C - Lublin

SURFACE CHARGE OF SOILS AND PLANT ROOTS
Józefaciuk G.
Electric charge of natural materials may be divided onto two general types:
permanent charge and variable charge. The permanent charge results from a substitution
of higher valence cations by lower valence cations in the atomic (macromolecular
or crystal lattice) structure, and a variable charge resulting from dissociation
and/or association of protons by surface acidic groups (Van Olphen, 1993).
Prevalence of permanent charge exposed on basal planes of the particles is a
common feature of most clay minerals. Variable charge occurs on surfaces of most
soil solid phase constituents: organic matter, aluminum, iron and silicon oxides,
edges of clay minerals and on plant roots surfaces, as well. Variable charge dominates
also on kaolinites that is located both on the edges and on the basal Al and Si
sites of the mineral (Ward and Brady, 1998). Contrary to the permanent charge, the
magnitude of the variable charge depends on the composition of the soil solution
(pH, concentration, ionic composition). The variable charge origins from dissociation
and association of hydrogen ions (protons) from/to surface functional groups
of acidic, basic or amphoteric character.
MINERAL SOIL CONSTITUENTS
The permanent charge of clay minerals origins from that higher valence
cations in the crystal lattice (for example silicon in the silica layers) are substituted
by lower valence cations (for example by aluminum) during the genetic processes.
This results in the “unsaturation” of oxygen bonds and excess of the negative
charge. The substituting cation should be of similar size to this being replaced,
therefore this process is called isomorphic substitution. Frequently the summarical
amount of the charge resulting from the isomorphic substitution is not equal to the
permanent charge of the mineral. For example a part of the lattice charge may be
neutralized by specifically bound cations inside the mineral particle structure (i.e.
in the interlayers), as this frequently occurs in illites (binding of potassium cations)
or may be blocked by multivalent polycations adsorption (Keren, 1986). The
amount of permanent charge is characteristic for the mineral and conditions of its
genesis. For most frequently occurring clay minerals this charge amounts from a
few (kaolin group) by few tens (mica group – illites) to hundreds (smectites, zeolites)
of centimoles per kilogram of the mineral. Permanent charge minerals dominate
in mineral soils of the temperate climatic zone, therefore soils of these regions
are called permanent charge soils.
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