Sirius Minerals shares plunged after the company announced a delay in its application to mine for potash at its flagship site in Yorkshire.The AIM-listed company said it had asked the North York Moors National Park Authority to defer a decision on its application.The shares were down 12.8% to 18.75p at 08:26 and have lost a third of their value in the last week.Sirius said it needed more time to deal with European environmental requirements and questions raised by a consultants' report. The authority was due to have considered the application on July 29th.The Yorkshire project was meant to launch Sirius as a leading potash producer. It claimed the mine would be the world's largest and best quality source of Polyhalite, which is a variant of potash.But the project has ran into trouble after the park authority's consultants, AMEC, raised various objections to Sirius's plans. Sirius said on July 16th that AMEC's report was out of date and contained errors.Chris Fraser, Chief Executive of Sirius, said: "Whilst a deferral will be frustrating to the thousands of supporters of the project, we must ensure the National Park is in a position to make a robust decision on the application and that all the key statutory agencies are comfortable with the extensive environmental information and commitments that we have provided."