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shoots are growing rapidly, the plant has the greatest requirement for nitrogen. Late summer and early autumn also have increased requirement because the vines have root growth and are storing reserves for winter.


Nitrogen deficit


Mycorrhizal fungi live on the roots of plants and form a symbiotic relationship. They extend microscopic straw-like filaments called “hyphae” into the soil where they extract, transport and can dramatically increase a host plant’s supply of nutrients and moisture.


to maintain but when present, the enhanced water and mineral uptake can be an important component of vine nutrition. Mycorrhizae can help to buffer a vineyard against localized or temporary water deficit conditions from the drying out off surface soils. Vine roots tend to be localized near the surface and can be quite vulnerable to drying conditions in mid summer. A grass cover crop increases the requirement for both water and nitrogen if the grass is to be maintained throughout the summer. This may be important on steep slopes, in order to maintain soil stability. Sod maintenance may double vineyard irrigation requirement by comparison with a non-irrigated alleyway. The advantage is that soil humus will be maintained or increased. Grasses consume nitrogen and can cause the overall soil nitrogen level to decline. If the vineyard has excess vigour, a


grass alleyway can be a benefit. Cover crop nutrition should be considered if it is to be maintained. Nitrogen requirements vary with


the growth rate of the vineyard throughout the season. During spring and early summer when roots and


30 British Columbia FRUIT GROWER • Summer 2013


during this period handicaps the vines’ winter hardiness and regeneration at the start of next year’s season. It can also limit fruit quality and cause fermentation problems because of low yeast available nitrogen (YAN). A factor I hadn’t considered until I


recently encountered it is that water uptake is dependent on the


contiguous channel of the vine’s xylem to maintain flow. Evapotranspiration causes a


pressure drop at the top end of the xylem tube leading into vine roots. So long as water pressure in the soil is higher than in the xylem, water will move upward through the plant. This depends on there being no break in the system to admit air, just as a pump will not work if there is a leaky pipe on the inlet side of the pump. When vines are hedged or topped,


the xylem is opened to admit air bubbles and the pruned parts of the vine have a temporary deficit until the wounds are healed. Whether or not this is an advantage will depend on the circumstances of the individual vineyard. Fruit development will be delayed, along with growth of the pruned shoots. In a perfect world we should be


able to create a table that lists the exact level of nutrient to be applied at each stage of vineyard growth. In a perfect world . . . — Gary Strachan is posted on LinkedIn.


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