Soils typically carry a negative charge on particle surfaces, and these surfaces attract and
hold positively charged base cations such as calcium, manganese, potassium, sodium, and
others. With only minor exceptions, soil particles hold hydrogen ions more strongly than
these other ions. This means that whenever one of the acidification reactions causes
hydrogen ion concentration in the soil to increase, some of the hydrogen ions will then
displace these base ions, which become susceptible to leaching out of the soil. Over time,
well-drained humid region soils become quite acid and devoid of base ions unless the
bases are replaced by nature or by man. Eventually, as this process continues, hydrogen
(H
+
) concentration on the surface becomes high enough that the mineral structures
themselves are attacked, releasing silica (Si
4+
), iron (Fe
3+
), and aluminum (Al
3+
) as well as
other ions contained in the minerals. Whereas the silica ions leach to lower levels, the
released iron and aluminum ions tend to come out of soil solution and form hydrous oxides
(sesquioxides) which remain in the soil. Eventually insoluble amorphous
3
hydrous oxides
of aluminum and iron, along with Kaolinite
4
, become the dominant solids in the soil.
Soils typically carry a negative charge on particle surfaces, and these surfaces attract and
hold positively charged base cations such as calcium, manganese, potassium, sodium, and
others. With only minor exceptions, soil particles hold hydrogen ions more strongly than
these other ions. This means that whenever one of the acidification reactions causes
hydrogen ion concentration in the soil to increase, some of the hydrogen ions will then
displace these base ions, which become susceptible to leaching out of the soil. Over time,
well-drained humid region soils become quite acid and devoid of base ions unless the
bases are replaced by nature or by man. Eventually, as this process continues, hydrogen
(H
+
) concentration on the surface becomes high enough that the mineral structures
themselves are attacked, releasing silica (Si
4+
), iron (Fe
3+
), and aluminum (Al
3+
) as well as
other ions contained in the minerals. Whereas the silica ions leach to lower levels, the
released iron and aluminum ions tend to come out of soil solution and form hydrous oxides
(sesquioxides) which remain in the soil. Eventually insoluble amorphous
3
hydrous oxides
of aluminum and iron, along with Kaolinite
4
, become the dominant solids in the soil.
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