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How do I alternate calcium hydroxide with potassium carbonate?

I've read on Sylvia's blog and other sources that it's a good idea to alternate calcium hydroxide with potassium carbonate (or hydroxide) to buffer up ph and add nutrients. 

I'm hoping that someone can help answer the following questions:

- Is there a particular food or scientific grade that is most suitable to use with fish?

- What quantity should I use per 100G? My tank is about 140G and my gravel growbed is W2'xL5'xD10"

- What is the frequency to alternate the two? Every time the ph needs to be buffered up or has anyone found a better schedule?



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Someone more experience can answer the best quantity to start with better than I can but it will depend. You will want to likely pre-dissolve it in your top-up water and you want to be sure you do not swing your pH by more than 0.1 points per day otherwise the stress on the fish can be hard on their immune system or worse, kill them.

Keep in mind hydroxides are extremely caustic, where carbonates are fairly mild in terms of pH. Another great alternative to calcium hydroxide is to just use some "hard" water. Check your well/municipal water for hardness as it may contain a lot of calcium carbonate already. How often to alternate potassium will be very depended on your plants demands. If you are just growing leafy greens you probably don't need to add any as there is already enough in the fish effluent. If you are growing zuchinni or cucumbers, you are going to be using a lot of potassium to buffer instead :) I think the main reason potassium isn't used unless needed is that it is more expensive. Your plants should be the ones dictating when you use one vs the other but building up a supply of both in the system of wont hurt either.

I hope that helps you a bit anyway.

I agree with Chris in that you can probably forgo the potassium hydroxide (KOH) and just use the bi-carbonate form (KHCO3) it is not as strong as the potassium carbonate (K2CO3) but may be easier and cheaper to acquire. it also doubles as a fish safe and organic gardening approved fungicide when used as a foliar spray (spraying the leaf surface).

I also agree that your buffering regime may have much do with what plants you wish to grow right off the bat. Zucchini and cucumbers (everything in the cucurbit family) use a LOT of magnesium (Mg) as well as potassium (K). Fortunately you can add a third type of buffer to your alternating regime in the form of dolomitic lime (CaO-MgO).

It is my humble and personal opinion that someone who knows that they want to grow things like cucumbers and tomatoes right off the bat, would benefit significantly from this 3 pronged alternating approach i.e calcium based buffer, potassium based buffer, and a magnesium/calcium based buffer. And again, whatever you decide to it slowly.  

Dear Chris and Vlad, thanks for the pointers!  

NYC Water does have erosion deposits of Ca (5mg) and CaCO3 (14mg) but I couldn't say that it's helping me.  My pH drops fairly rapidly.  I have to keep adding ph up every few days to keep my ph near 7.

Right now my tomato and eggplant are suffering the most, and my peppers are doing great.  I think I'm ok on Potassium because adding Maxicrop over time did nothing to improve the situation. I've also tried spraying on diluted Cal Mag to better understand the deficiency.  When I did this, the lower tomato foliage started to unravel from its shriveled state, but there's still interveinal chlorosis.  I've been leery of adding anything to the tank with the fish in it even if it's touted as organic. I'll print out these notes and see what my local hydroponics store carries.  I'm still not sure about quantity: are we talking about something like 1 tsp in the tank to start with?

Is the Maxicrop w/ or w/o iron? :) Iron is almost always deficient in all new aquaponics systems and I didn't see you mention adding any.

This is a handy chart you may find useful for troubleshooting other deficiencies:

Thanks for the table.  The maxi crop I use is just straight seaweed, without any iron.  I do add chelated iron every 3 weeks or so.  My pepper plants are my barometer, since they don't seem otherwise affected by nutrient levels.  The weird thing about the yellow leaves is that they're holding tight and not dying off.  The leaves at the top of the tomato plant are green, but the ones at the bottom are yellow with green veins.  The eggplant leaves are just all around pale.  

I'm sure a picture's worth a thousand words, so here's the latest one.  I added iron just after I took this picture:

You can see how obnoxious the peppers are in the middle.  The eggplant is to the left and the tomato is to the right. You can see the shriveled yellow leaves, and one lower branch behind it that remained mysteriously green .

Chris said:

Is the Maxicrop w/ or w/o iron? Iron is almost always deficient in all new aquaponics systems and I didn't see you mention adding any.

This is a handy chart you may find useful for troubleshooting other deficiencies:

Sorry to say you are beyond my experience. The only older leave issues I have dealt with first hand are K and Mg deficiencies. I was hoping you were going to say you didn't add iron but that would be in the newer leaves :P Vlad will likely have a better suggestion after he sees the picture.

Yeah, correcting a Mg deficiency (interveinal chlorosis on lower leaves) once it occurs is real difficult in my experience...Unlike the relative ease of correcting an Fe deficiency...The potassium becomes much more important when the tomato has set fruit. (Not that K is not needed now, just much less so)...

I'm not familiar with your pH up product, but maybe try adding something with a carbonate alkalinity...Or the old crushed seashells in a bag trick might help out with the hassle of having to add something every few days? The bacteria use up about 8mg of carbonate alkalinity for every 1mg of ammonia that they oxidize into nitrate... So it might be good to try any one of the carbonate (CO3) buffers mentioned...

The hydroponic store is probably the worst place you could go to...All of the buffers mentioned above are certified for use in organic gardening, but much more importantly,...they are fish safe. Epsom salt is also a good (and probably way cheaper than Cal-Mag fertilizer) source of Mg.

Peppers, relative to tomatoes and eggplant have a pretty low(er) essential element requirement. Hot peppers even less than sweet, or bell peppers...

You can use the search field to look up some of the many threads on buffering here...but I doubt anyone is going to be able to give you a gram amount, since it will depend on your water might take a gallon or whatever of your top up water and play around to see how much of whatever you decide to use it takes to get you up to a certain pH...and go from there...

2'x5'x10"should be 62.34 gallons total capacity...minus 38% displacement...minus the couple inches that the water line is below the rim of the grow bed...say about 30 gallons of water or so, in there...So 170 gallons total sound about right?

I just now saw the picture and the previous posts...that sure looks like one whammy of an Mg can try to correct that with dolomotic lime buffering (probably the better choice since you'll be killing two birds with one stone) or additions of Epsom salt (MgSO4-7H20)...Epsom salt is about 10% (9.94%) magnesium content by weight and a 20-30 mg/ L (milligrams per litre same as ppm in this case) might be a good target amount to try and take care of that. The leaves that are already affected may not respond, but any further damage may be averted.

The K deficiencies usually start on the leaf margins or tips, eventually turning to necrotic lesions all across the broad surface of the leaf if you let it. 

Be careful when foliar feeding "un-protected") Cal-Mag or any other thing, lest you expose your plants un-necessarilly to fungal pathogens. (Most fungal diseases can only take hold when there is free water i.e the leaf is wet. And in my experience it seems  that weak or deficient plants are somehow more easily apt to become infected...)...Potassium bi-carb is an organic/fish safe anti-fungal remedy when used foliar.

It really would not be a bad idea at all to get some potassium bi-carb and some Epsom salt or dolomitic lime. The D-lime you would just use like the pH up and target a given pH, while the Epsom salt you would target a threshold amount (by weight and/or mg/L by proxy) and is probably available at the grocery store, gardening store, tractor supply store etc...  

At 170 US gallons that'd be 643.52 litres (we'll say 650)

And a target threshold of at least 30 mg/ L (most hydro literature calls for 50 mg/ L, more for cukes, but you have fish that are pooping some out already, so 30) would be good to shoot for...

3 grams Epsom salt (remember that would only be 10% or 300 milligrams of actual Mg) in 1 litre of water is 300 mg/ L... At 10 litres of water that is 30 mg/ L.

10 goes into 650, 65 times so 3 grams times 65 is 195 grams of Epsom salt for a 170 US Gallon system...

Hope that made sense? 

Wow, really informative and invaluable stuff! I appreciate the level of detail and follow-up. I'm hoping that I'll have good news to share with you.  Thank you gentlemen!

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