The Chemistry Behind the 6.5 pH Sweet Spot
At pH 6.5, microbial nitrifying bacteria operate at peak enzymatic efficiency, converting organic matter and ammonium into plant-absorbable nitrate. Furthermore, phosphorus—which is famously insoluble—remains in its most bioavailable orthophosphate forms (\(\text{H}_2\text{PO}_4^-\) and \(\text{HPO}_4^{2-}\)).
When soil drops below 5.5, aluminum (\(\text{Al}^{3+}\)) ions dissolve into the soil solution, destroying root tip meristems. When soil rises above 7.8, iron oxidizes into insoluble ferric hydroxide, starving turf chloroplasts of the catalytic iron required to manufacture chlorophyll.