Awesome, I love all the thought you’re putting into your pond.

Are you getting powdery mildew on your peppers if you don’t use a fungicide? That’s another sign that your plants are doing too much work to reduce the sulfates carbonates and nitrates. Peppers should be able to naturally defend themselves against PM. Pathogenic fungi like acidic and oxidized conditions. Fungus breath in oxygen and breath out CO2 (opposite of plants), so when you force oxygen in the sap through sulfates carbonates and nitrates the fungus is happy…I think.

There’s not many good options for pH down, possibly no good options. I’ve put a lot thought into pH management, and quite frankly most of the experts are wrong. Acetic acid, lactic acid, and citric acid, will only drop the pH for a few hours, then they will raise the pH higher than the original pH. They have caloric value, so as soon as a microorganism consumes the organic acid, it will produce a carbonate, and raise the pH. So don’t use organic acids to lower the pH for irrigation. However, organic acids are good chelators/reducers for foliar spray!

Sulfuric acid will turn most of your dissolved calcium carbonate into calcium sulfate, which is soluble up to around 2000ppm…which will force your plants to drink calcium sulfate when they don’t actually want to. They’re going to spend a lot of energy/sugar to reduce the sulfur. I used sulfuric acid the whole last year on my water supply, and got worse plant health and no increase in sulfur in the sap. I’m not 100% certain about this, but there’s enough evidence for me to think that sulfuric acid is bad for agriculture in most cases.

Phosphoric acid may be the best option. It will form calcium phosphate which is not soluble, and just end up on the bottom of your pond. Do a test in a 5 gallon bucket of your water. Adjust the pH to 6 with phosphoric acid. Test the calcium level before and after (give it a few days for calcium phosphate to settle, and don’t stir). Test the carbonate level before and after too. You can use aquarium test kit made by API to test both calcium and carbonate (kH). While you’re at it, buy their Ammonia and Nitrate test kit. I’d bet that most of the urea you’re adding to the pond has been converted to Nitrate by the time it runs in your irrigation. Whatever urea did not get converted to nitrate in the pond, will definitely get converted to nitrate once it hits the carbonate rich soil. Nitrification happens very quickly when carbonate is available.

Possibly you FIRST treat your pond with phosphoric acid, to remove the carbonate and precipitate the calcium phosphate out of solution. Then you fertigate urea (don’t put it in the pond). The idea is that you get rid of most of the carbonate before adding urea. You don’t want to put urea in the pond because, whatever portion of urea that converts to nitrate and nitric acid (will dissolve the calcium phosphate). (I think the The order of this is important. To produce a bad result, you do that in reverse: First add urea to the high carbonate water. All of it will convert to nitrate. Then add phosphoric acid. You’ll just get an acidic solution with a lot of nitrate)

We are only farming 1/4 acre, but very intensively…I don’t know if this can work at your scale, but it’s worth considering. Here’s what we do:

from June-Nov: 50% RO water and 50% hard water(also about 150ppm hco3).

from Dec-May:and 50% rainwater collected in a pool/pond 25% RO water, 25% hard water from Dec-May
This removes about 2/3 of the carbonate load.

I suspect you have a small sulfur problem and a big oxidation (lack of energy/sugar) problem. Which is about where I’m at and I’m now focusing on the oxidation problem, and seeing better results.

Right now I’m playing with adding sugar in the fertigation and see if that is a way to cheat to get more reduction energy in the system, until my plants can get to a higher state of health where they can do all the reduction themselves.

I believe most sulfur exists in plants attached to amino acids. So I suspect that high protein cover crop, would be the best way to make reduced sulfur available. Foliar sprays of amino acids would work, but nobody can afford that.