Backyard Aquaponics
A place to discuss aquaponics

My IBC aquaponics system has been up and running since May. I have 30 tilapia and have actively growing basil, sweet peas, watermelon, and cucumbers. The fish are happy (no deaths, or disease & they eat well). The plants are growing well & flowering, but once fruit develops (baby watermelons & cucumbers), they turn black, die, and fall off the vine - so there's no fruit. Yet the vine keeps growing & flowering & repeating the same thing. The leaves are dark green at the growing end, but toward the roots, the leaves turn yellow, then black and die (see pics). As I said the fruit is also dying. Is this a deficiency or disease?

Using 3 different test kits I have, here's my latest parameters:

Using API's Freshwater Master Test Kit:

pH: 6.1 (using electronic meter)
Nitrates: 40 ppm
Nitrites: 0 ppm
Ammonia: 0 pmm

Using Nutrafin Test Kit:

Phosphate: 0 (Low / Non-existent)
Calcium: < 20 ppm (Low)
Free Iron (Non-Cheleated): .1 (High)
Free Iron (Toxic): 1.0 (High)
Carbonate Hardness (KH): 20 ppm (Low)
General Hardness (GH): 100 ppm (Moderately Hard)

Using Rapidtest Soil Test Kit (so may not be accurate since I have no soil)

Phosphorus: P3 (sufficient)
Potassium/Potash: K2 (adequate)
Nitrogen: N3 (sufficient)

Just yesterday, I added some crushed shells in a mesh bag to the water to help buffer the pH and keep it up (had dropped to 5.7 some days). It should also help with the calcium. I also added some Maxicrop Liquid Seaweed which should help bring up the Potassium/Potash. To address the high iron, I am working on replacing the tank water over time.

Any ideas what is going on & what else I should do to fix it? Thanks!

I'd say the majority of leaves are being lost to a nutrient deficiency. I think you might be lacking in both calcium and magnesium. The calcium would probably give you the equivalent of what you see with blossom end rot in tomatoes so your description of blackened melons makes sense. On the other hand it looks like the older growth is what's dying and that indicates a non-mobile nutrient like magnesium (calcium is mobile). It's also possible that one of these deficiencies is making the plant susceptible to disease which results in the melons turning black. Either way I think the solution is to up the calcium and magnesium. Have you done anything other than adding the shells that could affect either of these nutrients lately?

Scotty,

Thanks for the feedback. Adding shells a few days ago is the only thing I've done so far to address calcium. I have purchased some calcium chloride, but not sure how much to add (and I didn't want to overdo it since I just added the shells). Any recommendation on how much to add for 150 gallons of water?

As for magnesium, I don't know how to add that. Can I buy it separately & add it? If so, how much? If not, what do I do to bring that up?

Thanks,
Dave

We don't know exactly how much calcium is there to begin with, just that it's less than 20mg/L. I don't think you'll need anything other than the shells you are using for the calcium. Somewhere in the 35 to 65 ppm (mg/L) range should be good. I usually use a mesh bag to add the shells so that they can be removed if the pH climbs too high. Calcium chloride will allow you to get there without changing the pH much so go with that if you need to . It's possible to precipitate phosphate if you go too high (keep it below 100ppm), more is not always a good thing.

Hold off on the magnesium for now. Based on your hardness results I suspect you have enough. If you do wind up needing it, Magnesium can be added as epsom salts, which you can find cheap at the drug store. Spray application is the way to go, usually the container will have a recommendation for that but if it doesn't then 1 teaspoon per gallon should work. It can cause leaf scorch so don't over apply and you should apply it late in the day to avoid intense sunlight for the same reason.