IMPACT OF RUAPEHU ASH FALL ON SOIL AND PASTURE NUTRIENT STATUS .1. OCTOBER 1995 ERUPTIONS

Eruptions from Ruapehu volcano on 11 and 14 October 1995 covered large portions of the central and eastern North Island, New Zealand, with t ephra, including c. 17 000 km(2) of land in primary production. The th ickness and distribution of the tephra was mapped and it was chemicall y characterised in order to predict its effects on soil fertility. Tep hra from the two eruptions contained 3.0 and 0.7 % by weight sulphur ( S), respectively, 67-55 % of which was in an elemental form (S-0), wit h the remainder in the form of soluble sulphate. The S-0 in the tephra s was derived from the former, hydrothermal Crater Lake system in the active vent of Ruapehu. Sulphate was derived from Crater Lake water an d gases erupted with the tephra. Field and glasshouse investigations s howed that the tephra significantly raised soil sulphate levels. The i nitial elemental S component was very fine grained (65-99 % < 125 mu m diameter) and oxidised rapidly causing depression of soil pH. In a tr ansect of northern Rangitikei soil and pasture samples, S concentratio ns increased with increasing tephra thickness and remained above norma l levels for at least 8 months after tephra fall. Pasture Se concentra tions also rose briefly but returned to more normal levels for these s oils after 5 months. Other nutrient elements were deposited in useful amounts only under heavy tephra falls. Toxic elements were minor compo nents of the tephra sampled. As Crater Lake reforms following eruption episodes, future tephra eruptions from Ruapehu can be expected to hav e similar agronomic effects.