Geoscience Reference
In-Depth Information
Fig. 18.7 Relationship between pH and clay dispersion expressed as a the amount of clay
concentrated in runoff and b the total infiltration as a percentage of applied rainfall (Haynes and
Naidu
1998
)
compression of the double layer and flocculation of clays. Liming-induced
increases in pH lead to an increase in the negative charge on soil surfaces, and thus
to the ratio of negative to positive charges. This in turn causes a decrease in Al
3+
activity because of aluminum precipitation as hydroxy-Al polymers. As a result,
the dominant repulsive forces among particles lead to their dispersion. Under
leaching, the dispersed particles formed under liming may result in a surface crust,
clog the pore space, and/or be transported and redeposited at depth; these pro-
cesses irreversibly affect water and solute transport in the soil-subsurface system
(Roth and Pavan
1991
; Haynes and Naidu
1998
; Bednarek and Reszka
2007
).
Irreversible effects of liming on the soil structure can also be attributed to the
flocculation and cementing actions of applied lime and of newly precipitated iron
and aluminum oxides and hydroxides (Haynes and Naidu
1998
). The relationship
between pH
KCl
and clay dispersion for a Brazilian Oxisol, expressed as the amount
of clay concentrated in runoff and as the total infiltration (as a percentage of
applied rainfall), is shown in Fig.
18.7
.
Gypsum, a hydrated calcium sulfate, is a naturally occurring mineral that may
also be available as a byproduct material (e.g., phosphogypsum, from the phos-
phorus fertilizer industry). Gypsum has been used on agricultural soils to increase
the Ca
2+
content of acidic soils and to exchange Na
+
with Ca
2+
in calcareous
saline-sodic soils. Gypsum solubility depends on both the composition of soil
solution and the exchange phase, and is lower when applied as mined gypsum than
as a byproduct, in the widely used form phosphogypsum. Gypsum dissolution
follows first-order reaction kinetics. The electrolyte concentration of water flowing
through a layer of gypsum can be described adequately by a transport equation
which includes terms to account for advection, diffusion, and dissolution kinetics.
For a given rate of water application, the concentration of gypsum in the soil-
