For each unit increase in pH, the concentration of Mn^2- decreases by . . . . . . . . folds.

Microbial respiration and activity decide the availability of manganese ions in the soil and it is mostly low at neutral to alkaline pH, acidification of the rhizosphere has an important role in mobilizing soil Manganese ions. The pH change in the soil depends on the buffering capacity of soils, bulk soil pH, nitrogen sources and other factors.
For each unit increase in pH, the concentration of Mn^2- decreases by 100 folds.