Agriculture Reference
In-Depth Information
poor plant performance. Both the effect of toxicity and deficiency only becomes vis-
ible with severity; the micro nutrients generally have a much smaller range between
the two extremes than macro nutrients. A number of nutrients impact on reproductive
development of grapevines if deficient, while high N supply can lead to poor fruit set
and increase the sensitivity to botrytis (Keller
2010
). The insufficient supply of the
macro nutrient P leads to poor fruitfulness and fruit set, while a lack of K can reduce
bunch weight and uneven ripening. Localised Ca deficiency is seen as the cause of
the physiological disorder of the grape bunch bunchstem necrosis (BSN); with the
symptoms appearing after veraison and increasing during the ripening period. A de-
ficiency of the macro nutrients Zn, B and Mo negatively impact on fruit set, B leads
abnormal fruit set (millerandage), and Zn to poor fruit set (coloure), while a lack of
Mo has also been related to poor fruit set on acid soils (fixation).
The composition of grapes influences the winemaking process and the wine pro-
duced from these grapes and all nutrients effect crop quality directly. However,
the supply of the macro nutrients N, P and K alter must composition, excessive
K results in high juice pH and low colour in the ferment and wine. The amounts
nitrogenous compounds present in the must as amino acid and ammonium (NH
4 +
)
have a critical role in the fermentation process and the final wine composition (Bell
and Henschke
2005
). Yeast assimilable Nitrogen (YAN) consists of NH
4 +
and free
assimilable amino N (FAN), while the amino acid proline is classified as non as-
similable amino-N (YNAN). The uptake of nitrogen from the must is ordered, NH
4 +
is taken up first, followed by argenine and then the other amino acids present in
the must. The level of YAN impact on fermentation kinetics and desired/undesired
compounds produced during the fermentation and present in the wine. As indicated
before juice N content and composition impact on the wine making process, the
amount of N present in form of YAN which includes all assimilable amino and
NH
4 +
is critical. Low must YAN lead to a slow fermentation, high amounts of thiols
(eg. H
2
S) and undesired higher alcohols and low levels of desired esters and long
chain fatty acids. A fast fermentation is present when YAN levels are high and ethyl
acetate, acetic acid, volatile acidity is produced during the wine making process, the
final wine contains high levels of undesired ethyl carbamate and biogenic amines
(Bell and Henschke
2005
). This indicates appropriate level of YAN is required for
the must, the vineyard N application should be designed to produce the optimal
YAN content in the grapes, while further fine tuning can be conducted in the winery
with the addition of N supplements such as di-ammonium phosphate (DAP).
Assessing grapevine nutrient status is important to verify the requirements for
fertiliser application in a vineyard. The general appearance of grapevines in relation
to vine vigour and leaf symptoms is a first observation of how well a vineyard is
supplied with nutrients. However, visual deficiency (or toxicity) symptoms become
apparent after the productivity declines, some nutrients specifically reduce the yield
levels at key stages of reproductive development as mentioned previously. Deter-
mining the annual grapevine nutrient status with supporting information on soil
nutrient levels is an important to ensure optimal vineyard productivity and grape
composition. The appearance of visual deficiency symptoms vary between nutri-
ents and the severity of the lack of supply (Nicholas
2004
), in general most macro
