notes from the winemaker

I love Twitter. For example, biochar would not be on my radar today if I was not following @RandallGrahm on the platform. I remember reading an article on terra preta some time ago (might have been this article in National Geographic) but it is Randall’s sustained expression of enthusiasm for biochar as a vineyard amendment that has me thinking of it more specifically.

There is plenty of information on biochar available online to the interested reader. Briefly, biochar is a carbonized residue produced by pyrolysis of cellulosic biomass (such as crop residues–both field residues, and processing residues such as nut shells, fruit pits, etc.–as well as yard, food and forestry wastes, and animal manures) under conditions of limited oxygen. The products of pyrolysis are bio-oil, syngas and biochar. The first two products are combustible substitutes for petroleum-derived fuels. Biochar residue can be used to improve acidic and agriculturally-depleted soils. The feedstocks used greatly affect the utility and suitability of the derived biochar product for building or rebuilding any particular soil.

What got my attention was Randall’s tweet that application of biochar in the vineyard can increase the assimilable nitrogen in grapes. Quick background: in wine production, yeast depend on nitrogen sources in the juice to ferment cleanly and completely. Nitrogen in juice occurs in two forms: as ammonia ion (NH₄⁺) and as more complex nitrogen-containing organic compounds that the yeast can take up and utilize. The latter pool is generally referred to as “yeast assimilable nitrogen” (YAN) or as “assimilable amino nitrogen” (AAN).

The role of nitrogen in yeast nutrition is pretty well understood; I am not going to review it here. I’ll just say that nitrogen compounds are necessary for complete and clean-smelling ferments. Winemakers understand that some grape varieties and some vineyard locations yield juices that are deficient in both NH₄⁺ and AAN. Syrah is a classic example.

Some winemakers analyze the nitrogen levels present in every juice that comes into the winery, and supplement with di-ammonium phosphate (DAP–a source of NH₄⁺) and perhaps with yeast extracts–putative sources of AAN–to some prescribed minimum level in order to ensure a good fermentation. There is evidence that supplementing AAN not only aids in complete and clean fermentation, but also increases the level of pleasing aromas and textures in the finished wine.

Not surprisingly, such additions are anathema to “natural” winemaking adherents. I take a more practical approach to my process–clean and complete fermentation is a high priority for me and I am going to do what I can to assure it happens. However, another of my guiding principles is to strongly avoid adding anything to juice or wine that doesn’t taste good on its own. DAP tastes awful, and I refuse to use it except in cases of extreme need.

So anything that might increase the natural level of nitrogen compounds in grapes I know to be chronically deficient is of interest. I could fertilize the vineyard heavily, but I don’t for two reasons: 1) I’m simply adverse to using industrially-produced fertilizers, and 2) excess fertilization–especially with nitrogen–produces excessive vine vigor, which usually leads to poor wine quality.

So far in our vineyard operations we have been using small applications of low-N liquid fertilizers, compost teas and compost to keep our soils and vines healthy. Biochar looks like it could be a useful addition to our soil support regimen. Thanks, Randall–now I’ve got more research to do. After vintage.