Agriculture Reference
In-Depth Information
Shoots
Roots
Figure 5.6
Time trend in shoot and root
growth and N uptake by
grapevines (redrawn from
Freeman and Smart 1976,
Conradie 1980, and Lohnertz
1991).
Nitrogen
Time
before bud burst because of the potential for NO 3 loss by leaching. The best
time for application is from preflowering to preveraison. However, in warm cli-
mates, N applied after harvest can be effective because it increases the N reserves
that are utilized the following spring.
An adequate supply of N to the vine is most important for shoot growth and
photosynthesis, which produces the sucrose subsequently translocated to the fruit.
If the N supply is deficient, bud fertility and therefore yield are reduced, and the
N concentration of the berries will be too low. The critical measure of berry N is
the Yeast Assimilable Nitrogen ( YAN ), comprising NH 4 -N and amino acid N (ex-
cluding proline), which should be in the range 200-300 mg N/L in the juice. If
YAN is too low ( 140 mg/L), the fermentation rate will be too slow and may
become “stuck.” The wine maker can correct this problem by adding DAP (see
table 5.7) to the must. However, if the YAN is too high, the fermentation pro-
ceeds too rapidly and poor-quality wine is produced, particularly in the case of
red grapes. Too high a supply of N to the vine, especially when coupled with
plentiful water, can lead to excess vigor —a condition of long, straggling shoots and
a dense canopy that overshades the bunches and results in lower berry color and
flavor. We return to the topic of excess vigor in chapter 6.
Supplying the correct amount of N fertilizer at the right time is important
for quality control in grape production and wine making. Examples of how to es-
timate the amount N fertilizer required are given in box 5.4.
5.4.1.5
Residual Effects
A residual effect occurs when enough of a fertilizer element applied in one year re-
mains available for growth during the next year. The residual effect of N fertil-
izer in vineyards depends on the rate and form of N applied, its timing, vine root
development, the amount of rain and irrigation, and soil properties. Provided the
 
 
Search WWH ::




Custom Search