AGRONOMIC IMPACT OF TEPHRA FALLOUT FROM THE 1995 AND 1996 RUAPEHU VOLCANO ERUPTIONS, NEW-ZEALAND

Eruptions from Ruapehu Volcano on 11 and 14 October 1995 and 17 June 1 996 distributed at least 36 x 10(6) m(3) of sulphur(S)-rich tephra ove r the central and eastern North Island of New Zealand. The tephras add ed between 30-1500 kg ha(-1) S to at least 25 000 km(2) of land in pri mary production. Smaller but beneficial amounts of selenium (Se) and i n some areas potassium and magnesium were also supplied. Addition of S to the soils in the form of sulphate and elemental S resulted in a dr op in soil pH and an increase in pasture S contents within seven weeks of the eruptions. The soils affected by the tephra are naturally low in S and Se, but following the eruptions S was not required in fertili zer applications in many areas. The strongest and longest lasting effe cts of S and Se deposition were in high anion-retention soils particul arly Hapludands (moist, moderately weathered soils, derived from volca nic ash). Soluble fluorine concentrations within the tephras were low compared to historic Icelandic and Chilean examples. However, pastoral livestock deaths were apparently caused by fluorosis in addition to s tarvation when tephra covered feed. The Ruapehu tephra contained very low concentrations of other soluble toxic elements.