Assessing Wine Quality Relative to Soil Types: A Study of Terroir.
Thomas Rice, Joseph Montecalvo, and Andrew Schaffner. 1 Grand Ave, California Polytechnic State University, California Polytechnic State U. San Luis Obispo, Earth & Soil Sciences Department, San Luis Obispo, CA 93407
The relationships between soil type and wine quality, as measured by the inorganic and organic constituents in wines (post-fermentation, pre-barrel stage) and grape must obtained from Cabernet Sauvignon grapes grown on their own roots, were investigated. The two contrasting soil types are Calodo clay loam, a shallow Calcic Haploxeroll, and Zaca clay, a deep Vertic Haploxeroll. The vineyard site is Carmody McKnight Estate Wines, located about seven miles west of Paso Robles, CA. Grape must and the pre-barrel wine samples were obtained over three harvest years, 2004, 2005 and 2006. Statistical analyses demonstrate significantly higher levels of calcium (Ca), potassium (K), pH, and reducing sugars in the grape must and wine samples grown on the Calodo clay loam. Ammonia levels were higher in the wine samples grown on the Zaca clay. In 2004, color intensity measurements of the must and pre-barrel wine samples were higher for the Zaca clay samples. These data indicate that the shallow Calodo soils with lower water supplying capacities resulted in higher reducing sugar levels in the grapes. This relates to higher alcohol levels in the wines. There appears to be a relationship between the inorganic Ca and K levels in the Calodo clay loam and these same levels in the grape must and wine. The cause is not entirely clear, since there is no difference in these same nutrient levels between the two soil types. A higher K value in the grapes corresponds with higher pH levels in the grapes grown on the Calodo soils. The positive relationship between K levels and grape pH has been demonstrated by past researchers. Future research on soil type and grape quality relationships should incorporate cane and fruit yield data along with atmospheric and soil climate data.