Note that the article says
The black volcanic soil [of Aso] gives birth to some of the finest produce in Japan.
Including green tea!
Incidentally, the place name "Aso" (阿蘇) is the name of the region (roughly bounded by the huge caldera in which it's located), the name of the county, and the name of the city, which was recently formed by the merger of Ichinomiya Town, Aso Town, and Namino Village. These names all originate in the name of the volcano that looms over the area—Mt. Aso.