Growing good wine

The boom in the New Zealand wine industry over the past 20 years has seen the value of wine exports grow from just $3 million in 1985 to $302 million in 2004. Worldwide, New Zealand's wines win praise for their intense, fruity and crisp flavours, frequently outshining products from traditional wine-producing nations in international competitions.

That this success has come in one of the most demanding of horticultural pursuits is testament to the pioneering spirit and innovation that is the hallmark of New Zealanders. With little experience of growing wine grapes in New Zealand conditions, viticulturists have had to turn to science and experimentation to get the best from their crop.

Soil type, site conditions and aspect, rootstock, varietal suitability, plant nutrition and irrigation all need to be considered when establishing a vineyard. Though a high-value crop, grapes have traditionally been regarded as having a low nutrient requirement. This is particularly so for nitrogen: high levels of available nitrogen encourage canopy growth, while too little depresses yields. Increased canopy growth has two negative effects: fruit can be shaded and ripening delayed; and primary bud necrosis can develop, reducing crops by up to 50 percent. The challenge facing growers of wine grapes is to manipulate nitrogen levels so that yield is maximised without compromising quality.

Considerable research has been carried out into the interactions of nitrogen and grapevines, berries, juice, must and wine. Although this work has revealed some general trends, the complexities of the interactions mean that specific responses in individual situations may vary.

Nitrogen and the plant

Regardless of the grape variety involved, there appear to be two keys to managing the effect of nitrogen on growth. One is the amount of nitrogen supplied, and the other is the time at which it is delivered. Work carried out in Germany by Schaller showed how plant uptake of nitrogen varied throughout the growing season.¹ Prior to budburst, nitrogen uptake by White Reisling vines was relatively minor. Following budburst it increased to 50 g N/ha/day, boosting once more after the development of five to six unfolded leaves. Flowering produced a further spike, with rates of 600-800 g N/ha/day being taken up by the vines. The peak, though, came when the plants were bearing pea-sized berries, when 1800-2000 g N/ha/day was deposited in the vines. After this the rate declined, with a slight lift at veraison. Interestingly, nitrogen was mobilised from within the plant to supply the growing portion of the vine, even though sufficient soil nitrogen was available. Schaller notes that applying fertliser nitrogen early in the season, when uptake rates are slow, may increase the risk of loss through nitrate leaching.

In Australia, Bell and Robson investigated the effect of supplying fertiliser nitrogen to Cabernet Sauvignon vines of low nitrogen status.² Applying 0, 50, 100, 200 or 400 g N/vine (as liquid urea) by dripper irrigation, they showed that moderate amounts of nitrogen (100 g/vine) had a beneficial effect on vine productivity, improving vine vigour and vine nitrogen status (as shown by petiole nitrate concentration). Applying higher rates of nitrogen did not produce further benefits. During the course of the three-year trial, Bell and Robson noticed that the phosphate status of the vines was declining. They suggested this was due to decreased availability of soil phosphate as a result of increased soil acidity. This occurred with the addition of 50 g N/vine or more, although the higher rates did not cause any greater acidity than the lowest rate.

Nitrogen and the wine

Numerous studies have shown a relationship between nitrogen supply and wine quality. High rates of nitrogen on Pinot Noir grapes have been shown to decrease anthocyanin levels in juice and wine, and increase pH. Reduced anthocyanin levels have also been shown in other cultivars, such as Merlot and Tempranillo. High pH is associated with lower quality as a result of microbial instability and brownish hues in red wine.

In white wine varieties, increased nitrogen supply is again associated with raised pH, and also with increased malate concentration. Higher levels of citrate and tartrate have also been observed for some varieties at some sites. That researchers investigating one cultivar grown in multiple sites have noted different responses to nitrogen treatments underlines the importance of environmental conditions and viticultural practices.

In an extensive study, Treeby et al. showed that the effect of nitrogen applications on wine quality can be affected by amount of nitrogen, time of application, rootstock and cultivar.³ Shiraz grapes were fertilised with nitrogen at 0 or 40 kg N/ha/year for three years, then harvested. Wine was made from these samples according to standard practice, although DAP was not added to the must. Levels of yeast assimilable amino nitrogen (YAN) in grape juice (needed as a source of nitrogen for yeast during fermentation) were highly responsive to nitrogen applications, and not influenced by the timing of those treatments (both summer and autumn applications produced a significant effect). However, when nitrogen was applied post-harvest, rootstock did affect the outcome, with grape juice YAN from vines on Teleki rootstock not improved by the nitrogen treatment. In contrast, Schwarzmann and Ramsey rootstocks appeared to utilise post-harvest nitrogen efficiently.

With higher YAN, fermentation rates increased, and 50% depletion of total soluble solids (the point at which skins were pressed off) was achieved in less time, the quickest time being 182 hours for Ramsey rootstock treated with nitrogen, and the slowest being 274 hours for Teleki without nitrogen. Levels of residual sugars in the final wine indicated dryness was attained.

When the wines were assessed by a taste panel, a significant interaction was noted between rootstock and nitrogen applications. Overall, wines made from Schwarzmann-grown grapes fertilised with nitrogen during the autumn, and Ramsey-grown grapes receiving nitrogen during the summer were found to be inferior to all other treatments.

Given that so many factors influence yield and wine quality, it is not surprising that research tends to underline the importance of individual response characteristics to fertiliser strategies. Rootstock, time of nitrogen application, quantity of nitrogen applied and the pre-fertilisation nitrogen status of the vine are just some of the factors that need to be considered when looking to improve wine quality. In the end, each winegrower must examine their own practices and results, learning from the experience of others but tempering that with a good dose of judgement.

References

1. Schaller, K. (2000), Intensive viticulture and its environmental impacts: nitrogen as a case study, Acta Hort 512, pp. 39-46.
2. Bell, S-J. & Robson, A. (1999), Effect of nitrogen fertilization on growth canopy density, and yield of Vitis vinifera L. c. Cabernet Sauvignon, Am. J. Enol. Vitic., vol. 50, pp. 351-358.
3. Treeby, M.T., Holzapfel, B.P., Pickering, G.J. & Friedrich, C.J. (2000), Vineyard nitrogen supply and Shiraz grape and wine quality, Acta Hort 512, pp. 77-92.


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