Agriculture Reference
In-Depth Information
phosphorus (P), potassium (K), and sulfur (S) are not specific for grapevines.
They have been adapted from other orchard and agricultural crops, mainly
because plant analysis has been more favored for grapevines, especially for estab-
lished vineyards. Nutritional deficiencies and toxicities can be corrected with
appropriate fertilizers (including organic inputs), with soil amendments (such
as lime and gypsum), and in some cases by changing soil pH.
A soil's supply of N and K deserves special mention. For N, there must be an
adequate supply for healthy growth, flowering, and fruit set, as well as to provide
a concentration of yeast assimilable N in grape juice of 250 to 300 mg N/L for
optimum fermentation. Mineral N is produced naturally through the mineraliza-
tion of organic N, and table 3.2 in chapter 3 gives a qualitative estimate of the
net N supply from different organic sources. When this supply is inadequate, as
in sandy soils in relatively hot climates, it can be supplemented with fertilizers.
However, soils of high organic C content and C-to-N ratios below 20 to 25 may
produce more mineral N than a vine needs for balanced growth—the condition
of excess vigor ensues. The extent to which excess vigor is a problem depends on
the grape variety and the style of wine to be made. It is especially undesirable for
the production of high-quality red wines, because too much shading of the grapes
during ripening retards the full development of color and phenolic compounds.
On the other hand, a white grape variety such as Sauvignon Blanc needs a good
supply of N and water to develop the full aromatic potential of the fruit, for the
making of wines meant to be consumed without ageing. Wines made from other
white varieties such as Riesling and Semillon benefit from ageing and the use of
fruit from less vigorous vines, provided the vines have not suffered severe water
stress around veraison.
Too much N relative to K in vines, under variable weather conditions in
spring, can lead to symptoms of “false K deficiency.” Conversely, in soils contain-
ing illitic clay minerals, release of K
+
ions from within the clay mineral lattices can
result in too much K being available and taken up by the vines. This can lead to a
high juice pH, lower acidity, and color instability in red wines. Thus, especially for
red grape varieties, soils with a low N mineralization potential and small propor-
tion of illitic clay minerals are to be preferred.
Although micronutrient chemistry is complex, some generalizations can be
made as to the effect of pH on micronutrient availability: that is, (a) the solubility
of Fe, manganese (Mn), zinc (Zn), and copper (Cu) decreases with an increase in
soil pH; (b) boron (B) availability reaches a maximum at pH(H
2
O) of 8, above
which it decreases; and (c) molybdenum (Mo) availability is least in acid soil
and increases with an increase in pH. Another consistent trend is that active Al
increases as the soil becomes more acidic, resulting in an inhibition of root growth
and P uptake. However, vines have survived, if not thrived, on soils of pH(H
2
O)
as low as 4 for many years, as evidenced by the old bush vines shown in figure 6.5.
By a similar reasoning that moderate water stress at a critical physiological stage
