This is where alkalinity and fertilizers used come into play.
Here's a little bit more of the conversation if you're interested, useful stuff if you're growing in
ProMix:
"___ think of PRO-MIX as a filter. It can filter out elements from the water, but it cannot filter out pH. As elements, such as bicarbonates, nitrates and other elements accumulate in the growing medium, they start to change the pH of the growing medium. The buffering capacity of PRO-MIX does not change overtime, but the exchange sites can be loaded with new elements that can change the pH of the product. Again the exchange sites (or cation exchange) is much lower by volume compared to soil, so the pH of PRO-MIX can be changed within a few weeks after planting. I would not look at the buffering capacity of the PRO-MIX to be significant.
The fertilizer used has to have a neutral effect on the pH of the growing medium. In commercial horticulture this would mean that the potential acidity or potential basicity of the fertilizer should be near zero. If it is much above this value, then it will cause the pH to change. For example, if the fertilizer has a potential basicity above 50 lbs calcium carbonate equivalent , the pH of the PRO-MIX will go up over time. If the fertilizer has a potential acidity higher than 100 lbs calcium carbonate equivalent, then the pH will decline over time."
