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PostPosted: Feb 27th, '19, 07:53 
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Hello friends! :wave:

I've added phosphoric acid to my vermiponic system to move my pH from 7.5 to 7.0 (pH is high because I was using municipal water to flush my worm bin and the water has a lot of carbonates- 8.2pH). The Salifert Magnesium test shows that I have zero Magnesium in my system (plants are not showing magnesium deficiency, but I don't want to wait till they do)


I've been running a tilapia aquaponic system for 2 years now and a few vermiponic systems for a 1 year now.

For the vermiponic systems, I'm essentially flushing water through a worm bin every few days and adding it to the vermiponic system. The health of my plants has been exceptional, basil even seems to tolerate 40F nights in the vermiponics, while other basil in aquaponics and soil died. My theory is that there's more bacterial and fungal life in the vermiponics that is doing some extra work to make nutrients available.


My veggies aren't showing any problems, but I just started growing cannabis (for the first time) and I want to grow it to perfection. So I've been monitoring nutrients and adding supplements. yes, I might be just overcomplicating my life, but please don't leave those sort of comments. I really want to understand the chemistry.

yesterday: before adding calmag and epsom salt
EC=800 us/cm
pH=7.0
KH=89ppm
ammonia= .25ppm
nitrite= .5ppm
nitrate = 20ppm
calcium =60ppm
magnesium = 0ppm

Then, I theoretically added enough calmag and epsom salt to bring my magnesium from 0ppm to 200ppm. I am targeting 50ppm, but it was not budging so I quadrupled the dose. Next day here's my numbers

Today:
EC=770 us/cm
pH=7.3
KH=107.4ppm
ammonia= .0ppm
nitrite= 0ppm
nitrate = 5ppm
calcium =140ppm
magnesium = 0ppm

note the drop in nitrogen is what I would consider a normal drop from plant uptake in 1 day. Reservoir is only 12 gallons, so nitrogen goes up and down every few days when I redose with fresh worm leachate.

You can see the calcium went up as expected, but no budge in magnesium. Also the pH went up.

Is my magnesium being eaten up by the phosphoric acid, raising the pH and causing no gains in magnesium? Other theories?

How do I get my magnesium to 50ppm without detriment to other nutrient levels? (Yes, I know I need to wait some time for the carbonates to be taken out of the worm bin so that the pH naturally goes down...until that happens, what do I do?)

Thanks for any help!


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PostPosted: Feb 27th, '19, 08:39 
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You probably already have enough magnesium in the system since you weren't seeing any deficiencies. My guess is that this is being bound into something that's available to the plants but not detectable to your kit. If you're concerned that this may not be available to the plants then spray apply the epsom salts to the foliage (I wouldn't feel comfortable doing it at this low of a temp though).


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PostPosted: Feb 27th, '19, 08:49 
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Oh I should add that the cannabis 12 gallon vermiponic system is in a greenhouse attached to my house. Temperature is 60F at night and 75F during the day. C02 is 800ppm (atmospheric is 410ppm) Humidity is 60-70% when there's no light and 30-50% when there is light. I have grow lights on from midnight to 9am and sun from 9am-4pm.


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PostPosted: Feb 28th, '19, 02:16 
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Good to know it's inside the greenhouse and the temp is warm enough to spray. If you're going to try to up the magnesium levels (which I don't really think you need to), I would spray with epsom salts a bit before 4pm when the sun is no longer at it's peak intensity. Epsom salts can burn the leaves if over applied or done at the wrong time of day when there is intense sunlight. The spray application rate is usually on the container.

Using the spray application methods avoids any interactions with ions or organic compounds in the system water and can also be useful when you experience pH related nutrient lockout (like of iron for instance).

I don't grow any cannabis so I can't tell you if this is a good idea for this crop (might leave a residue on the leaves). Someone else here might know or you may want to ask in one of the online forums specific to this crop.


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PostPosted: Feb 28th, '19, 14:45 
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scotty435 wrote:
Good to know....


Thanks Scotty435 for the tip. I probably will go the route of foliar sprays next time. I've been meaning to learn more about that...

I still would like someone to explain the chemistry. Anybody out there understand what's going on?


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PostPosted: Feb 28th, '19, 21:14 
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I doubt that anyone here will be able to give you a good answer on this question but I have a couple of questions. At what point are you adding the cal-mag and epsom salts - to the leachate before adding to the system water or directly to the system water? Assuming your top up water has no organic compounds, If you dissolve epsom salts in top up water to make a solution of known magnesium concentration does your kit detect the expected level of magnesium? If it does, adjust the pH with phosphoric acid, is the magnesium level the same after adjustment with phosphoric acid (this should tell you if magnesium binding to phosphoric acid is the issue)? I would not expect phosphoric acid to be the issue since both it and magnesium would probably exist as ions in solution. If the past two tests come out the same and as expected then I think this will tell you if the magnesium is either being taken up by the plants or bound to an organic compound and I think it would be a tough guess to know what that was.

Hope this helps.

EC meters aren't particularly helpful in AP, the chemistry is more complex than regular hydroponics so they aren't capable of measuring what's really there or it's availability to the plants :dontknow:


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