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Reuse of Ash

Key re-use options for volcanic ash include:
  • Incorporating (by tilling or ploughing) ash into pasture, garden or cropping land. This is viable in most cases. The depth of the ash deposit is a key determinant of viability, along with the availability of resources such as tilling machinery.
  • Mixing ash as a component of mortar or concrete. This is viable in some cases, and depends on the physical chemical properties of the ash.
  • Use as a landscaping fill is viable in most cases.

Incorporating ash into soils

Incorporation of ash into soil has the potential to improve soil physical and chemical properties. A key factor in rehabilitating ash-covered pasture is ashfall depth. For falls of <5 cm depth, generally no cultivation will be required and rainfall will rework the ash into the soil. For depths of 5-15 cm, tilling ash into soils is generally viable and effective. For depths of >15 cm, a case by case approach will be required to evaluate the viability of rehabilitating the land. These thresholds may vary depending on climate. Benefits for soils:
  • An ashfall deposit can provide a mulching effect and slow down evaporative losses of water.
  • Incorporating coarse (sand-sized or larger) ash can improve soil aeration and drainage. However, too large a proportion of coarse ash can lead to root zones drying out. Proportions depend on the organic matter of the soil.
  • Some ash compositions, particularly basaltic and andesitic ash, can contribute useful quantities of nutrient elements such as sulfur, selenium and magnesium to soils.
  • Ash has successfully been used as a beneficial additive in seed-raising mix.
Cautions for soils:
  • Some ash contributing to soil in a high proportion can lead to soil imbalances.
  • The acidic surface coating on ash can lead to soil acidification, particularly for soils lacking buffering capacity.
  • Ash with excessively high concentrations of sodium (such as ash from submarine explosive eruptions) has the potential to change the nutrient balance of soil by displacing other base cations (calcium, magnesium, potassium). This can lead to short to medium term base cation deficiencies and long-term poor soil structure. Calcium-based soil amendments may be required to return base saturation to optimal levels.
See also the Agriculture section for detailed impacts that expand on these cautions when adding ash to soil.

Ash as an admix to mortar and concrete

  • Ash can be used as an additive to mortar or concrete depending on its properties.
  • Important properties include grainsize, friability (mechanical strength of ash grains), and ash surface coatings.
  • In all cases, technical expertise should be sought to determine the suitability of ash deposits for intended purpose.
  • Some examples include use of ash in hydraulic mortars (e.g. Santorini ash and pumice clasts are used in harbours and the Suez Canal) and local production of concrete blocks (e.g. ash from the 2017-2018 Ambae eruptions).

Ash as landscaping fill or infill

  • Ash deposits have successfully been used as landscaping fill in many locations globally.
  • Infilling of quarries has also been successfully achieved.
  • Standard local requirements for stabilization, compaction and testing of fill for a given purpose should be sought and followed.
  • Fine ash may be prone to liquefaction when subjected to earthquake shaking. Expert advice should be sought when fill is being proposed as a building or infrastructure foundation.