Soil samples taken after the Hunga-Tonga Hunga-Ha'apai volcanic eruption and tsunamis showed that seawater had directly impacted soil salinity and crop growth in the Kanokupolu area on western Tongatapu and by September 2022 the salt had moved down to the subsoil.

A Tonga Soils Partnership Roadshow held on 1 June in Nuku’alofa was attended by soil scientists and researchers from Manaaki Whenua Landcare Research (New Zealand) and CSIRO (Australia). They discussed the impact of the tsunami and ash deposits on soil properties and function across Tonga.

Local soil scientist, Dr. Halavatau said that the seawater directly impacted soil salinity and crop growth in the Kanokupolu area after the event and by September 2022 the salt had moved down to the subsoil.

"However, the impact on the soil remains a concern due to the salt changing its structural stability. The amount of ash deposited and the direct impact on short term soil function was limited."

He said soil monitoring and evaluation was critical in this assessment. Fortunately, the team had conducted an assessment in 2018 which enabled direct comparison, and had restored historical data collected in the 1960-70s into an online database which allow further comparisons. 

This showed that soil organic carbon has declined over the last 40 years, which indicated that farming system management is not sustaining Tonga’s key asset, he said.

Empowering farmers 

Helen Fonua, a Senior Development Programme Coordinator at the New Zealand High Commission, said the soil partnership that emerged from disasters like the HTHH events offered a model for other Pacific countries around soil partnership.

"At the heart of the partnership we have to promote, sharing of knowledge, sharing of expertise and sharing of opportunities for engagement from the wise heads of soils science to the new scientists that will emerge and support laboratory services, extension services and agronomy/forestry in the Kingdom."

She said New Zealand in partnership with Manaaki Whenua-Landcare Research, the Ministry of Agriculture Food and Forest, alongside soil scientists here in Tonga, collected more than 100 core soil samples from 25 historic sites and nine tsunami impacted areas. 

"Establishing the Tonga Soil Partnership is one approach to returning our communities here in Tonga back to living in harmony with the land.

“Healthy soil is life, and healthy soil sustains healthy communities. This is not something that one group can protect and manage. The integration of stakeholders is essential, we need coordination across public, private, community and research to ensure that our lifeblood, our healthy soils and clean water, sustains our people and provides good income opportunities."

She said the recent survey compared results against these baselines and provided vital information for infrastructure, farming and forestry that wouldl ensure Tonga prospers for another 50-years and restores vital connections with the fonua (land).

"Farmers and land owners are central to our purpose here. We want to empower them with these findings and ensure that they can succeed this season and next, and that their legacy can be passed on to the generations of farmers that follow."

In 2025, Tonga will host a Pacific Week of Agriculture and Forestry.

Dr Viliami Manu, CEO for the Ministry of Agriculture, Food and Forest (MAFF) said Tonga is very fortunate to have this meeting and the experts to tell inform them of the impacts of the volcanic ash and the tsunami, on the soils of Tonga.

He also acknowledged the assistance from the Governments of New Zealand and Australia by providing their scientists to assist Tonga.

This initiative is a partnership between MAFF, Manaaki Whenua Landcare Research, CSIRO, MORDI and local soil scientists (Dr Halavatau and Sione Foliaki), supported by New Zealand’s Ministry of Foreign Affairs and Trade.