How do limey, high pH soils produce more acidic, low pH wines. What's the mechanism?
Answer From Expert Roger Bohmrich MW
This is a terrific question which takes us into the murky area of the link between soil and wine characteristics. I am not a geologist, botanist or chemist, but I do try to rely on the research and observations of true scientists. Unfortunately, the conversation about wine often seems to be filled with supposed "truths" which have been passed from one person to the next, gaining in validity along the way. With that said, let's turn to specifics.
As is often the case, drawing a straight line from a single factor to a precise single result is frequently not possible. It is often said that the limestone (calcium carbonate) soils which are found in many French wine regions assist in boosting acid levels in the wines. This idea has been embraced by winemakers in other parts of the world. The calcium carbonate renders the soil basic or alkaline and scarce in potassium. Consequently, it is claimed, the grapes retain a higher level of acid (and lower wine pH). Calcium may also enhance the coloring of red grapes (or the anthocyanins). To point out that this subject is a mine field, consider the words of a geologist, Alex Maltman (Vineyards, Rocks, and Soils - an absolute must-have reference). He writes that, for Riesling in Germany, "limestone soils generally produce a wine of higher pH and lower perceived acidity than does sandstone or slate, which largely contradicts the thinking in France." Other elements which might affect ripening and thus a wine's acid content could be soil temperature and surface color. And many other influences. It's not so easy to narrow this down.
A 2015 Oregon study (Retallack & Burns) found an inverse relationship between soil and wine pH with Pinot Noir. Complicating factors: depth and age of soil also affect pH. The authors did conclude that "one can taste some aspects of soil in wine, especially acidity." However, winemaking influences often obscure the relationship. Wine can be fascinating and frustrating!
