Effects on Pastures

Ashfalls greater than 100-150 mm (4-6 in) typically result in the complete burial of pastures and soil. Where soil burial is complete, the soil will become sterile because it is deprived of oxygen; existing pasture species and crops and most soil microorganisms will die. Where ash is as thick as about 50 mm (2 in), plant survival and re-growth will be dependent on several factors, including the chemical nature of the ash, compaction of the ash after the eruption, degree of continuing disturbance, amount and reliability of rainfall, and length of plant stalks at the time of ashfall.

The impact of weather conditions on ash thickness

The timing of rainfall influences the survival of pastoral plants after ash covers an area. Wet ash will consolidate to approximately one-third of the original thickness of dry ash. If it rains soon after an eruption (within 2-3 days) plant survival may be improved because of the compaction. On steep slopes, rain will wash ash into gullies and low-lying basins, leading to increased erosion and deposition in some areas (for example, deposition often occurs at the base of steep hillsides). Wind erosion may also pile ash into 'ash dunes', if the ash is not already consolidated or incorporated into the soil profile.

Duration of ash burial

When ashfalls lead to the complete burial of pastoral plants for 5-7 days, it is likely that all plants will die, as also occurs with heavy silting and flooding. Even if ash is removed within 5 days, plants may still die from burning if the ash is acidic.

Benefits of ash cover

It has been reported that when ashfall is made up of fine grained particles with thicknesses less than 50 mm (2 in) the ash acts like a mulch, increasing pasture yield (Wilson, 2009).