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 tephra, including c. 17 000 km(2) of land in primary production. The thickness and distribution of the tephra was mapped and it was chemically characterised in order to predict its effects on soil fertility. Tephra 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), with the remainder in the form of soluble sulphate. The S-0 in the tephras was derived from the former, hydrothermal Crater Lake system in the active vent of Ruapehu. Sulphate was derived from Crater Lake water and gases erupted with the tephra. Field and glasshouse investigations showed that the tephra significantly raised soil sulphate levels. The initial elemental S component was very fine grained (65-99 % < 125 mu m diameter) and oxidised rapidly causing depression of soil pH. In a transect of northern Rangitikei soil and pasture samples, S concentrations increased with increasing tephra thickness and remained above normal levels for at least 8 months after tephra fall. Pasture Se concentrations also rose briefly but returned to more normal levels for these soils after 5 months. Other nutrient elements were deposited in useful amounts only under heavy tephra falls. Toxic elements were minor components of the tephra sampled. As Crater Lake reforms following eruption episodes, future tephra eruptions from Ruapehu can be expected to have similar agronomic effects.