). I read the Wikipedia entry for the chemical, and it talks about how it can bind certain metals. I have also seen some de-chlor products say that they precipitate heavy metals.

So bottom line question is: Am I precipitating my chelated iron out when I de-chlor the water?

I'm not too good at chemistry myself, but thiosulfate is a reductant and can change a ferric iron (Fe+3, an oxidant) to ferrous iron (Fe+2, a weaker oxidant), but it's not strong enough to change it all the way to solid iron (Fe, a stronger reductant).

But this process only takes place when all the chloramines are gone, because (I think) chloramine is a better oxidant than ferric and ferrous iron..

I looked it up, and it turns out that plants only absorb ferrous iron, so what I just wrote suggests that the de-chlor could actually have a positive effect on iron absorption by plants from an iron solution.. So I'm probably looking at the wrong characteristics.

From what you've said Xzobie, I would have agreed.

Which leaves us then with what is causing Janet's problems.

Janet apart from the initial indications of iron deficiency what leads you to believe you are suffering on-going problems?

And how are you measuring the effectiveness of the chelated iron addition, are the differences that apparent?

1) My cuke and squash leaves show the deficiencies, and in fact, growth for them is very lackluster, with stunted leaves as well as the coloration problems. With the addition of chelated iron, I am just starting to show some recovery in coloration, but I'm not convinced that will last. I have some seedlings (not sure what they are, maybe cole family) that are also showing the pale new leaves and are also not recovering. Broccoli transplants are also poor. On the other hand, tomato, eggplant, beets and basil are doing wonderfully.

2) My iron test always reads zero.

OK, another random thought...maybe my iron test is defective, and the problems with cukes and squash has more to do with low light?

Possible Janet.

Is your system high flow ?

I turn over the volume of the tank about 3 times per hour. Is that what you mean, Rupe?

My understanding is that the plants will quicly take up the iron provided and therefore you are unlikley to get iron readings - except shortly after you put it in.

I expect it would be better to put little in often than a lot in less often. I was putting about 3ml into my 1500 litre water (500 litre grow-bed volume) system every 3 weeks or so. I haven't doen so for a while - but should put some in. I don't think EB has put much in at all over the life of his system - but he does add seasol now and again (not sure if that has any iron in it).

It is possible as you say that the deficiency is not iron at all. Potassium is one we have been concerned about over time. Has your system any salt in it - cause the word was that salt reduces the uptake of potassium.

I would have to test for salt. At one point I had maybe 2ppt salt, but I've changed water out since then. I will test tomorrow and see if there is still residual salt. It's past my bedtime, and the fish are already asleep.

OK, salt test shows 0.8 ppt of salt. Pretty low, but I could change out some more water for fun. pH is 6.8, nitrate 200, just a trace of nitrite, ammonia 0, Iron 0.

It could be that squash and cole crops really hate shade (and light is a known problem for me), and really like iron. The eggplants are the most beautiful trees you could want, and each of them has set its first fruit. Tomatoes are setting fruit, and the Australian tree fern has sent out new fronds in no time at all. The basil is delicious.

If the problem was potassium, I should see purple in the leaves, not yellow.

Maybe I should just stop with the self-study in chemistry? That and pull out the squash and broccoli?

I would say you have many plants in one system that may not work together...broccoli and tomatoes....Broccoli is a very heavy feeder.

Thanks, Hayden. So I'm not completely nuts. I solved the problem--removed the squash and planted out more lettuce, basil and shiso seeds. I'll remove the broccoli soon, but I don't want to rip out all my plants at once.

I tend to agreed with Hayden in as much as both broccoli and tomatos are heavy feeders and at the stage of "flowering" heading/setting fruit would have high demand for available trace elements.

Flower drop, failure to set fruit, slow growth are all common symptoms of insufficient trace element availability in soil based growing.

Still with nitrates being at a level of 200, I would have thought that the availability of trace elements would have been correspondantly/proportionally high enough to supply the plants.

Perhaps this is not the case like many systems here in OZ, because (correct me if I'm wrong Janet) the Talapia are being feed a mostly vegetarian diet??

Most fish pallets are made from "animal/fish" sources and contain levels of minerals and trac elements within them.... perhaps at levels greater than that contained in the vegetables being feed to the talapia?

The squash I would have thought was perhaps more water dependant.

Hopefully you'll see some improvement, certainly with the nitrate levels you have Janet you'll need to keep something growing to remove them.

Do you always run that high with your nitrates?

doesn't high N (200ppm) work to discourage flowering and fruiting in favour of continued vegetave growth, even in the presence of sufficient P and K?

Duun, i'm asking

Hi janet how did you do your salt test !

F&F, I have an actual salt test kit. Pond hobbyists use salt to keep algae growth down, so often maintain a low level of salt. It's a test-tube-n-drops thing.

Rupe, I am also suspicous that the more vegetarian diet of tilapia means fewer trace elements are available in the fish poo. I feed mostly pellets, but they are the lowest protein ones I could get. Interestingly, although I am getting fruits, I also have a number of flowers that bloomed and were pollinated, but haven't swollen into fruit yet. After a long delay, they do start to swell. Very interesting clue! The high nitrates are not on purpose. I'm trying to get that down, but hate to do big water changes.

Steve, you are right that hi N will make the plant put out leaves and neglect blooms and fruit.

I transplanted in some Silver Beet (Swiss Chard), and have seeded some basil. Also transplanted some foliage houseplants into the beds.

Perhaps because of the vegetarian diet (and lower light), I will have to emphasize foliage plants, and rely less on fruiting plants. Either that, or buy chelated iron by the gallon jug.