Wandering around a bonsai show a while ago I noticed one of the traders selling basalt as a soil improver. I wanted to understand what exactly it improves, how, and when it could be useful.
What is Basalt?
Basalt is a fine-grained extrusive igneous (volcanic) rock formed from the rapid cooling of low-viscosity lava exposed at the surface of a rocky planet (eg. Earth).ref More than 90% of all volcanic rock on Earth is basalt, and this comprises about 13.5% of the Earth’s current land surface.ref The product available to gardeners is a powder produced as a by-product of quarrying basalt for aggregate used in roads, concrete and railway ballast. Basalt quarries are found worldwide, wherever volcanic rock is present.
There are three types of basalt. Tholeiitic basalt is rich in iron and poor in alkali metals (such as sodium & potassium), and includes most basalts of the ocean floor, large oceanic islands, and continental flood basalts such as the Columbia River Plateau. Alkali basalt is relatively rich in alkali metals and is characteristic of continental rifting and hotspot volcanism. High-alumina basalt has greater than 17% alumina (Al₂O₃) and is intermediate in composition between tholeiitic basalt and alkali basalt.ref
How Does Basalt Act as a Soil Improver?
Around half of the composition of basalt is SiO₂ (Silicon Dioxide, the same compound that makes up quartz and sand, but here chemically bound in silicate minerals). Basalt also contains other elements including aluminium (Al), iron (Fe), magnesium (Mg), potassium (K), titanium (Ti), and calcium (Ca) with the relative amounts of these depending on where the basalt was quarried. As you may remember from Nutrients for Trees, most of act as nutrients for our bonsai so there is the potential for basalt to be a useful additive. Research suggests that the main role basalt can play is as a slow-release multinutrient remineraliser and a mild liming agent (increasing pH).ref
The micronutrient effect is because nutrients and trace elements are released as the basalt powder weathers over time. Which ones are released will depend on the basalt source and composition.
The liming effect has three causes: 1/ when the silicates are dissolved they react with hydrogen ions and remove them from the soil (hydrogen ions determine acidity). 2/ bicarbonate is formed from CO₂ & water reacting with released Ca²⁺/Mg²⁺, which buffers the soil against further acidification and 3/ aluminium in the soil (another source of acidity) is displaced and precipitated out into an insoluble form.
The released Ca²⁺, Mg²⁺ and K⁺ (nutrient ions beneficial for trees) are then held on the substrate’s exchange sites (its cation exchange capacity) rather than leaching away with watering — basalt’s pH effect actually helps with this retention too. This makes these nutrients more available to roots over a period of time instead of washing away.
Which Trees Might Benefit from Basalt Addition?
Although most of the relevant research has been done on crops rather than trees, the implication is that species which would benefit from basalt supplementation are those on acidic, leached, or highly weathered soils where Ca/Mg/K depletion and Al³⁺ toxicity limit growth. It might be considered a good addition if the substrate hasn’t been changed in a long while with the nutrients having been depleted through constant watering. Although best practice here would be to repot versus add anything.
It might also be considered for species which appreciate or tolerate higher pH and require nutrient supplementation. To find out which species these are (for European species), you can look at this spreadsheet – the Reaction column lists the plant’s soil pH preference.
Among species commonly used in bonsai, lime-lovers are a minority. Some species are happy enough to be in moderately alkaline soils since they evolved in higher pH environments such as the Mediterranean basin, Crimea and Caucasus. These include European Ash (Fraxinus excelsior), Field Maple (Acer campestre), Field & English Elms (Ulmus minor & procera), Oriental Hornbeam (Carpinus orientalis), European Spindle (Euonymus europaeus), Box (Buxus sempervirens), Olive (Olea europaea), Mastic (Pistacia lentiscus) and Italian Cypress (Cupressus sempervirens).
For Mediterranean species in particular the benefit isn’t only the pH effect but also that the slow release of silicon and trace minerals matches the mineral-rich, free-draining substrate these species evolved on.
Which Trees Would NOT Benefit?
Basalt supplementation will increase the pH of the bonsai medium, so should not be used for any tree which prefers acidic soil, such as azaleas, camellias or any species originating in Japan where surface soils are generally acidic (pH 4.5 to 5.6).ref Most plant species tend to prefer mild acidity or neutral pH, so adding basalt should only be considered for trees which you know actively prefer or tolerate alkalinity.
In conclusion, basalt is a niche addition rather than a general-purpose supplement. Repotting is the better answer for tired soil. Micronutrients are better supplied via a balanced fertiliser or seaweed extract, as this acts on a much shorter timescale and contains a wider range of nutrients. I personally also add biochar which provides better nutrient ion retention capacity, encourages beneficial microorganisms and assists with water retention.