When particle size turns sand into a sponge
Three pots, the same sandy soil but different rock powder treatments led to strikingly different plant growth. What’s going on here?
In our Carbon Drawdown greenhouse experiment on enhanced rock weathering (ERW), we are comparing several feedstock materials on the same soil, this example shows pots with a soil from a field called Danskenfeld. The photos, taken about three months after the experiment started, show a striking difference in plant growth and coverage.
Left: vegetation is dense and reaches the pot’s edges
Right: Growth is clustered around the irrigation ring, with sparse cover and even un-germinated seeds elsewhere.
What’s driving the difference?
The only difference between these pots is the type of rock dust mixed into the soil, everything else - soil type, irrigation, temperature, and light - is identical.
Left (lush growth): Glacial Rock Flour (GRF) from Greenland. This material is incredibly fine, with a d90 of only 4 microns (!). (The D90 means 90% of its particles are smaller than 4 µm).
Center (sparse growth): Peridotite (d90 = 223 microns).
Right (sparse growth): Limestone (d90 = 361 microns).
The standout of this group with GRF isn’t only chemistry - it’s the physical texture. GRF from Rock Flour Company is rich in fine silt (German: “Feinschluff”), which refers to mineral particles that are typically between 2-6 micrometers in size. This material milled naturally by glaciers in Greenland over thousands of years until it was dredged from the sediments of glacial river deltas. The energy necessary to mill our other feedstocks to this microscopic size would be astronomical.
This superfine grain size doesn’t only make this rock dust especially suitable for enhanced weathering but it also has other advantages as we see here in a sandy soil: Sandy soil has large particles with large pore spaces (macropores) between them. Water drains through these large pores very quickly due to gravity, making soils dry out fast.
When you add fine silt to sand, something remarkable happens: The tiny silt particles fill the large gaps between the bigger sand grains. This doesn’t clog the soil (as often seen in clayey materials); instead, it creates a new network of smaller, medium-sized pores called mesopores.
These mesopores are the key. They are small enough to hold water against gravity through a process called capillary action - the same "paper-towel effect" that draws water upwards. This process creates suction that pulls water laterally away from the irrigation ring, distributing it evenly throughout the pot. At the same time, the network maintains good hydraulic conductivity, allowing water to keep flowing to where it's needed.
The result is even moisture throughout the pot, leading to uniform seed germination and healthy plant establishment. The coarser rock dusts like Peridotite and Limestone lack this high concentration of fine silt, so the water-spreading effect is much weaker.
Why it matters
This shows that for ERW to be successful on sandy soils, particle-size distribution can be as important as mineral composition. Adding a material rich in silt can:
Improve water availability for plants, leading to a better ground cover and healthier growth.
Increase root-to-mineral contact, which is crucial for accelerating the the chemical weathering that captures CO₂
Help make carbon removal more reliable and effective in real-world agricultural settings.
So beyond the direct benefit of carbon removal, adding rock dusts to soils can have significant co-benefits, like improving the water supply for crops and boosting soil and plant health.
