Why is it that everywhere I read about high levels of calcium carbonate in soils and the resultant higher pH of the soil, inevitably 'produces high acid wines' shows up? What is the chemical reason for this? I know that nutrient uptake is effected by the soil pH and that soils with a pH too low or too high will inhibit specific nutrient uptake but this still doesn't seem to explain why acid is higher in higher pH soils. Help please.
Hi Samantha, this is a really great question, thanks for posing it. There's not one simple, clear-cut explanation for this, because there are so many factors that affect fruit pH, but here is a little insight into a few mechanisms that may cause increased acidity in limestone-based soils:
Background information on fruit acidity
Before verasion, tartaric and malic acids accumulate in the fruit. After veraison, fruit acidity decreases both though (1) breakdown of malic acid and (2) potassium ions (K+) from the soil and transported into the fruit and exchange places with H+ ions, which are pumped out of the fruit, resulting in deacidification. Acidity is essentially a measure of H+.
So, higher fruit acidity can result from (1) more acid accumulation before veraison, (2) less breakdown of malic acid, or (3) less potassium exchanging with H+, also known as cation exchange.
Effect of rootstock
Most rootstocks are not adapted to limestone soils, so specific rootstocks with Berlandieri parentage must be used. These rootstocks also happen to be inefficient at potassium uptake from the soil, so there is less cation exchange in the fruit, and more acidity is retained.
Effect of limestone soils
Limestone is calcium carbonate, so limestone-based soils have a lot of calcium, which results in a lot of calcium uptake by the vine. Calcium serves MANY purposes within the plant, it’s an important building block for cell walls, and it’s involved with photosynthesis, defense from predators, as well as a number of hormonal responses including signaling for water stress and ripening. High calcium is correlated with an increase in the accumulation of both tartartic and malic acid. The exact mechanism for the increase is not well understood, and it is likely the result of several different cause-effect relationships, but it has been observed empirically.
Effect of basic and clay soil
As Geoff noted, it’s not only limestone that enhances fruit acidity. Basic soils, clay soils, and soils with a lot of organic material (humus) are rich in calcium and magnesium, both of which result in higher acidity in the fruit.
It is indeed counter-intuitive that high pH soil would result in low pH wine, but the key here, is to understand that it is the concentration of cations in the soil and taken up by the vine that is important for fruit acidity. Keep in mind that this explanation only looks at a few factors that affect acidity, and there are probably examples of vineyard systems where these observations do not hold true.
Speaking of cations, I found this short video very helpful:
Love the video. Thanks!