The truth about volcanic soil and olive tree water stress

There have been some claims made that growing olives near a volcano can increase the nutrition available to the trees via the volcanic soil. It has also been claimed that trees growing near a volcano can offer unique challenges, increasing the amount of polyphenols that end up in the trees’ olives.

Nutrition of soil near volcanoes

While volcanic soils do tend to aid crop growth by counteracting soil erosion and adding a fresh source of nutrition to the surrounding soil, there is an assumption here. The assumption is that the volcano is periodically ejecting material—without this ejection there is no way for volcanic soil to form (Source). Take Tuscany for example, the last volcanic activity here was 300,000 years ago, with Mount Amiata officially declared extinct.

Does water stressing an olive tree increase polyphenols?

Depriving an olive tree of water in a controlled way can induce stress. This stress has been shown to increase polyphenol concentration, but it seems to only be in the plant’s leaves, which does not help in the context of olive oil (Source). Water stress actually seemed to have the opposite effect on polyphenols within the olives themselves, causing a decrease in the total amount of polyphenols contained in the resulting oil (Source).

It does seem to be true that intentionally adding in volcanic material by hand does seem to reduce some negative effects caused by water stress (Source). But this study only looked at tree growth factors like number of branches and density of leaves, and did not consider polyphenols at all.

Conclusion

Soil around active volcanoes can be less eroded and have more nutrient availability, which could help to grow plants and crops, but there is no evidence of these effects in areas like Tuscany where volcanic activity is dormant or extinct.

Reducing water availability to an olive tree can stress it and cause an increase in polyphenol density in its leaves, but this is not useful for olive oil production. Water stress seems to decrease the polyphenol count in the fruit itself, leading to an oil with lower polyphenol count compared to a tree with adequate water availability.