The Soap-Nut Tree

By Stephen Elliott

First Published in Samara  Issue: 40, 2024 (The newsletter of the Millennium Seed Bank Partnership (MSBP)

School children, visiting the community tree nursery at Ban Mae Sa Mai, in northern Thailand chatter excitedly as they carefully pot up tree seedlings. With slithers of wood, they gingerly prise the tiny seedlings from modular germination trays and transfer them into 9-inch-tall black polythene bags. By the end of the activity, rows of polybags, containing the future forest, are lined up for watering … and it's time for lunch. But first, the children must wash their hands. At the nursery’s wash station, however, there’s a surprise. Instead of soap, they find the brown, wrinkled, leathery husks of forest-tree fruits, about the size of cherries.

Instructed to crush the husks in water and rub them on their hands, the children are intrigued to see their hands become covered in a slippery, soapy lather as the husks gradually soften in the water. Thus, the children have discovered the magic of the soap-nut tree. With hands now squeaky clean, they are ready to enjoy their lunch.

Sapindus rarak fruits - nearly ripeThe fruits, with which the children cleaned their hands, were those of Sapindus rarak, commonly known as the soap-nut tree. Found throughout Thailand, this briefly deciduous tree thrives in upland evergreen forest ecosystems. Research by FORRU-CMU identified S. rarak as an acceptable framework tree species, for restoring upland evergreen forest ecosystems more than 20 years ago. Its saplings have high survival and growth rates, when planted out on exposed deforested sites. They can even resprout after being burnt by wildfires, making the species an excellent choice for forest restoration projects, particularly in fire-prone sites.

FORRU-CMU and RGB Kew, in collaboration, also recently identified S. rarak as a prime candidate for studies on population genetics and seed thermotolerance under the GTSBU  project, due to its wide distribution and its high and rapid seed germination. Multiple germination tests, performed by FORRU-CMU 1990’s, resulted in germination rates exceeding 80% in full sunlight, without the need for pre-treatment, and a median dormancy period of around 50 days.

The lather-making properties of the tree’s fruit comes from saponins, a group of natural chemicals, commonly produced by trees in the family Sapindaceae, most probably to deter seed predators. But the same chemicals have sparked cottage industries across Thailand, which are  transforming the soapy fruits into organic shampoos, cosmetics, clothes detergents and household cleaners. Several  women’s cooperatives have successfully established themselves by making such products, even marketing them in supermarkets and online. Thus S. rarak is quietly empowering women across Thailand, both socially and economically, as well as promoting sustainable forest management, to secure supplies of these valuable fruits.

Increasing scientific knowledge of this useful species may also foster its socio-economic benefits, by maintaining the species’ genetic diversity and thus its resilience to climate change. This will enable its wider inclusion in restoration projects and secure its contribution to village economies and the empowerment of Thailand’s rural women.