Agriculture Reference
In-Depth Information
the amount of nutrients supplied at normal application rates is small, the vines
(and soil microorganisms) may benefit from growth-regulating compounds in the
extracts.
Summary Points
5.8
This chapter on nutrients and their supply in fertilizers is summarized as follows:
■
Vineyard productivity depends on soil fertility, which is determined by the
nutrient-supplying power of the soil. Although visual symptoms in vines can be
observed in the field, deficiency or excess of macro- and micronutrients in
grapevines is usually diagnosed by
plant analysis
or
tissue testing
. The
critical value
is the nutrient concentration at which the vine no longer responds to further ad-
ditions of that nutrient.
■
A basal leaf, opposite a bunch near the base of a shoot, is sampled on a vine at
full flowering. Some 75-100 leaves from vines of one variety in a block should
be collected to make a
representative sample
for analysis. Petioles are preferred to
leaf blades. A further sample can be collected at veraison to check on doubtful re-
sults. Samples should not be taken soon after any sprays or dusts have been ap-
plied.
■
When a vineyard is being established, soil analyses, especially for P, micronutri-
ents and pH, must be used to assess nutrient requirements. Soil samples (to 50-
cm depth) can also be analyzed to measure the amount of mineral N (NH
4
and
NO
3
) in the root zone. However, soil N supply during the growing season is
mainly determined by net mineralization of organic N.
Green manuring
with legu-
minous cover crops produces a short-term increase in soil organic N. The rate of
N mineralization can be estimated from the organic N content and decay coeffi-
cients.
■
Both plant and soil tests must be calibrated against the grape yield response to a
particular fertilizer. From this, the quantity of nutrient required is assessed. By
comparing the value of the product with the cost of individual fertilizers, the most
appropriate form and optimum quantity of fertilizer can be determined.
■
Yields and soil properties often vary widely within one vineyard. This variation
can be measured annually and mapped in a GIS, which enables specific manage-
ment to be applied within blocks, or between blocks, to remove yield and/or grape
quality constraints. This is called
precision viticulture
.
■
The amount and timing of N fertilizers (mainly Ca(NO
3
)
2
, urea, and NH
4
NO
3
)
must be carefully matched to the vines' needs and to the period of most rapid
root growth. Too much N leads to excess vigor and the risk of surplus NO
3
be-
ing leached below the root zone. Too little N leads to reduced yields and to low
yeast assimilable N
(
YAN
) in the fruit, which in turn can lead to “stuck fermen-
tations.” The optimum
YAN
in grape juice is between 200 and 300 mg N/L.
■
Soluble N fertilizers can be applied in irrigation water (
fertigation
), which is the
best way to avoid volatilization of NH
3
from urea spread on the soil surface, es-
pecially for calcareous soils.
■
P can be supplied as
water-soluble fertilizers
(SSP, TSP, MAP, and DAP) or as in-
soluble
reactive phosphate rocks
(RPRs). To dissolve, RPRs depend on the soil to
supply H
ions and moisture. As they dissolve, they have a small liming effect.
However, the main liming materials are ground limestone and chalk (CaCO
3
),
