Wines produced from biodynamically grown grapes have received increasing attention. Similar to organic agriculture, biodynamics eliminates synthetic chemical fertilizers and pesticides. The primary difference between the two farming systems is that biodynamics uses a series of soil and plant amendments, called preparations, said to stimulate the soil and enhance plant health and quality of produce. Whether these preparations actually augment soil or winegrape quality is unclear and controversial. A long-term, replicated, 4.9-ha study was initiated in 1996 on a commercial Merlot vineyard near Ukiah, California, to investigate the effects of these biodynamic preparations on soil and winegrape quality. The study consisted of two treatments, biodynamic and organic (the control), each replicated four times in a randomized, complete block design. All management practices were the same in all plots, except for the addition of the preparations to the biodynamic treatment. No differences were found in soil quality in the first six years. Nutrient analyses of leaf tissue, clusters per vine, yield per vine, cluster weight, and berry weight showed no differences. Although average pruning weights for both treatments in 2001 to 2003 fell within the optimal range of 0.3 to 0.6 kg/m for producing high-quality winegrapes, ratios of yield to pruning weight were significantly different (p < 0.05) and indicated that the biodynamic treatment had ideal vine balance for producing high-quality winegrapes but that the control vines were slightly overcropped. Biodynamically treated winegrapes had significantly higher (p < 0.05) Brix and notably higher (p < 0.1) total phenols and total anthocyanins in 2003. Biodynamic preparations may affect winegrape canopy and chemistry but were not shown to affect the soil parameters or tissue nutrients measured in this study. We recommend that you consult the full text of this article, which was published in the American Journal of Enology and Viticulture 56: 367-376.