TOUGH economic conditions have forced Incitec Pivot to close its Geelong single superphosphate plant.

The company said drought and falling commodity prices had caused a sales slump of its single superphosphate products, used mainly for pastures.

SSP sales were 55 per cent down on the five-year average, said Gary Brinkworth, Incitec Pivot's general manager of Australian Fertilisers.

But he assured farmers who use SSP that future supplies were assured with its Portland plant continuing production.

Present stocks of 300,000 tonnes of SSP combined with ongoing production at Portland were sufficient to meet forecast market demand in the long term.

"These are very testing times for farmers and it is clear many have decided to defer fertiliser purchases until there is an improvement in commodity markets and rainfall patterns," Mr Brinkworth said.

Twenty-nine jobs will be lost at the North Shore plant which will close on June 30, but IPL will continue to employee about 50 people in the Geelong area in distribution and business support functions.