Geoscience Reference
In-Depth Information
Equation (5.3) indicates an inverse relationship between ρ for a sandy or silty soil and the volu-
metric water (soil solution) content, θ (= φ S ). Although this inverse relationship is usually found for
most soils, instances do occur when this relationship does not hold, implying that other factors need
to be taken into account. There are two fairly intuitive effects that θ can have on ρ. First, given set
concentrations for the dissolved ions within the soil solution (ρ W is now assumed to be constant), a
change in θ causes a change in the total number of charge carriers (dissolved ions) per unit volume
of soil, in turn altering the soil's capacity to distribute electric current as defined by ρ (or σ, EC).
Waterlogged soil conditions produced by an irrigation event conducted to flush salts from the soil
profile, followed next by complete gravity drainage, is one agricultural scenario corresponding to
ρ W remaining constant, while θ changes from near-saturation to field capacity. Under this scenario,
ρ gets larger as the soil drains.
Regarding the second effect of θ on ρ, as θ varies significantly, so too does the continuity of soil
solution through which electrolytic current is transferred. These changes in the soil solution conti-
nuity in turn alter the soil's ability to transfer electric current as quantified by ρ. To further empha-
size this second θ effect, a substantial decrease in soil wetness (lower θ) reduces the thickness of the
soil solution films covering solid soil particles, thereby lengthening the electric current flow travel
paths within the soil solution (increased tortuosity), and as indicted by Equation (5.2), diminishing
the overall capability of the soil to convey current (higher ρ). Wet soils near saturation or at field
capacity will exhibit good soil solution continuity for distributing current, but for extremely dry
soils, soil solution films may not completely cover all the solid surfaces, thus severely reducing the
connectivity of travel paths for electrolytic current flow. In an example discussed previously, soil
drying reduces the amount of soil solution present (lower θ), narrowing and lengthening the soil
solution conduits for electrolytic current flow, which in turn almost always results in an increased
ρ value, even though there is a drop in ρ W due to evapotranspiration concentrating dissolved ions in
the remaining soil solution.
Up to this point, the discussion regarding soil resistivity has focused on sandy or silty materials
containing no clay minerals and organic matter. Most soils, however, have significant amounts of
clay minerals (layered aluminosilicates) and organic matter. Soil organic matter can be divided into
two components. The first component includes only a few percent of the total soil organic matter
and includes living organisms (worms, bacteria, fungi, etc.) and nondecomposed substances such
as dead plant roots. The second component, representing the large majority of soil organic matter,
is the stable, decomposed residue called “humus” (Bohn et al., 1985). It is the stable, decomposed
residue portion of the soil organic matter which affects the overall soil resistivity, ρ. Clay minerals
and organic matter often coat the larger sand- and silt-sized particles, resulting in the sides of soil
pores being dominantly composed of these clay mineral and organic matter materials, consequently
giving these materials a much larger impact on soil processes than would be inferred based on their
weight percent alone. The general manner in which the previously discussed factors influence ρ W
and θ, and likewise ρ, is the same regardless of whether a soil does or does not contain clay miner-
als and organic matter. The major difference regarding soils containing clay minerals and organic
matter is that given sufficiently wet conditions, there is an additional effect caused by the presence
of clay minerals and organic matter, which tends to enhance electric current flow.
Substitution of ions different from the ones normally composing the clay mineral crystal lattice
and the functional groups that are a part of the humus chemical structure typically yield large num-
bers of discrete negatively charged sites on the surfaces of clay mineral and organic matter particles.
Cations are electrostatically attracted and become attached at the negatively charged surface sites.
The quantity of the negatively charged surface sites per unit amount of dry soil is referred to as the
cation exchange capacity (CEC). Given a sufficient amount of soil solution covering the clay mineral
and organic matter surfaces, these attached cations are exchangeable and often displaced by other
cations temporarily present in the soil solution. The displaced cations move freely within the soil
solution adjacent to clay mineral and organic matter surfaces and can then displace cations at dif-
ferent negatively charged exchange sites. The displacement and movement of cations from exchange
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