The Impact of Differences in Soil Texture within a Vineyard on Vine Vigour, Vine Earliness and Juice Composition

By: M. Trought, R. Dixon, T. Mills, M. Greven, R. Agnew, J. Mauk, and J. Praat

In: Journal International des Sciences de la Vigne et du Vin. 42(2):67-72. 2008

• ·When there are substantial differences in soil texture in a vineyard, it is likely there will also be differences in fruit maturity, manifested as both overripe and unripe flavors, in the fruit at harvest.  These differences in fruit composition from individual vines are then blended to obtain a single vineyard composition at the receiving winery.  To the current authors, understanding the contribution of individual vines to this single composition will help to manage the differences to achieve an optimum fruit style.

• · With this goal in mind, these authors chose to study the influence of soil variability on vine performance and fruit composition.  Their approach was to document vine variability (trunk diameter, shoot numbers, date of main phenological stages, cluster Brix distribution), on the one hand, and soil variability (electrical conductivity, soil profiles, air and soil temperature) on the other.  They then studied the relationship between the two sets of variables.

• · The trial site was a Sauvignon blanc/SO4 commercial vineyard in Marlborough, New Zealand, trained to a 4-cane vertical-shoot-position which was hedged 3 times in the season to maintain a very narrow canopy.  After measuring the trunk diameter, the authors classified all the vines from 8 rows into 5 size categories: extra-small (XS), small (S), medium (M), large (L), and extra-large (XL). For the soil electrical conductivity, they used an EM38 electromagnetic sensor, able to detect changes in conductivity which, in turn, reflect changes in gravel composition, on the one hand.  The soil profiles were obtained after excavating pits 1.5 m deep next to vines belonging to each size category.

• · Results:

1) Vines with XS trunk circumferences grew in soils where the gravels reached the surface, and conductivity values were low.  In contrast, vines with XL circumferences grew in silty loams with no stones at least in the top 2 meters, and had high conductivity values.

2) XS vines flowered earlier (about 3 days) and progressed through veraison earlier (about 7 days). At harvest, their fruit achieved sugar targets earlier (about 11 days), and had higher pH, and lower TA, compared to larger vines.

3) Leaves in the fruiting zone of XS vines senesced earlier (60 days) and their chlorophyll levels were lower than extra-large vines.

4) Whereas the distribution of berry density (Brix) of XS vines was approximately normal, with a mean of 21.6^oBrix, the Brix distribution of these vines was negatively skewed, in that they had more berries with lower Brix than expected, around a higher mean, 24^oBrix.

5) The authors calculated that a vineyard with 25% XS vines would have a fruit Brix range between 18 and 27, with a mean of 21.8^oBrix.  In contrast, a vineyard with 75% XS vines will have a Brix range of almost 19.0^o, with a mean of 23.8^oBrix.

Thus, the proportion of small vines in a vineyard can make a big difference.  Even though harvest fruit composition has traditionally focused on the mean values of soluble solids, it has been suggested that the variation around the mean may be important in determining final wine style.  In the authors’ words, “The higher the proportion of gravely soils in the vineyard and hence the higher proportion of extra-small vines, the riper the fruit will be on a particular date, and the more likely the wine to exhibit riper (i.e. more tropical) flavor and aroma and less unripe (i.e. herbaceous) characteristics.

* Values with the same letter not significantly different (p<0.05)

Author: Bibiana Guerra, Editor: Kay Bogart. This summary series funded by J. Lohr Vineyards & Wines.