Wine grape quality as affected by water and N supply in cool climate vineyards (Brown Brothers Miliwa)

A field experiment to study the causes of excess vigour in grapevines was carried out over three years using nine year old Sauvignon Blanc vines at Brown Brothers Whitlands vineyard in the Upper King Valley, Victoria. The vineyard soil was a Red Ferrosol up to 1.5 m deep that had a very high potential to mineralize nitrogen under favourable conditions (18 kg N ha^-1 day^-1 in the top 10 cm). High mineralization rates were also observed in similar ‘cool climate’ soils at T'Gallant on the Mornington Peninsula and Hoddles Creek in the Yarra Valley. Over all sites 78% of the variation in mineralization potential could be accounted for by variation in total soil N, which ranged from 0.3 to 0.7% N. Depth-placement experiments with ¹⁵N-labelled (NH₄)₂SO₄ and a nitrification inhibitor showed that 85% of the mineral N taken up by vines came from the 0-15 cm soil layer, so control of the soil water content in this layer offered the dual opportunity of decreasing water and N uptake during critical periods, such as between fruit set and veraison (when grapes start to soften before ripening).

During the three years of the experiment, vine vigour and grape yield were not consistently decreased by withholding irrigation. Although mineral N, mainly as nitrate, remained plentiful (270-290 kg N per ha) during the growing season at Whitlands, its uptake by the vines could be reduced up to fourfold when irrigation was withheld in a dry summer, such as 2002-03. However, the effect of withholding irrigation was not as marked during the wetter seasons of 2001-02 and 2003-04. Lower N uptake in the non-irrigated vines in 2002-03 was associated with a lower yield and a berry juice pH of 3 compared with 2.95 for irrigated vines. The titratable acidity and Baume (a measure of the sugar concentration of the grape juice) of the non-irrigated vines was 9.8 g/L and 12.8% respectively, compared with 12.4 g/L and 12.3% for the irrigated vines. These were desirable changes in grape juice quality for the non-irrigated vines. A vigorous inter-row cover crop of perennial ryegrass decreased both water and N uptake relative to a grass mulch. White clover in the inter-row had no significant effect on current N uptake, but may have contributed to the naturally high soil N contents in the original pasture from which the vineyard was created.

Drainage through the profile occurred mainly in winter and spring and ranged from 133-148 mm in a dry year to 302-316 mm in a wet year. Because of the large concentration of nitrate in the profile, this drainage could result in substantial leaching of nitrate.

Funded by: Brown Brothers Milawa Pty Ltd, UM

Research Team: Mrs Lilanga. Balachandra (UM), Emeritus Professor Robert. E. White (UM), Dr Robert Edis (UM), Dr Deli Chen (UM), Mr Mark Walpole (Brown Brothers Milawa)

Contact for further information: Mrs Lilanga Balachandra, lilanga@unimelb.edu.au