Excerpted from my book, “Small Spaces, Big Yields”:
Why does soil pH drop during flowering? The plant is doing it! Here’s how it works: nitrogen is available to the plant in three forms, ammonium nitrogen, nitrite andnitrate. As the soil decomposes, the ammonium nitrogen is transformed by bacteria into nitrate, changing the ratio from “high-ammonium-to-low-nitrate” to “low-ammonium-to-high-nitrate” (stopping along the way as nitrite).
Chemically, ammonium is expressed as NH4+ and nitrate as NO3–; you can see that ammonium is a cation (positively charged) and nitrate is an anion (negatively charged). (Lots of diagrams on the internet get this wrong, confusing ammonia with ammonium. Ammonia is not ionic and is a gas; it is not available to the plant.) What happens in the pot is that the plant roots secrete another cation, H+, into the soil which it exchanges for the ammonium (+ exchanges for +), or an anion, hydroxy, OH–, for nitrate (- exchanges for -). H+ is acidic (in fact “pH” is the measure of the activity of solvated H) and OH– is alkaline—the process of ammonium uptake will acidify the soil over time, while nitrate uptake has the opposite effect.
The acidifying effect is exacerbated during flowering as the plant increases its uptake of K and Mg, which also trade an H+ for their absorption (P uptake has thereverse effect). This is why soil, depending upon its composition, can be rapidly acidified during peak flowering. The fix for this is to add dolomitic lime to the soil. Just water some in and give it time, it will correct.