Agriculture Reference
In-Depth Information
conditions. The list is not exhaustive but it serves to illustrate the important
processes. The properties of the elements are summarized in Table 7.6 and
Table 7.7 gives the important redox equilibria.
Zinc
Zinc occurs in soils exclusively in the
+
2 oxidation state and in solution as
the B-type cation Zn
2
+
. It is weakly complexed by the main functional groups
in organic matter, but under sulfate-reducing conditions forms insoluble sulfides
(ZnS, p
K
=
24
.
7). In intermittently submerged soils, such as wetland rice soils,
ZnS probably generally does not form because FeS and FeS
2
are precipitated
at a higher pe
+
pH and hence will form preferentially if the redox is poised
by Fe(II) (Sajwan and Lindsay, 1986). Under such conditions Zn
2
+
forms solid
solutions in oxides and clay minerals (see below). Hence it tends to be highly
immobile under anaerobic conditions, but under acid oxidizing conditions it is
released in soluble and mobile forms.
Zinc solubility in soils tends to show a minimum at near neutral pH
(Figure 3.14). At low pH the free ions are only weakly sorbed on charged soil
surfaces, but at pH
>
7, as concentrations of dissolved organic ligands increase,
soluble Zn—organic complexes may form, raising the total concentrations of
Zn in solution even though the activity of the free ion may be extremely small
through sorption reactions. At high pH Zn
2
+
forms solid solutions in Ca and Mg
carbonates, and mixed hydroxy-carbonates, so it is immobile and unavailable to
plants in alkaline or calcareous soils.
Zinc deficiency is widespread in wetland rice affecting up to 50% of the
area (Katyal and Vlek, 1985; Welch
et al
., 1991; Batten
et al
., 1992; Neue and
Lantin, 1994). Zinc relations in rice have therefore been studied extensively. The
deficiency is most often associated with poor drainage and perennial soil wetness.
The soils typically have weak profile development, reflecting the poor drainage,
and much of the Zn is in primary minerals or in other highly insoluble forms. It
is also often associated with high soil organic matter content, high pH and high
Mg:Ca ratios in the soil.
All of these factors are present in the toposequence at Tiaong, Quezon Province,
Philippines shown in Figure 7.6, which has been used for many years by the IRRI
to screen rice for Zn deficiency tolerance (Quijano-Guerta
et al
., 2002).
The toposequence is on the gently sloping foot-slope of a young inactive vol-
cano, Mt Banahaw. The drainage is poor across the toposequence due to perennial
upwelling of artesian water. The extent of Zn deficiency increases down the slope,
as do soil wetness, organic C content, CaCO
3
content and CEC. The pH, clay
content and extractable Zn are uniform. The upwelling water contains high con-
centrations of Ca
2
+
,
Mg
2
+
,
HCO
3
−
and H
4
SiO
4
of volcanic origin. As it reaches
the surface, CO
2
degasses causing the pH to rise. This results in precipitation
of CaCO
3
and possibly
de novo
synthesis of Mg smectities, on and in which
