Agriculture Reference
In-Depth Information
reduced and dissolves. The increases and subsequent decreases in Fe
2
+
in solution
coincide with the increases and decreases in HCO
3
−
, suggesting that insoluble
ferrous carbonates are formed. The ion activity products of well-known ferrous
carbonates and hydroxides are exceeded up to 10-fold in the four soils in the
early stages following flooding. Evidently precipitation of these compounds is
inhibited, probably as a result of adsorption of foreign solutes, such as dissolved
organic matter, on nucleation sites. However, once a sufficient supersaturation
has been reached, amorphous solid phases are precipitated, and these later re-
order to more crystalline forms. The changes in Fe(II) in the solid phase are
consistent with this, the more soluble pools showing peaks roughly matching
Fe
2
+
in solution but the HCl-extractable Fe(II) continuing to increase over time.
Mixed Fe(III)-Fe(II) compounds form initially upon flooding with a progres-
sively greater proportion of Fe(II) as reduction proceeds. In the Maahas, Nueva
Ecija and Tarlac soils, which contain 2:1 clays, some of the Fe is structural in clay
lattices. Reduction of this structural Fe causes a net increase in the negative sur-
face charge on the clay, resulting in increased CEC and decreased clay swelling
and surface area (Stucki
et al
., 1997). Further, in soils that undergo intermittent
reduction and oxidation, as all the soils here do, a large part of the easily reducible
Fe is present as coatings of oxyhydroxides on clay surfaces (Brinkman, 1985),
and these are dissolved during soil reduction. Where positively charged oxyhy-
droxides neutralize negatively charged sites on the clay, dissolution of the coat-
ings will cause the net surface negative charge and hence CEC to increase (Roth
et al
., 1969). The rapid increase in CEC in the early stages of reduction and its
subsequent stabilization (2.7) are consistent with the changes in pH and Fe.
The changes in
f
L
in the four soils following flooding roughly parallel the
changes in CEC and Fe. Increased CEC will cause increased anion exclusion
from narrow pores, decreasing
f
L
. Decreased clay swelling and interlayer space
with reduction of structural Fe will also increase anion exclusion and exacerbate
the decrease in
f
L
. There will also be changes in pore geometry with dissolution
of ferric oxyhydroxides coatings, but these will be complicated by subsequent re-
precipitation in initially amorphous but later crystalline forms. Poorly crystalline
compounds lining soil pores should increase the tortuosity of the diffusion path-
way, but as they revert to more crystalline forms with smaller specific surfaces,
f
L
should increase. Although the changes in diffusion impedance due to changes
in redox are smaller than the changes due to water content
per se
, they will be
important in some soils.
The Impedance Factor for the Solid Phase,
f
S
In Equation (2.18)
f
S
is defined in relation to
D
L
: it takes account of all factors
decreasing the mobility of the sorbed solute from the mobility it would have
in free solution. This includes the binding of the solute to the surface and the
limited thickness of the layer of water molecules associated with the surface,
