New Zealand Journal of Agricultural Research abstracts
Impact of Ruapehu ash fall on soil and pasture nutrient status 1. October 1995
eruptions
SHANE J. CRONIN
M. J. HEDLEY
Department of Soil Science
Massey University
Palmerston North, New Zealand
R. G. SMITH
Ravensdown Fertiliser Co-operative Ltd
P. O. Box 448
Feilding, New Zealand
V. E. NEALL
Department of Soil Science
Massey University
Palnerston North, New Zealand
Abstract 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 km2 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
(S0), with the remainder in the form of soluble sulphate. The
S0 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
um 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.
Keywords Ruapehu volcano; tephra; soil fertility;
hydrothermal Crater Lake; elemental S; Se; impact on pastures
New Zealand Journal of Agricultural Research, 1997, Vol. 40: 383-395
0028-8233/97/4003-0383 $7.00/0 (c) The Royal Society of New Zealand
1997
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