Agriculture Reference
In-Depth Information
tained in soil) may be increased under waterlogged conditions, because of the faster
weathering of ferromagnesian minerals.
Note that H
ions are consumed in reducing reactions, so the soil pH tends
to rise. This can affect the availability of micronutrients, and also P. For exam-
ple, in waterlogged soil, reduction of Fe
3
in iron oxides releases adsorbed P or
P from insoluble FePO
4
. However, at pH
6, Fe
2
compounds become less sol-
uble, and a metastable iron oxyhydroxide, Fe
3
(OH)
8
, precipitates. This precipi-
tate provides a highly reactive surface for the readsorption of P, and certainly all
the dissolved P is readsorbed when aerobic conditions return and Fe(OH)
3
pre-
cipitates.
At very low
E
h
values (fig. 5.9), SO
4
2
is reduced due to the activity of ob-
ligate anaerobic bacteria of the genus
Desulphovibrio
. This reaction depletes the
available sulfate supply. As the concentration of H
2
S increases (reaction 5.15), fer-
rous sulfide (FeS) precipitates and slowly reverts to
iron pyrite
(FeS
2
). Iron pyrite
is a mineral found in present-day estuarine and marine sediments. Over geologi-
cal time, such sediments have transformed into sedimentary rocks, often with as-
sociated coal deposits. For example, FeS
2
occurs in the
cendres noires
that have
been regularly applied to build up the Chalk soils of the Champagne region in
France (section 1.3.3.2). When exposed to air, FeS
2
oxidizes according to the re-
action
FeS
2
15/4O
2
7/2O
2
Fe(OH)
3
4H
2SO
4
2
(5.16)
In chalky and calcareous soils, the H
2
SO
4
formed reacts with CaCO
3
to cre-
ate microsites of lower pH in the mineral soil, where Fe is more available.
Organic Viticulture
5.7
Organic viticulture involves growing grapes without the use of industrially man-
ufactured fertilizers (sometimes called “chemical fertilizers”) and herbicides, pes-
ticides, or fungicides for weed, pest, and disease control. France is probably the
home of organic viticulture and “organic wine,” but the practice is becoming more
popular in many of the world's wine regions. This process has also been referred
to as “sustainable viticulture” or “alternative viticulture.” An ideological variant of
organic viticulture is “biodynamic viticulture,” which follows the same basic tenets
as organic viticulture, but in addition holds that the moon and stars influence the
vine and growth of its parts at different times of the day and year. The scientific
basis of these beliefs has not been established.
Strictly, a vineyard can be called “organic” only if it has been registered, in-
spected, and certified by a supervisory organization such as Ecocert, Nature et
Progrès, or Terre et Vie in France, the Organic Foods Production Association of
North America, or the Organic Vignerons Association of Australia. Normally, a
three-year conversion period is required for vineyards to be certified as organic.
Certified organic production means that grapes are grown without insecticides,
herbicides, fungicides, and chemical fertilizers, other than those approved by the
certifying agency. Nor can genetically modified vines be grown or genetically mod-
ified yeast used in the fermentation to produce organic wine.
Many vignerons have not adopted true organic certification, which is an ex-
acting standard, but have moved toward minimum intervention in the vineyard
