Study compares biodynamic and organic soils
With the lack of published research on biodynamic viticulture, Dr. John Reganold was eager to participate in a side-by-side comparison of organic and biodynamic viticulture in northern California at the McNab Ranch, owned by Jimmy Fetzer.
The 150-acre vineyard was certified organic in 1996 when the study began, and certified by the Demeter Association as biodynamic in 1997. Reganold specifically wanted to measure the effects of the biodynamic preparations on soil quality and wine grape quality.
Four randomized and replicated plots of biodynamic and organic vines were delineated within a 12-acre block of Merlot grapes. The soils were the same, and the block was managed the same. The only difference between the plots was the addition of the biodynamic preparations.
Roads going into the rugged terrain vineyard were paved to minimize dust, a portable chicken house was built to transport chickens into the vineyard during the day, from where they were locked up at night for protection from coyotes. An erosion area was converted to an annual stream, planted with riparian vegetation. Wildflowers and other native vegetation extended from vineyard rows into the surrounding area to serve as corridors, attracting beneficial insects into the vineyard.
Soil samples were collected for six years and analyzed for active microbial biomass, soil respiration, soil texture, organic matter, earthworm populations, and more.
“Perhaps six years is insufficient time for treatment differences to manifest in soil,” he said. “Or maybe there just weren’t any differences.”
Only the top six inches were tested in the study. Also, the preparations were added at a minimum, with some preparations added only once during the six years, while others were added once a year.
“Some say that we might have seen more differences if we added more preparations.”
Leaf tissue nutrient analyses also showed no differences after seven years. Because grapes were thinned to a targeted 4.5 tons per acre throughout the block, there were no differences in yields.
The only difference in vine performance that was observed in the study was pruning weights, he noted. Yield-to-pruning-weight ratios indicated that the biodynamic vines had an ideal balance of a ratio near 5.0 for three years, while the organic yield-to-pruning-weight ratio averaged 6.4, indicating the vines were slightly overcropped.
Analysis showed that the biodynamic grapes had significantly higher tannins in 2002, and significantly higher tannins, total phenols, total anthocyanins, and Brix in 2003.
“But we don’t know if there are biological differences in the grapes,” he said. “You would have to make the wines exactly the same, side by side, in the same barrels.” Such controlled winemaking between the two groups of grapes was not done.