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What is ‘Rock Dust’ (Enhanced Weathering) and how it can Affect Soil Health and Carbon Sequestration

19 December 2024

What is 'rock dust'?

Rock dust, or enhanced weathering (EW) mainly comprises igneous (volcanic) material rich in magnesium and calcium (essential for enhanced weathering), such as basalt, olivine and dunnite which are crushed to a powder for application to soils.

Rock dust on soils - the concept

Figure 1 outlines the concept of EW which has lead to ‘rock dust’ getting more attention in terms of its potential for enhancing carbon storage when applied to soil. Rock is sourced (usually from quarrying), crushed and applied to land. A rock begins to weather (break down), it takes atmospheric CO2 (and water) to form bicarbonates. These are eventually leeched from soils, transported in watercourses and deposited in oceans. This therefore creates a pathway for long-term carbon storage.

Rock dust process

What evidence is there relating to rock dust and our soils?

EW is a novel area of science and so compared with other land management practices has significantly less evidence which quantifies the true potential of this method. Most studies to date have been laboratory-based experiments or assessing scenarios with different computational modelling approaches. There are some field studies looking at rock dust applications but it can be argued that overall there is insufficient evidence to provide robust guidance at this stage. As rock dust is considered a by-product and not a waste, there are no regulations regarding its application to soil at present.

From the evidence we have so far, what are the pros and cons of applying rock dust to soils?

Potential positive impacts of rock dust applications

  • The weathering process is genuine and can generate carbonates in soil systems
  • There is a liming effect in most cases which may contribute positively to soil health.
  • Increased solubility of trace nutrients and P and K are often observed which may be beneficial for soil fertility

Potential negative impacts of rock dust application

  • Lack of field data and long-term experiments (extrapolation is uncertain)
  • Weathering is dependant on temperature, rainfall and soil type and so large variations exist across Scotland (with limited local studies)
  • The stability and persistent of carbon is unclear. Carbon sequestration potential depends on a range of soil-specific factors (range in pH, texture, moisture and nutrient composition), as does nutrient release and other co-benefits.
  • Soil respiration is often not considered, which may offset any carbon gains (https://www.sciencedirect.com/science/article/pii/S0048969724064532 )
  • Long term carbon storage through soil-to-sea transport is only conceptual. There is no direct evidence for this with difficulties in quantifying the true precipitation of carbonate minerals and time it would take.
  • Experimental studies show high application rates (50-100 t/ha) but it is unclear whether applications should be repeated and if so how frequently.
  • Depending on rock size/granularity there are practical implications in terms of potentially blocking/damaging farm machinery during applications
  • Positive effects on fertility are likely due to a liming effect from the weathering of rock dust minerals. Once the soils balances out, what next?
  • Potential for metal accumulation depending on the sources of rock dusts used. Ultramafic rocks that weather quickly often have high trace elemental concentrations (Cr, Ni, Cu, V, Co etc.)
  • Knock on effects to wider environments. Once applied there is little control over its fate, particularly where soil leachates may go and potentially sensitive areas (E.g. peatlands) that may be negatively impacted. There are also considerations to air quality when aping the ‘dust’
  • May be uncertainty around who owns the carbon credits within  schemes. E.g. How/whether rock dust is represented in the standards, what will the price/rates be. What will the duration of contracts be considering this is a strategy for long-term

Sarah Buckingham, SAC Consulting

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