Agriculture Reference
In-Depth Information
Box 1.5
Complex Formation and Podzolization
As indicated in the discussion of lessivage, polyphenols can form soluble
organometal complexes or
chelates
with the normally immobile Al
3
and Fe
3
cations. Typically, ferric iron (Fe
3
) is reduced to ferrous iron (Fe
2
), with the
concurrent formation of a soluble ferrous-organic complex. Under anaerobic
conditions, decomposition of the organic compound supplies the electrons that
drive the reduction process. Because H
ions are also involved in the reduction,
the formation of the complexes is favored by low pH. Thus, chelation and subsequent
translocation of Fe and Al are more obvious on acidic parent materials and under
wet surface conditions. The overall process, called
podzolization
, is best developed
under conifers (and heath-type species) in wet climates on acidic, sandy soils.
decomposed. Conversely, the litter of elm and ash, and to a lesser extent oak and
beech, is more readily ingested by earthworms and becomes mixed with soil as fe-
cal material. The result is a more uniform distribution in the A horizon, to a depth
of 20-30 cm, of well-decomposed organic matter that is called
mull humus
(fig.
1.9b).
Earthworms are the most important of the soil-forming fauna in viticultural
regions supported both by small insects and larger burrowing animals (rabbits,
moles, and voles). Because of their burrowing, earthworms create large pores that
facilitate soil drainage, and so are very important in vineyard soils. The relation-
Soil with (a) mor humus and (b) mull humus (White 1997). Reproduced with permission
of Blackwell Science Ltd.
Figure 1.9
(a)
(b)