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Ash fall can have serious detrimental effects on agricultural crops and livestock depending mainly on ash thickness, the type and growing condition of a crop, the presence of soluble fluoride on the ash, and timing and intensity of subsequent rainfall. Fluorine poisoning and death can occur in livestock that graze on ash-covered grass if fluoride is present; it may be advisable to sample and analyze ash or ash-coated vegetation to determine whether this potential hazard exists for livestock in areas covered with ash as thin as 1 mm. Shortages in feed due to ash burial and access to quality water after an ash fall can also lead to death of livestock.

Survival of agricultural crops and pasture is often severely limited when ash thickness is greater than 10-15 cm (4-6 in). Predicting the potential crop losses from ash fall, however, is difficult and usually exaggerated because of the great variety of environmental and plant conditions that exist in tropical and temperate areas during and after ash fall of varying thickness.

The abrasiveness of ash can damage farm machinery and equipment, but increased maintenance and a few precautionary actions can signifiantly reduce the cost of keeping the machinery in working condition.



Ash fall can adversely affect crops and livestock in a variety of ways, but it is very difficult to predict exact consequences and associated costs of potential ash damage or mitigation measures. The information in this section identifies a range of known effects of ash fall on acricultural crops and livestock that can serve as a rough guideline. The information below is incomplete, however, and is not applicable to all situations because of the wide range of ash thickness and type and status of crops that can exist in different parts of the world at the time of an an explosive eruption. Furthermore, there is a lack of detailed accounts of the effects of ash fall on individual farms in different regions, including the ways that farmers have attempted to reduce the damaging consequences to their crops and livestock.

Additional information and case studies are needed to improve the usefulness of this section. If you have information and knowledge of case studies that can help the Ash Web Team prepare new material on the effects of ash on agriculture and livestock, please contact the Ash Web Team. Through your support and contributions, this Web site can be significantly improved to help farmers and others deal with future volcanic ash falls.


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Survival of livestock exposed to ash falls || feed availability || water quality || seasonal influences || ash toxicity || other factors || evacuation ||

Feed availability
Pasture land for sheep, New Zealand
Pasture land for sheep near Mount Ruapehu, New Zealand; photo courtesy of GNS, New Zealand.

When ash falls destroy pastures, livestock need to be supplied with all of their feed in order to survive in the short-term. The supply of dry feed must be maintained until the livestock are either evacuated or slaughtered, or pasture is re-established.

Even with very light ash falls that do not destroy existing pastures, animals can be forced off their feed, requiring supplementary feed. For example, if the ash contains a high level of fluorine adsorbed onto the tiny particles, and livestock consume both ash and fluorine, there is a risk of fluorosis.

Livestock in this region were affected by fluorosis when grazing grass contaminated by ash from the 1995-1996 eruptions. More than two thousand animals died.

Water quality

Where there is a significant ash fall, clean water will likely be in short supply. Natural water sources and man-made ponds may be temporarily contaminated by ash, and water-pumping equipment can be damaged by the abrasive ash. Restoring quality water supplies for livestock is typically a high priority if livestock are to stay on land affected by ash fall.

Seasonal influences on survival (metabolic and nutritional demand of livestock)

The 1995-1996 eruption of Mt. Ruapehu in New Zealand (temperate region, 39o S latitude) clearly showed that ash fall in early spring can have a significant impact on sheep, beef, and dairy farms. On dairy farms, milk yields were severely depressed in early lactation (yields were actually depressed during the entire lactation). On sheep and beef farms, lamb and calf survival and thrift were poor as ewes and beef cows reduced or stopped lactating. Wool quality is likely to be severely affected where sheep are close to shearing; the entire wool clip could be rejected.

Ash toxicity

Ash falls may be poisonous to livestock and result in clinical diseases, including hypocalcaemia, fluorosis, forestomach and intestinal damage, and secondary metabolic disorders.

Fluorine aerosols in the eruption column and cloud that become attached to fine ash particles pose a potentially significant threat to livestock. As smaller ash particles have large surface areas relative to their mass, the fine particles can transport significant amounts of soluble fluorine onto pastures. The smallest ash particles travel the greatest distance from a volcano; thus a thin layer of fine ash only 1 mm thick can contain potentially toxic amounts of fluorine. Livestock ingest fluorine directly as ash is consumed along with pasture feed and soil.

Fluorine poisoning has occurred in several Icelandic eruptions, and was recently responsible for the deaths of over 2,000 grazing animals following the 11 October 1995 eruption of Ruapehu, New Zealand (Cronin and others, 2003). An immediate toxic dietary intake of Fluorine is >100 μg g -1 for grazing animals, but a lower concentration may cause sickness. Cattle can tolerate around 40 μg F g -1, and sheep up to 60 μg F g -1. Under normal winter conditions in New Zealand sheep ingest 260-275 g of soil per day, and sheep foraging for feed covered with ash are likely to take in even more (Cronin and others, 2003).

Chronic fluorosis causes death. Before death, however, the poisoning causes lesions in the nose and mouth, and hair to fall out around the mouth. Other symptoms include nutritional and stress related diseases, convulsive seizures, pulmonary odema, and kidney and liver changes. A tooth condition known as “spiking” may also occur, causing outgrowths to develop on molars and making chewing difficult.

When toxic levels of fluorine on pastures are identified, it is recommended that livestock are removed from the affected areas until sufficient rainfall has leached the fluorine from the ash.

The high sulphur concentration adhered to the ash may induce copper and cobalt deficiencies in the long term.

Other factors

Evacuation of livestock

When pastures are subjected to ash falls, evacuation of livestock to areas with good quality feed and water may be prudent. Even after evacuation, long-term inhalation of ash and exposure to fluorine may result in reduced productivity. In some cases, stock may not recover in the long-term, with humane slaughtering being the best option.

Where ash falls affect a large area, evacuation of stock would be extremely difficult due to the logistics of moving large numbers of stock and sourcing feed in areas unaffected by the ash. This may result in large losses of livestock through dehydration and starvation.

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Effects on Pasture || ash thickness || ash composition || secondary impacts ||

Ash thickness

Ash falls greater than 10-15 cm (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 micro-organisms will die. Where ash is as thick as about 5 cm (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 ash fall.

The impact of weather conditions on ash thickness

The survival of pastoral plants is influenced by the timing of rainfall 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 ash falls 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.

Impacts on plants and soil from increasing ash thickness (based on limited historical observations or recent eruptions). The impact on some aspects, including soil composition in particular, will be varied depending on ash composition.

Thin burial (< 5 mm ash)

  • No plant burial or breakage.
  • Ash is mechanically incorporated into the soil within one year.
  • Vegetation canopies recover within weeks.

Moderate burial (5 - 25 mm ash)

  • Buried microphytes may survive and recover.
  • Larger grasses are damaged but not killed.
  • Soil underneath remains viable and is not so deprived of oxygen or water that it ceases to act as a topsoil.
  • Vegetation canopies recover within next growing season.

Thick burial (25 - 150 mm ash)

  • Completely buries and eliminates the microphytes.
  • Small mosses and annual plants will only be present again in the local ecosystem after re-colonization.
  • Generalized breakage and burial of grasses and other non-woody plants;
    some macrophytes of plant cover do not recover from trauma.
  • Large proportion of plant cover eliminated for more than one year.
  • Plants may extend roots from the surface of the ash layer down to the buried soil, thereby helping to mix the ash and the buried A horizon. This is generally accomplished within 4-5 years.
  • Vegetation canopy recovery takes several decades.
  • Mixing of new ash into the old soil by people or animals greatly speeds recovery of plants.

Very thick burial (> 150 mm ash)

  • All non-woody plants are buried.
  • Burial will sterilize soil profile by isolation from oxygen.
  • Soil burial is complete and there is no communication from the buried soil to the new ash surface.
  • Soil formation must begin from this new "time zero."
  • Several hundred (to a few thousand years) may pass before new equilibrium soil is established, but plants can grow within years to decades.

Ash composition

The acidity and nature of the ash (and leachates derived from the ash) varies between volcanoes and eruptions. Ash falls can lead to elevated soil sulphur levels and lowered soil pH. These changes in soil composition can reduce the availability of phosphate and other essential minerals and alter the soil's characteristics to such an extent that arable crops and pasture plants will not survive. Where there is acid rain following an eruption, pastures will be scorched and die.

Ash interaction with soil will have variable effects on pH, soil nutrients, capacity for cation exchange and micro-organism activity dependant upon the ash composition and leachate composition. To date little research has been published discussing these dynamics in detail.

Waikato farm Pastural land in the Waikato Region, near the Taupo Volcanic Zone, New Zealand (Photo from GNS collection)

Secondary impacts

Ash falls also affect insect populations, which are severely limited by ash falls greater than 2.5 cm (1 in). This may have a beneficial effect if pastoral and crop-pest populations are reduced due to the ash. Highly mobile insects, many of which have a dense covering of body hairs which can trap the tiny ash particles, such as honey and pollination bees, are more susceptible to ash than smooth-bodied insects such as beetles.

Fine ash can also serve as an effective mulch, improving water retention of soil.

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Effects on forestry

Young forests are most at risk from ash fall; stands of trees less than 2 years old are likely to be destroyed by ash deposits thicker than 100 mm (Neild and others, 1998). Ashfall alone is not likely to kill mature trees, but the accumulated weight of ash can break large branches in cases of heavy ashfall (>500mm). Defoliation of trees may also occur, especially if there is a coarse component of ash-sized particles or larger tephra and during heavy ash fall.

Impacts on forests are not expected to be significant until ashfall exceeds 100 mm. Branch damage may begin to occur in younger trees at around this level, with an increase in damage occurring as levels of ash increase. Access to forests will also be disrupted as roads may be blocked. Little long term damage is expected to ensue. Depths of around 500 mm of ash or more will cause major damage to forests. Extensive branch breakages will occur, and access to forests will be severely impeded. Access will not be possible at all for logging trucks. The area will be reusable, but the existing forest environment will be substantially altered, the burial of young trees a major part of this. Planting directly into basaltic ash is possible for many species, including Pinus Radiata. Planting directly into more silicic ash such as rhyolite is however more problematic, due to nutrient issues such as nitrogen and calcium deficiency (Neild and others, 1998).

Ashfall on forest Plantation forestry on the slopes of Mt Etna during the 2002 eruption, unaffected by several millimetres of fallen ash; photo courtesy of S. Barnard.

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Effects on arable crops || plant development || weight of ash || Grain and cereal

Ash fall effects at different stages of plant development

Horticultural land Horticultural land in Hawkes Bay, New Zealand, downwind of the Taupo Volcanic Zone. (Image from GNS collection)

Periods when crops are most at risk (from research in the temperate regions of New Zealand): (Neild and others, 1998)

Pea: from emergence until end of flowering.

Squash: during the initial stages of growth and flowering.

Tomatoes: during seed emergence and flowering stages.

Sweetcorn: during the early stages of growth.

Pipfruit has three danger periods:

Stonefruit is also susceptible at the same times as pipfruit, except that the early fruit development period is 4-6 weeks after blossoming, when sensitive fruit skins could be damaged, and show russet or deformation in severe cases.

Kiwifruit is also at risk at, and 6-8 weeks after, blossom. There would also be a problem at harvest time. As kiwi fruit cannot be washed prior to packing, the hairy nature of the fruit would make ash removal very difficult.

Grapes have three main periods when damage could occur:

Vinyards and horticultural land Mixed agricultural land including abundant vineyards in the Gisbourne region of New Zealand, downwind of the Taupo Volcanic Zone. (Image from GNS collection)

Citrus fruit

The 2002 eruption of Mount Etna, Italy, resulted a light ash fall in Catania (3 mm). The light dusing of ash nevertheless adhered to the skin of citrus, which rendered fruit unfit for juice production because it was not economically feasible to separately clean each fruit before processing.

Ash on fruit Ash from Etna volcano on the surface of citrus fruit made it uneconomical to produce juice because it was too expensive to clean the fruit (image courtesy of La Sicilia).

Weight of ash

The weight of ash on leaves affects plant survival, increases harvesting costs, and reduces yield. Lucerne and pea crops, regardless of stage of growth, would either fail or have poor yields from ash falls of 10 mm (0.4 in) or greater. These plants have abundant delicate leaves and stems which are easily damaged by ash, which can reduce the rate of photosynthesis and make the crop susceptible to lodging.

Grain and cereal crops (especially corn)

The timing of the ash fall will affect the chances of survival of grain and cereal crops. For example, when corn is in a vegetative period during the first two months of growth, light ash falls are unlikely to affect the expected yield. Heavy ash falls, however, bury much of the plant and change the soil characteristics sufficiently to result in crop failure. The most critical period for corn yields is between three weeks before tasselling to two weeks after pollination. Even light ash falls during this period could result in barren stalks and crop failure. Damaged stalks are also more susceptible to disease, which may also reduce yields.

Corn requires many heat units for a crop to reach maturity. An eruption could delay crop maturity if sunshine hours were reduced during the eruptive period. ash fall near crop maturity will make harvesting difficult and reduce the quality of grain. Ash collected within and among the spikes will cause some contamination of the harvested grain. A high proportion of ash will be removed in the cleaning procedures already used in flour mills if ash falls are light.

Acid rain

Rainfall interacting with volcanic gas within the ash plume may produced acids which fall as acid rain. Continued degassing at the vent may lead to ongoing acid rain even after ash fall ceases.

Acid rain damage to coffee leaves Acid rain damage to coffee plantation leaves 15 km downwind of Poàs volcano, Costa Rica. Frequent rainfall in the region mixes with volcanic gas to produce acid rain. (Peterson and Tilling, 2000)

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Case Studies || New Zealand || Italy ||

As a result of a less than 5 mm (0.2 in) ash fall on the Rangitaiki Plain (Taupo) during the 1995 Ruapehu eruption, approximately 2,000 ewes and lambs (2.5% of the area's sheep population) were killed by eating ash-affected pastures. Autopsies of the dead animals suggest fluorine poisoning or pregnancy toxaemia as the cause of death. Three Ayrshire dairy cows died at Atiamuri in June 1996. It was reported that they had stopped eating and showed signs of lethargy after swallowing quantities of ash; toxic levels of fluorine were found in the dead animals' blood. The Department of Conservation also reported the death of a number of wild deer in the Kaimanawa Ranges, located downwind from Ruapehu, following the two largest October 1995 eruptions (possibly up to 5% of the sika deer population). Livestock losses from the eruption of Ruapehu in 1995 were greatest in lactating ewes, grazing short pasture.

During the eruption, ash falls of 2 mm on pastoral land elevated soil sulphur levels and lowered soil pH by 0.2 - 0.3 units compared to pre-eruption values. One benefit was the reduction in sulphur requirement in annual fertilizer applications for many farms.

Etna, Italy

The crops grown in this area consist largely of citrus fruits, vegetables and grapes. All of these were ready to be harvested during the course of the 3 month eruption. Local produce was still sold in markets, but a covering of ash had to be washed off. This was not an easy task as the ash adhered to the fruit to such an extent that a simple rinse under a hose would not remove it.

Grapes needed to be washed individually before consumption to avoid ingesting fine ash. This effectively ruined the grape crop economically, even though the fruit itself was not damaged. This was the case for citrus fruit.

In some cases the skin of the fruit was reportedly pitted, however it was the adherence of the ash to the skins that again contributed much to the economic ruin of the fruit. Also, the usual mechanical processing of the fruit was stopped because the abrasive nature ash would have damaged machinery. Furthermore oranges destined for fruit juice production could not be used, as the inclusion of skin/peel in the manufacturing of juice was not possible with the coating of ash that was difficult to economically remove. Half of the orange crop in the province of Catania was destroyed by ashfall. Most of the orange crop received less than 3 mm of ash.

Italy’s federation of farmers also estimated about 80 percent loss of vegetables (unspecified sorts, but mostly leafy crops) over both the immediate area around Etna (the province of Catania) and in the neighbouring province of Siracusa. Seventy-five percent of agricultural jobs were also lost as produce could not be harvested. Total estimated cost to the region was 140 million euros.

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Rehabilitation options for pasture and arable crops

The ability to rehabilitate pastoral land and arable crops is mainly dependent on the thickness of the ash covering. The following table summarizes some of the options available at different thickness' of ash.

Options depending on ash thickness and season
Based on observations from Mount St. Helens (1980), Ruapehu (1995-6) and others. Possibly most applicable to temperate countries.

Thin burial (< 5 mm ash)
  • Impacts of ash negligible.
  • Rehabilitation of the land not necessary.
  • In short-term, ash is washed and consolidates to 1-2 mm.
  • Increased supplementary feed may be required if stock off their feed.
Moderate burial (25 - 30 mm ash)
  • Pastures destroyed from ash burning, rehabilitation is similar to the 25-50 mm scenarios; otherwise, rehabilitation will be similar to that after a severe dry period.
  • Pastures can be re-established either through conventional cultivation or undersowing.
  • Where ash is up to 25 mm deep, incorporation of the ash through ploughing is the most suitable method.
  • Hill country rehabilitation will be slower, as material cannot be incorporated into the soil profile.
  • Rainfall will improve the rate of recovery as the ash is eroded.
  • Oversowing with fertilizer will be necessary, due to the inherent lower fertility of the soils and also where pastures are weakened/destroyed by the ash.
  • Extra supplements are required to maintain stock numbers until pastures recover.
  • Greenfeed crops and high producing annual ryegrasses could be established where the eruption occurred late summer, to provide increased feed in the winter until permanent pastures could be established.
  • It is important to maintain farm operations, especially in terms of providing good quality water and maintaining farm machinery.
  • The costs of re-establishing pastures after an eruption are similar to a severe drought.
Moderate burial (25 - 30 mm ash)

Rehabilitation will be greatly influenced by the time of year of the ash fall and the nature of the ash.

Late winter/early spring in temperate climates

  • Most critical period for dairy, sheep, and beef (pasture covers are low and supplementary feed has largely been used).
  • Management options will be to mob stock up, move them through longer pasture areas of the farm to shake/remove ash off the plants. These areas can then be grazed with some brought supplementary feed such as meal and hay if available.
  • It will be difficult to procure sufficient grazing to de-stock affected farms at this time of the year. However, some de-stocking of the farm may be possible by sending stock to the works or for grazing in other parts of the country.
  • Paddocks that had been intended for cropping or pasture renewal could be cultivated and sown in fast growing annual crops, including annual ryegrasses, feed oats and barley.
  • Soil fertility is likely to decrease in the short term, requiring higher fertilizer inputs but not necessarily for all elements.

Summer/autumn in temperate climates

  • Management options are eased by the ability to de-stock lambs, prime cattle and cull dairy cows to the works. Reserves of hay or silage are at the greatest and greenfeed crops such as corn, choumollier or swedes will be of sufficient maturity to supply a substantial amount of feed.
  • Good quality water is essential, increased water pump maintenance and cleaning of troughs will be required for farms on deep well bores and reticulated systems.
  • Farms taking supplies from streams or dams, provision of good quality water for both human and stock consumption will be more difficult. Outside assistance may be required in the short-term (until streams clear and dam water can be tested clear of toxic chemicals).
  • Rehabilitation of any farm will be dependent on the financial resources of the farmer and the robustness of the farm business.
Thick burial (50-100 mm ash)
  • Rehabilitation will be influenced by the farm contour, availability of suitable machinery, finance and human resources.
  • Ash does not dissolve or percolate into the soil profile, therefore tillage with high inputs of fertilizer is required (providing a medium for establishing ryegrass/white clover pastures).
  • Ash falls of 50 mm will have serious financial implications in the year of the ash fall and also the following season.

Land able to be cultivated

  • Ploughing as deep as 20 cm (8 in) gives best results, as the ash layer is mixed with underlying soil. Incorporation of ash will still result in changes to the soil characteristics, such as greater soil moisture, lower fertility and permeability. Large scale cultivation will be expensive.
  • Costs include re-grassing, fertilizer and high machinery maintenance costs due to the abrasive nature of the ash increasing wear and tear.
  • Rehabilitation of land affected by ash is similar to development of sand country where the initial requirement is establishment of any species tolerant of the conditions to stabilize the ash and build up fertility.
  • Re-establishment of pastoral species, dependent on the nature of the ash. With very acidic ash, liming could be required, along with high fertilizer inputs to create a soil medium, conducive to pastoral growth.
  • Initially, acid tolerant species may need to be planted and species more tolerant of severe conditions. For example, Marram grass, lupins, Yorkshire Fog and Lotus.
  • These species tend to be lower yielding than the existing ryegrass and clover pastures. Once soil fertility and organic matter levels increase, more productive species may be established.

Land not able to be cultivated

  • Rehabilitation will be a slow and costly process.
  • Oversowing of low fertility species with fertilizer inputs may be required.
  • It may be un-economic for land to resume pastoral use; other land use may be appropriate.
  • Rehabilitation will be dependent on the financial resources of the farmer, which may be extremely limited after the financial toll of the eruption.


  • May require de-stocking of the land for at least 6 months.
  • Rehabilitation will require re-stocking, but may not be physically possible where the eruption devastates a large area.
  • Slaughtering of stock may be the only option, which will result in a loss of valuable stock of high genetic merit.
  • Until the ash consolidates, quality water for stock will be scarce.
  • Extra expense will be incurred in maintaining water pumps (affected by the abrasive nature of the ash).
  • The physical removal of the ash from buildings, yards, roadways will be required. Click here for removal methods
Very thick burial (100 - 300 mm ash)
  • Rehabilitation extremely difficult and likely to take generations.
  • Soil sterilized below ash.
  • Ash too deep to be incorporated using conventional cultivation techniques, including ploughing, discing or rotatilling.
  • Restoration dependent on removal of the ash layer or a much longer time frame of re-colonization of the ash layer.
  • Re-colonizing agents need to be adapted to harsh environments and will vary with climate.
  • Possible re-colonizing agents are Lupins and Lotus species (that fix nitrogen) along with Marram grass.
  • Initially, restoring basic facilities such as roads, water supply, power, and effluent systems are required before restoration can occur.
  • Immediate requirement, relocation of the affected residents and provision of adjustment programs.
Extremely thick burial (> 300 mm ash)
  • Land un-farmable for many generations.
  • Rehabilitation at an extreme cost.
  • Medium-term (20 to 40 years), rehabilitation unlikely to be economic.
  • Long-term, alternative land uses need to be explored (i.e. forestry).
  • Immediate requirement, relocation of the affected residents and provision of adjustment programs.

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Blong, R.J., 1984, Volcanic Hazards: A sourcebook on the effects of eruptions: Academic Press Australia, 424 p.

Cook, R.J., Barron, J.C., Papendick, R.I., and Williams, G.J., 1981, Impacts on agriculture of Mount St Helens eruption: Science, v. 211, p. 16-22.

Cronin, S.J., Hedley, M.J., Neall, V.E., and Smith, R.G., 1998, Agronomic impact of ash fallout from the 1995 and 1996 Ruapehu Volcano eruptions, New Zealand: Environmental Geology v. 34, p. 21-30.

Cronin, S. J., Neall, V. E., Lecointre, J. A., Hedley, M. J. and Loganathan, P. (2003) Environmental hazards of fluoride in volcanic ash: a case study from Ruapehu volcano, New Zealand Journal of Volcanology and Geothermal Research, vol.121, p.271-279

Folsom, M.M., 1986, Ash on range and forest lands of eastern Washington: Local erosion and re-deposition, in (eds needed) Mount St Helens: Five years later: Eastern Washington University Press, p. 116-119.

Gregory, N.G., and Neall, V.E., 1996, Volcanic hazards for livestock: Outlook on Agriculture, v. 25 (2), p. 123-129.

Thorarinsson, S., 1979, On the damage caused by Volcanic eruptions with special reference to ash and bases, in Sheets, P.D. and Grayson, D.K. (eds.), Volcanic activity and human ecology: Academic Press, New York, p. 125-159.

Neild, J., O'Flaherty, P., Hedley, P., Underwood, R., Johnston, D., Christenson, B., and Brown, P., 1998, Agriculture recovery from a volcanic eruption: MAF Technical paper 99/2. (Available online at http://www.maf.govt.nz/mafnet/publications/volcano-eruption-impact/httoc.htm).