Chemistry of Vineyard Soils

THE ROLE OF SOIL INORGANIC CHEMISTRY IN DETERMINING WINE GRAPE COMPOSITION

Dr Doug Mackenzie is a Visiting Fellow in the Department of Earth and Marine Sciences at The Australian National University where he has been carrying out research on the cation chemistry of vineyard soils and how this relates to wine grape composition.

The soils and riesling grapes of the Heggies and Pewsey Vale vineyards, in the northern Adelaide Hills region, South Australia were the subject of the first phase of the study (see Mackenzie and Christie, 2005). The results show that riesling grape properties such as sugar content (Baumé) and titratable acidity (TA) of the juice are correlated with several plant-available trace elements in the soil, notably Ca, Sr, Ba, Pb and Si. Increasing soil clay content also influences grape juice TA (positively), pH (negatively), and sugar content. Elements such as Si, Na, K, Mn, B, Rb, Cr, Cu, Sn, As, Pb and V are significantly more concentrated in the grape juices than in the 'host' soils. However, concentrations of several cations, including Cs, Ni, Zn and, most notably Ca, Sr, Ba, are lower in the grapes than in the host soils. Ca - and probably also Sr and Ba - is accumulated in grapevine leaves as highly insoluble calcium oxalate, thus limiting the amount of Ca transferred to the fruit. Calcium has functions in plant cell walls and in cell membrane permeability, and is an enzyme co-factor. Therefore, it is possible that Ca (along with Sr, Ba and Pb) influences cell reactions (such as formation of sugars) by affecting the cell membrane ion-exchange process and enzyme reactions.

Stage 2 of the project is on cabernet sauvignon vineyards of the Clare area and of Coonawarra, South Australia. In this part of the study, the grape-juice properties total polyphenols (tannins) and total anthocyanins (colour-producing compounds) are being assessed, along with sugars (measured in units of Brix), TA and pH, from a much larger number of sample sites (vines and adjacent soil-sampling sites) than were involved in Stage 1. Preliminary interpretation of results indicates that grape juice from vineyards with low-Ca soils (Clare Valley):

Brix increases as plant-available Mg and Ca of the soil increase;
Brix increases as total Mg (as MgO) of the soil increases;
TA increases as plant-available plant-available K and Mg of the soil increase;
pH increases as total Mg (MgO) and possibly K (K₂O) of the soil increase;
total polyphenol content increases as clay content of the soil increases; and
total anthocyanin content increases as clay content of the soil increases.

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