Agriculture Reference
In-Depth Information
Complexes with
Soil biomass
Primary mineral
insoluble organic
matter (soil humus)
weathering
Plant
[M ions
M-organic ligand and complex]
uptake
solution
M-inorganic
complex
Surface adsorption
Redox reactions
involving Mn, Fe,
Se, As, and Cr (solution
and precipitation)
M ions
M-ligand complexes
Occlusion
Solid state
in surface
diffusion into
precipitates
soil minerals
(MnO
2
, Fe
2
O
3
)
Figure 5.8
Possible reactions involving a micronutrient in the soil (White 1997). Reproduced with
permission of Blackwell Science Ltd.
Conversely, MoO
4
2
is most strongly adsorbed at low pH on sesquioxides
by a ligand-exchange mechanism analogous to that for H
2
PO
4
(section 4.5.4).
B is adsorbed the same way, as B(OH)
4
. Because this ion only occurs at pH
8,
B becomes less available at high pH levels.
Lime-induced Fe chlorosis
is a common problem in vines growing on soils
formed on chalk or limestone. The visual symptoms of chlorosis were introduced
in section 5.2.1. French scientists have found that the total CaCO
3
content of the
soil is not as important as the “active” CaCO
3
, which is in a finely divided form
(0-0.02 mm) (Baize 1993). The active CaCO
3
content is measured by extraction
in ammonium oxalate and can be related to the Fe extracted by EDTA to give a
chlorosis-risk index
(
CRI
), where
Active CaCO
3
(%)
CRI
10
4
(5.10)
(Extractable Fe)
2
(ppm)
Most of the American species of
Vitis
that provide phylloxera- and nematode-
resistant rootstocks are more sensitive to lime-induced chlorosis than
V. vinifera
.
Of the common rootstocks, only those derived from
V. berlandieri
show low sen-
sitivity to chlorosis. Fe chlorosis can be treated by adding Fe-chelates to the soil
(see appendix 5), but more commonly Fe, Mn, and Zn deficiencies are treated by
spraying vines with soluble sulfates. Cu is applied in the fungicidal spray Bordeaux
Mixture, which contains lime and CuSO
4
. In older vineyards, Cu may accumu-
late to potentially toxic levels, especially in acid soils, because of the repeated spray-
ing and strong complexing of Cu by soil organic matter.
Because of precipitation in insoluble compounds and strong sorption on sur-
faces,
micronutrient losses from leaching
are very small. The exceptions are B, which
is weakly adsorbed up to pH 8, and Fe and Mn, which can be leached in their
