Studies on nutrient and canopy factors that affect the potassium content, pH and colour of wine
University of Stellenbosch. Department of Soil Science
Engelbrecht, G A
Agenbach, G A
Normally pH is considered to be the most useful index of acidity in wines. Furthermore, it is one of the most important factors related to wine quality. During the last two decades winemakers around the world have encountered high wine pH values under certain conditions.
In the Western Cape similar problems have been reported. High pH values have been linked to soils with a granite parent material. It has also been established that high potassium (K) levels in the soil may lead to luxury K uptake by plants. Since a high level of K in must and wine could lead to higher wine pH values, high pH wines may not be unexpected on soils derived from granite.
High pH values are known to cause a number of problems in wine: the activity of undesirable bacteria increases; the colour intensity of red wine decreases; the free SO2 content of wine decreases; red wine?s capacity to mature is restricted; and there is a greater susceptibility to oxidation and biological decay. The colour stability as well as the taste may also be influenced by pH, which also plays a role in the potassium bitartrate precipitation process.
Efforts are therefore being made around the world to lower the pH of wines. A viticultural approach to the problem might be deemed more appropriate, in which the solution is looked for in the vineyard itself.
One of the ways explored by researchers to restrict K uptake by vines is the use of different fertilisers or to restrict K fertilisation. An increase in the concentration of one ion species in the soil solution may lead to a decrease in the concentration of another ion species in the plant. This phenomenon is known as a cation antagonism which results from the fact that, while the cation composition in the plant may change, the total amount of cations remains essentially the same. In the case of vines the divalent cations, Ca and Mg, might be favoured at the expense of K if suitable conditions could be created in the soil. In restricting the uptake of K it might be possible to restrict K translocation to the berries.
Another way of lowering wine pH is canopy management. Most research shows that a high canopy density and consequently overshadowing has a detrimental effect on wine quality. It has also been suggested that high canopy density leads to higher K and pH levels in wine and must.
To study the effect of fertilisers and canopy management on the K and pH values of wine and must, field trials were established in the Paardeberg area. Four fertiliser treatments consisting of a control, 5 t/ha gypsum ( CaSO4.2H2O) and equivalent quantities of Ca(OH)2 and MgSO4.7H2O have been studied in combination with three treatments designed to achieve different densities of canopy. A parallel trial with vines in pots was also established with similar fertiliser and canopy treatments. The trials are still in progress and results are expected to be published during 2002.
Agenbach, G, Saayman, D and Frey, M V. Potassium supply and canopy management effects on quality of red wine from granite-derived soils near Paardeberg. Golden Jubilee Congress Soil Science Society of South Africa, Stellenbosch.