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Fertiliser Fallout from Ruapehu

Scientists from Massey University are advising farmers on how to deal with the impact on soil fertility from last year's Ruapehu eruption. Dr Vince Neall, Mike Hedley and Shane Cronin have been studying the effect of the eruption on soil fertility in the Hawke's Bay and Gisborne regions since the two major eruptions on October 11 and October 14 last year.

Hedley says that sulphur is the most important to farmers of the different elements found in the ash. The eruption on 11 October, which mainly spread its ash in Gisborne, consisted of approximately 3.6% sulphur, while the second eruption, which affected the Hawke's Bay region was around 1% sulphur.

"We found several elements which play a role in soil fertility, but the amounts of sulphur most directly influence a farm's fertiliser policy," he says.

Hedley says it will be important for farmers to adapt their topdressing practices this year, to take account of the effects of the ash.

The scientists are co-operating with Ravensdown Fertiliser field officers to study the long-term effects of the ash fall, which will largely depend on how long it takes for the elemental sulphur to oxidise.

"At the moment we are investigating the particle size of the sulphur in the ash, which partly controls how quickly it is being oxidised. The slower it oxidises to plant available sulphate, the longer it will provide an alternative to fertiliser sulphur," says Hedley.