Scientists mixed just 3% volcanic ash into forest soil in a UC San Diego experiment; coleus plants grew roughly three times as much as plants in untreated soil

Believe it or not, the same volcanic material that can ground entire fleets of aircraft and bury towns in ash could actually be helping your garden flourish. In a new greenhouse study, researchers from UC San Diego found that adding a small amount of volcanic ash to forest soil nearly tripled plant growth. This was due to the ash altering the micro-ecosystem beneath the soil.

According to a Scripps Institution of Oceanography article by Robert Monroe, adding just 3% of basaltic ash to the soil helped triple the biomass in coleus plants while simultaneously changing the bacteria and fungi surrounding the plants’ roots. This is great news for a generation who have been desperately turning their homes into jungles, creating compost on their balconies and trying to breathe life into tired urban soil.

The soil wasn’t just fed; it was reprogrammed


While your immediate instincts may be to think that volcanic ash acts like Miracle-Gro by providing nutrients to the soil, a 2025 study titled “Tripled plant productivity and soil microbiome restructuring in a greenhouse volcanic ash fertilization experiment,” published in the journal Ecosphere, shows an interesting phenomenon. Rather than volcanic ash aiding plants with increased levels of iron and magnesium, the study found that it increased the numbers of helpful plant growth-promoting bacteria and fungi while decreasing the numbers of parasitic nematodes.

The addition of 3% volcanic ash acted as a sort of tipping point: once that threshold is crossed, the whole ecosystem changes and becomes far more fertile. As Hubert Staudigel, one of the study’s lead authors, explained, this runs counter to the popular assumption that volcanic soil is fertile simply because lava rock is mineral-rich, the real driver is the microbiome shift, not direct mineral feeding.

Other studies back this up

This links to other studies which show how quickly life can begin to thrive after a volcanic event. In a 2020 study titled “Metagenomic analysis reveals rapid development of soil biota on fresh volcanic ash,” published in the journal Scientific Reports, researchers looked at the growth of functional microbial communities in ash from Sakurajima volcano in Japan. They found that ash mesocosms developed diverse communities of microbes similar to those in forest soils within just a couple of years, with rapid growth in genetic diversity over that same period.

A separate 2025 study titled “Pedogenesis of volcanic ash soils determines the soil microbial pool and its contribution to nutrient availability,” published in the European Journal of Soil Biology, looked at how the geochemical weathering of volcanic ash affects the composition of microbes in the soil and how those microbial shifts, in turn, affect nutrient cycling. The researchers found that the maturity of volcanic soils, rather than their age, determines how well the microbial community can provide available nutrients to plants. It suggests that volcanic minerals act less like a direct feed and more like a reset button.

Why should this matter

While you may want to put this into the 'cool science, not my problem’ category, most Americans do not have gardens near an active caldera. However, tired, over-farmed soil is a real problem in the US. Volcanic ash, as a natural and cheap soil amendment, has many benefits compared to using synthetic fertilisers (which are very energy-intensive to create and can lead to run-off pollution). The Scripps team mentions that volcanic ash can be used as a cheap solution for regenerative agriculture on poor soils. It could also be a small help in carbon capture due to healthier soils with lots of microbial activity that will usually store more carbon.

This was just a four-month study looking at one ornamental plant species and one type of ash. No one is saying you should go out this weekend and dust volcanic ash over your tomatoes. Real farmland, different climates, and crops will all need testing, and basaltic ash isn’t something you can simply buy in bulk online. Still, it’s a useful reminder that nature had a solution to tired soil long before humans tried to engineer one. The 2025 study adds to a growing body of research on soil health and sustainable agriculture, even if the findings are still early-stage. All it needed was a little bit of ash and fire.