The Gemtree Comparative Trial – 4 years on

In 2008, University of Adelaide and GWRDC PhD candidate Luke Johnston established a trial at Gemtree vineyards, McLaren Vale to investigate the three-year conversion period from conventional to organic viticultural practices. The replicated trial compares organic, biodynamic, low and high input conventional management systems for their impact on soil properties, vine performance and grape and wine quality. To better understand the long-term impacts of these management systems beyond the conversion period, GWRDC is funding the project for a further three years.

In terms of soil properties, organic carbon levels have not changed between management systems however there are higher levels of microbial respiration coming from soils in the organic and biodynamic systems compared to conventional systems.  It is likely this is because plants have been allowed to grow under the vine throughout the year, so associated carbon compounds released from the root systems, and labile carbon made available through plant senescence, has enhanced soil microbial activity.

The yields in 2012 were reduced significantly in the organic and biodynamic treatments compared to the low- and high-input management systems.  The most likely cause for this was the under-vine plant growth (weeds) that persisted through the growing season, and was only restricted by mowing.  Vegetative growth, measured by cane weights and number, was also significantly less on the organic and biodynamic treatments.

There were no significant differences found in nutrient content, TA (acidity), pH, Baume and anthocyanin levels between management systems. However, total phenolic levels in the grape berries were significantly higher in the biodynamic system.  An important question that many growers have is – what is the impact of these management systems on wine quality? To answer this question wines were made from the trial and presented to a group of McLaren Vale winemakers for tasting.  Analysis of the descriptors used showed that wines produced from organic and biodynamic vines were described as being more rich, textural, complex and vibrant than conventional treatments.

The continuation of the trial for a further three years is critical to understanding the impact on soil quality, vine growth and wine quality of the different management systems.  Measurable differences in soil properties are now becoming apparent, vine growth is being impacted and the wine quality appears to reflect these altered management strategies.  The following two years are therefore likely to provide some exciting outcomes from this large-scale trial.

For those requiring more up-to-date information about this trial and from Australian practitioners, Luke Johnston will convene an organic viticulture workshop at the Australian Wine Industry Technical Conference to be held in Sydney from 13-18 July 2013.

Acknowledgements

The support of the GWRDC, University of Adelaide, Melissa and Mike Brown and Troy Elliker, Gemtree Vineyards, the wine tasters, Peats Soil and Garden Supplies and the McLaren Vale Grape Wine and Tourism Association are gratefully acknowledged.

For more information contact Elise Heyes, GWRDC Program Manager, elise@gwrdc.com.au

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