Rocks and soils contain bases that can neutralize the acids in acid rain. The water running out of the soil becomes less acid and the soil itself becomes more acid. If the soil has large pools of bases, like the soils derived from Paleozic rocks shown in blue on the map, this can go on indefinitely. But if it has only limited pools of bases, like the soils derived from the granite rocks (I) shown in cream, the exchange reactions can deplete the base pools rapidly.

In watersheds with a high neutralizing capacity, like those of the northeast Adirondacks, acid rain has had little effect on either the soils or the surface waters. The soils are still fertile, plant growth is good, and almost no lakes have been acidified. In watersheds with low neutralizing capacity -–as in the west Adirondacks where acid deposition is most intense – the story is different; there acid rain has depleted the base reserves of the soils and acidified lakes. The trees on the depleted soils seem to be reproducing poorly; acid-related toxics, particularly aluminum and mercury, are moving through the acidified watersheds; and both fish and fish-eating animals are accumulating these toxics and suffering from them.