There are various different acids that commercial growers use to lower the pH of alkaline irrigation water, but the reason I suggested vinegar in the other thread is that it is often already in the house or is at least easy to get from a supermarket so more convenient for a small test. That (the teaspoon of vinegar to a litre of irrigation water) was really just for an initial trial on a plant or two to see if they would respond by producing greener new leaves than the untreated ones.
If you're going to amend the soil with organic material I would tend to do it long enough before planting that it has time to settle.
