Agriculture Reference
In-Depth Information
•
The thermal regime of the vine's microclimate is influenced by the soil
, through
surface color, stoniness of the surface (affecting heat absorption and
conduction during the day, and reradiation at night), and clay content
(affecting water holding capacity, which in turn affects the soil's heat
capacity). Heat storage by the soil is particularly important in cool-climate
vineyards where the maintenance of warm soil into the autumn is necessary
for root growth and the continued synthesis of cytokinins and their supply
to the ripening fruit. In the ripening berries, cytokinins are involved in the
accumulation of sugars, which are transported from the leaves. Sugars in
excess of the amount needed for respiration and cell growth are used in the
synthesis of flavonoid compounds that are associated with skin color,
flavor, and aromas.
•
The soil water supply is important
. The key factors are the volume of soil
exploited by the roots (mainly determined by rooting depth and planting
density), the rate of water supply to the roots at
m
values between
10
and
400 kPa, and the rate of drainage of surplus water from the root
zone (
10 kPa). In dryland vineyards, the soil's structure and
texture and the depth to groundwater control the water supply. Optimal
conditions have been identified, such as a soil strength
2 MPa at the field
capacity (section 7.1.2), an infiltration rate
50 mm/hr (section 7.1.1), air
capacity
10% (section 3.3.3), and a high value of
RAW
plus
DAW
(120-130 mm/m depth) (table 6.3). However, the influence of these soil
conditions on the vine is modified by the underlying parent material in the
case of older vines growing on porous and fractured rock, such as limestone
in the Côte d'Or, St. Emilion, or the Coonawarra, or on schists in
Beaujolais and the Hérault district of Languedoc-Roussillon. Furthermore,
the soil water supply can be augmented by irrigation and manipulated by
RDI
or
PRD
to minimize the problem of excess vigor. Experience in
regions with hot summers in California and Australia suggests that the
most critical period for regulating the water supply to enhance quality is
after fruit set until veraison. But in the cooler regions of Europe, regulation
of the water supply during the long ripening period after veraison is found
to be more important.
•
Good drainage is important,
especially in spring and early summer. A wet
soil at this time of year slows the rate of soil warming and inhibits root
growth. The best vineyards are consistently found on freely drained soils,
such as form on limestone, limestone marls, or chalk in the Côte d'Or, St.
Emilion, Champagne, and the Coonawarra, or on alluvial gravels and
sands, and colluvial deposits, as in St. Emilion-Pomerol, the Médoc, Napa
Valley, Barossa Valley, Maipo Valley of Chile, and the Hawkes Bay and
Marlborough regions of New Zealand. Soil drainage can be improved by
pumping where water tables are high (e.g., the deep sandy clays in St.
Emilion) or by underground pipes in heavy clay subsoils (e.g., Carneros,
parts of Pomerol, and the Médoc).
•
A balanced nutrient supply is important
, in particular of N and K (the latter
especially so in hot, dry climates). Too high a supply of mineral N during
the period of most rapid uptake between flowering and veraison causes
excessive canopy development. The consequences are potentially too many
bunches, large berries, and overshading of bunches. Shaded leaves produce
m
