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Dear Dr. Vinny,
I know that soil is a key component of terroir, and that old vines are highly regarded, so how is it that these soils can keep producing excellent wines for 100 years or more? Don't the soil's nutrients get depleted? You can’t plant tomatoes in the same dirt year after year and have great tomatoes …
—Diana, Glen Mills, Pa.
Great question. You’re right that terroir refers to a medley of variables, including soil, climate, topography and biodiversity, all of the things that create a vineyard's “sense of place.”
The theory behind crop rotation is to prevent nutrient depletion and control pests. Your tomato plants might attract certain insects or diseases, as well as drain a specific set of nutrients from the soil. You're right that tomato plants won't perform as well in the same plot of dirt year after year.
Grapevines are a bit different from tomato plants, however. For starters, your tomato plants aren’t going to keep producing year after year. But grapevines will, and the older they get, the deeper their root systems reach. This gives the vines a chance to access new nutrients, unlike most vegetables that only take nutrients from the topsoil because their roots are shallow. Moreover, once grapevines are planted, it takes at least three or four years before the grapes are mature enough to be turned into wine. Crop rotation is really only a relevant practice for annuals like vegetables—no one rotates their apple orchards.
However, some grapegrowers do rotate the cover crops in their vineyards. Cover crops help with soil erosion, water retention, pest control and can increase organic matter and nutrients in the soil. You may have seen the yellow mustard that carpets the vineyard floors of Napa Valley in the winter, but there are other types of cover crops, like alfalfa, clover, grains or radishes. Some rotate their cover crops; others use a mixture of different types simultaneously for diversity.
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