I could spend hours at my local garden store. I find it infinitely relaxing to look at what plants they have in store and imagine the possibilities, the layouts, the nooks and corners I could create in my garden. It’s not unheard of for me to go up and down the same aisle four times. Just to have one more look.
There is one area, however, that I actively avoid. It’s small and tucked in between the garden perennials and the houseplant section.
Yup, it’s the fertilizer aisle. I don’t have anything against fertilizers, especially since this store is pretty good at stocking a variety of organic options and it’s moving away from synthetic fertilizers.
I am simply of the mindset that we should feed our soil, holistically, rather than just give some energy shots to our favorite plants.
So I was pleasantly surprised when, on one such excursion down the fertilizer aisle, I discovered volcanic rock dust – also known as lava meal.
Now I’m nothing but an over-researcher and an under-buyer (if that’s a thing). So I took a photo of the box to look further into whether applying ground lava rock in my garden was a good idea.
Reader, I was back at the store the following weekend. Because what I found was convincing enough for me to try it. I’ve been applying this soil conditioner on my garden ever since. In fact, I’ve mentioned it as my preferred soil conditioner that I spread around my hostas in the spring.
And if you’d also like to know more about it, allow me to share what I learned with you.
What is volcanic rock dust?
Volcanic rock dust, lava meal, lava flour or lava feed is ground volcanic rock. It’s used as an organic, plant-based soil conditioner in organic agriculture.
But luckily, I’ve noticed an increased popularity and adoption among home gardeners as well. And I say luckily because it’s sold in such large packaging for agricultural use that would make it impractical for home gardening use.
Now that it’s becoming more popular with home gardeners, we’ll be able to find it in stores in more manageable packaging.
It comes in a few shapes and sizes: finely ground dust (aka flour or meal), lava rock pebbles, lava gravel or larger pellets.
Is rock dust good for the soil?
To answer this question, I’ve been reading some studies (which I will link at the bottom of the article) and it turns out that yes, rock dust is a good soil conditioner. Or soil improver, if ‘conditioner’ sounds a bit too shampoo-y.
I say conditioner because in order to call it fertilizer, “officially”, it would need to have an NPK number. Which it doesn’t. Because it’s so much more than that.
The packaging of the lava meal I bought lists the following: Silicon dioxide, Iron, Calcium oxide, Magnesium oxide, Potassium oxide, Phosphoric acid, Boron, Cobalt, Copper and Zinc. Depending on whether they have extra additives, lava meal conditioners will contain most minerals and trace elements that plants need.
Rock dust adds all these elements to the soil, rather than us adding them separately (for example, when we’re aiming to solve iron or zinc deficiencies in plants.)
These minerals and trace elements help improve the structure of the soil as well as help the plants become more resilient to stressful factors. The mineralization of the soil, in turn, will make it more hospitable to beneficial microorganisms.
I don’t know about your garden, but in my area stress comes in so many different forms for plants – either prolonged drought or too much rainfall, or sweltering heat during the day followed by cold nights.
So how do I apply this rock dust?
Here’s how I do it.
Super important to mention upfront: Do not spread this around on a windy day! It is a really fine powder, so it’s very light. The chances of re-enacting a movie scene from a bad 80s comedy where one gets a gust of ashes to the face are not zero. Don’t chance it!
So on a still day, I grab a handful of this and sprinkle it around my garden. The brand that I bought comes with a handy little cardboard envelope to make measuring it and spreading it around easier.
But you can improvise this using any cardboard roll (say, from the kitchen towels). If you use your hand to measure, please put on some gardening gloves before you grab a handful.
Always follow the instructions on the packaging for the right quantity, as it can differ for fruit, vegetables or potted plants.
I usually focus on particular plants that might benefit more from the extra minerals, such as peonies, hostas, roses and other large shrubs. But I also sprinkle around annual heavy feeding veggies, such as tomatoes and cucumbers.
Once I’ve applied the volcanic feed, I then water the area well to help the lava dust settle into the ground.
I also try to incorporate it into the soil by gently scratching it (very surface level digging), then covering it up with more soil or compost. This will prevent it from blowing away if it gets dry and windy in the first days after the application.
The process is the same if you apply it to potted plants. But read the instructions carefully to adjust the quantity.
What plants shouldn’t we add lava meal to?
In the informative segment Think Out Loud on Oregon Public Radio, science writer Ann Leslie Davis explains how the application of volcanic rock helps farmers de-acidify their soil.
Soil turns acidic when all the organic material is removed from fields at harvest time. In addition, farmers apply nitrogen fertilizer to boost plant growth in the short term. This leads to the acidification of the soil in the long term. The soil then becomes so acidic that the plants can no longer absorb nutrients through their roots. Another unwanted side-effect is that beneficial microbes end up dying.
What farmers have to do then is apply this volcanic rock on their land in order to restore the pH balance and bring their soil to a more basic state.
You may see where I’m going with this.
If you’re growing plants that do well in soil that’s more acidic, adding lava meal around them might decrease the acidity of the soil. So don’t spread it around plants such as hydrangeas, rhododendrons, azaleas and blueberries, for example.
On the other hand, if you’ve been adding too much nitrogen-rich fertilizer to your garden and have acidified your soil too much, adding lava meal could be part of the solution. It is much better than adding lime, because lime also interferes with the absorption of other elements.
Rock dust as a compost accelerator.
The sources that I’ve been reading, as well as the packaging that it comes in, list one other use for this crushed volcanic rock.
We can use it as a compost accelerator in case our compost is a bit too slow to “cook.”
This generally happens with cold compost when we don’t get the balance of greens and browns right; or when we add too much material in one go. It’s also a common occurrence when we layer large clumps of wet grass clippings. These that act like a sealant and limit air circulation. The compost then turns into a slimy, smelly mess.
This is when we can use lava meal as a compost accelerator. Sprinkle some in the compost and mix it around. Repeat if necessary.
I should mention that this is a short-term troubleshooting solution rather than a long-term compost ingredient. It would get very expensive very fast to keep balancing our composting process with lava meal.
Where to buy volcanic rock dust
Volcanic rock dust is still a relatively new product for home gardeners so if you can’t find it in the fertilizer section of your local garden center, it’s available on Amazon here from Greenway Biotech in a variety of different sizes.
Do you want to read more about adding volcanic rock to improve soil? Here are a few sources to get you started:
To Reverse a Troubling Trend, Farmers Are Adding Rocks to Their Fields on Modern Farmer
Volcanic ash as multi-nutrient mineral fertilizer: science and early applications on Research Gate. The reference section of this academic paper provides a deep dive into the topic.
How volcanic rocks may help farmers grow crops on Oregon Public Radio
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