Geoscience Reference
In-Depth Information
subsurface electrolyte solution increases with the amount of gypsum that is applied
(Kemper et al.
1975
; Oster
1982
).
Application of gypsum as a reclamation product to counter soil and subsurface
domain acidity may, however, enhance the dispersion of soil-clay colloids in
highly weathered acidic soils, leading to irreversible changes in the soil-subsur-
face system. A possible mechanism for the enhancement of clay dispersion caused
by gypsum application was given by Roth and Pavan (
1991
). They considered that
gypsum-induced clay dispersion in acidic soils is due to Al monomers and/or
polymers, which act as binding agents, being displaced in the presence of SO
4
2-
from exchange sites by Ca
2+
. An alternative mechanism to Ca
2+
-induced dis-
persion, following application of gypsum, is the specific adsorption or ligand
exchange of SO
4
2-
which can change the clay particle charge characteristics
(Uehara and Gillman
1981
; Rao and Sridharan
1984
).
The favorable effects of gypsum for reclamation of saline-sodic soils were
determined in field experiments carried out as early as the beginning of the last
century (e.g., Hilgard
1906
; Kelley and Arany
1928
), and discussed over the years
in comprehensive reviews and books (e.g., US Salinity Laboratory Staff
1954
;
Bresler et al.
1982
; Oster
1982
; Shainberg and Letey
1984
). Water transmission
properties expressed in terms of both hydraulic conductivity (HC) and infiltration
rate (IR) may be altered by irrigation with sodic water that originates from a saline
aquifer, drainage, or sewage effluents. Both HC and IR are controlled by soil-
subsurface clay content and mineralogy, as well by the Na
+
concentration of the
incoming water, due to Na
+
-induced changes to the physical configuration of the
solid phase. Gypsum amendments applied directly on the land surface or added to
the incoming water may change the water transmission properties by exchanging
Na
+
with Ca
2+
in sodic soils, or by preventing Na
+
adsorption on the soil solid
phase caused by irrigation with sodic water.
Hydraulic conductivity of sodic soils is improved following gypsum applica-
tion. In a laboratory column experiment, Shainberg et al. (
1982
) measured the
hydraulic conductivity of two sodic soils having an exchangeable sodium per-
centage (ESP) of 20, by leaching soils with distilled water to which equivalent
amounts of CaCl
2
and gypsum were added. The effect of varying application of
CaCl
2
and CaSO
4
on the relative hydraulic conductivities of noncalcareous
(Golan) and calcareous (Nahal Oz) soils, as a function of cumulative effluent
volume, is shown in Fig.
18.8
. It can be observed that addition of gypsum yielded
a higher hydraulic conductivity than addition of CaCl
2
.
When applied in the field, gypsum dissolution in incoming water controls the
efficiency of this amendment on the HC. Due to the relatively low dissolution rate
of gypsum, long-term dissolution in water occurs so that HC changes are irre-
versible on a human lifetime scale.
Infiltration rate, defined as the volume of water penetrating the soil-subsurface
cross section per unit surface area, is controlled by the aggregation status of the
upper soil layer and the potential formation of a crust (seal). Irreversible changes
in soil structure may occur when clay particles become dislodged due to an
increase in the Na
+
fraction in soil and a decrease in the electrolyte level of the soil
