A LOBSTER fisherman who was sceptical about the agricultural benefits of a £1.5bn polyhalite mine scheme has grown a 350lb pumpkin using just the mineral and water.

Sirius Minerals, due to resubmit a planning application to the North York Moors National Park to mine 250 million tonnes of the mineral from a site near Whitby, claims polyhalite vastly increases crop yields and helps solve the rapidly rising global demand for food.

After a number of universities, including Durham, conducted crop trials using the mineral and traditional fertilisers, the firm said last month that polyhalite increased tomato yields by 96 per cent and improved disease resistance and plant strength.

Campaigners, including the North York Moors Association, opposed to the plant being sited within the national park, have questioned the claims and whether there is significant demand for the mineral.

Matthew Cutt, of Sleights, near Whitby, said that after reading about the claims he was given a lump of the mineral found near the proposed mine site and set about growing a pumpkin from seed with eight-year-old daughter, Charlotte.

Mr Cutt, who grew a 70lb pumpkin using fertiliser from a garden centre last year, said after planting the seed in March and feeding it with the crushed mineral fortnightly, he was staggered by the plant's growth as it developed a 30ft-long vine and a pumpkin five times the size of his previous effort.

"When you listened to the claims they didn't seem believable, so I thought it would be interesting to find out how well polyhalite works.

"It has been unbelievable, the colour is fantastic, the pumpkin looks like it has been blown up and would now need four men to move it."

He said he had previously backed the proposed mine as it could create up to 1,000 jobs in North Yorkshire and Teesside, but the experiment dispelled his doubts about the mineral's value.

While the UK heaviest pumpkin record stands at 700lbs, Mr Cutt said his fruit was continuing to grow and he remained undecided about its future.

A spokesman for Sirius Minerals declined to comment on the results of Mr Cutt's experiment.

Chris Fraser, the firm's managing director, has previously pointed towards its sales commitments for millions of tonnes of polyhalite in place across four continents as evidence of a large and growing market for the mineral.