A tsunami and volcanic ash devastated Tonga’s soils and crops. Our soil scientist, Dr Ben Macdonald, was asked to investigate whether they could recover.

When submarine volcano Hunga Tonga–Hunga Ha’apai erupted powerfully on 15 January 2022, Tonga faced a crisis threatening all areas of life.

A tsunami was the first thing to hit the small Pacific nation. A blanket of volcanic ash soon followed and rained down across the region.

Afterwards, the Tongan Ministry of Agriculture, Food and Forests contacted Australian Department of Foreign Affairs and Trade (DFAT) for funding and assistance.

Not long after, our soil scientist Dr Ben Macdonald received a call from them.

“There was an immediate need for medicines, water, and housing supplies to rebuild from the damage. However, an important second tier need was re-establishing food production to prevent famine,” Ben said.

“The Tongan government realised the salinity of the tsunami and ash from the volcanic eruption could impact the nation’s longer-term food security. And they knew we could assist with assessing the damage to their soils.”

Our involvement came through an existing soil research collaboration, which Ben leads. This project involves several Pacific Island nations, including Tonga, Fiji, Tuvalu, Kiribati and Samoa. These nations share a need to improve crop yields, which already suffered from a lack of soil nutrients. The fear was that this volcanic event had caused irreversible damage to the nation’s soil, and therefore crops.

Flying soil from Tonga

To assess the damage, our soil scientists needed to access soil samples from impacted areas.

With travel to the area majorly disrupted by the eruption, we contacted DFAT and the Department of Agriculture Water and Environment (DAWE) to arrange for soil samples collected by the Tongan authorities to be analysed in Australia.

“They were terrific in working with the Australian Defence Force to secure passage on a flight to Australia,” Ben said.

“The existing collaboration we have with the authorities helped to facilitate transferring the material to CSIRO’s quarantine labs.”

DFAT provided funding for the analysis.

The crops and soil at Tonga covered in ash.
Recovery of the nation’s soils will take time, but there is good news for Tonga’s crops.

Analysing Tonga’s volcanic soils

The existing research project allowed researchers to compare how the tsunami and ash had changed the soil’s composition.

Thankfully they had recently collected pre-eruption samples, and also had access to decades of historic surveys.

CSIRO soil scientist Dio Antille helped analyse 24 soil samples that arrived at our labs.

For starters, we found the tsunami negatively impacted the soil. This is due to the salinity of seawater that washed over the land.

“There were significant changes in the chemistry of the soil due to increased salinity and sodicity. This changed the pH of the soil and may compromise the short-term ability to grow crops,” Dio said.

“However, the soils in these areas are very permeable. Given the rainfall they have, the salts are expected to move down the profile below the root zone.

“We know from previous work in Indonesia after the 2004 Boxing Day Tsunami that the soil is likely to recover from salinity in five to 10 years. This time depends on rainfall and management practices for soil amelioration,” he said.

In better news, deposits of volcanic ash could potentially improve the soil condition in the longer term. Volcanic ash is normally rich in cations, which may help restore the fertility of soil.

However, the impacts to Tonga’s soils – both positive and negative – won’t be evenly spread.

Due to the wind direction at the time of the eruption, the northwest of the main island of Tongatapu was more affected by ash deposition. Meanwhile, the lower part of the island was more affected by the tsunami.

Farming is starting to recommence in Tonga. We are starting a new project with the Tongan and New Zealand authorities to monitor the ongoing impacts to the soil and assess the rate of soil recovery.

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