Muddy
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Several years ago I did extensive experiments on ways to lower the starting pH of Pro Mix. After trying sulfur, cotton seed meal and aluminum sulfate, I found the AS to be the most effective and fastest way to lower the pH. I always applied it at the rate of 3 grams per gallon of Pro Mix. This worked well, dropping the pH from the starting 6.9 - 7.0 to about 6.3. As the soil dried out the pH would slowly rise to about 6.7, allowing the plants to experience the full pH range for optimum nutrient uptake. Recently I purchased a bale of Pro Mix without myco as it was all my supplier had. Since I'm growing soil less with chemical nutes I wasn't concerned about not having myco. I added my usual amount and had problem the whole grow with low pH. It was dropping as low as 5.2. It still rose to 6.7 as it dried but the low pH caused some problems with P and K being unavailable at that low pH. Consequently my plants suffered a bit and my yield was off. So with my current grow I cut the amount in half, thinking that would suffice. Wrong. I'm still getting low pH on this grow as well. So I filled a pot with straight Pro Mix, not AS, and watered it with 6.4 pH water. Two days later I tested it and it was at 6.4. As the soil dried out it slowly rose to 6.7. So it appears that at least with Pro Mix without myco, aluminum sulfate isn't needed. I wouldn't expect that the added myco would have any effect on pH, but without buying some with myco to test I can't say for sure. For those of you who are using my method I would strongly suggest you test your Pro Mix before adding any aluminum sulfate.
