APATITE IN THE PYROCLASTIC FLOW DEPOSIT (1990-1995) OF THE UNZEN VOLCANO, JAPAN, AND ITS UTILIZATION BY BUCKWHEAT

Abstract

The dacitic pyroclastic flow deposit (1990-1995) of the Unzen volcano, Japan, contained 1.0-1.6g P₂O₅ kg^-1 of total phosphorus (P). Apatite, a major chemical form of P in the pyroclastic flow deposit, was found as discrete particles and inclusions in various minerals. The dissolution of P from apatite was dependent on pH and chelating organic acids similarly to those in other fresh tephras. However, the ratio of the dilute H₂SO₄-soluble or plant-available P to the total P content was lower in the Unzen pyroclastic flow deposit than in the fresh volcanic ash deposits because of the coarse particle size. Among the tested plants, buckwheat grew best and went to seed without a P fertilizer due to partial dissolution of apatite at a soil pH around 4.