ST. PAUL  — The Minnesota Pollution Control Agency postponed its plans to issue Thursday a long-awaited and politically sensitive preliminary recommendation on whether changes to the state’s water quality standards are needed to protect wild rice from sulfates.

The agency gave no explanation for canceling the release except to say it wasn’t ready. Agency spokesman Dave Verhasselt wrote in an email to reporters late Wednesday that the agency would provide an update “when we can in the coming weeks.”

The Minnesota Chamber of Commerce is criticizing the methodology and conclusions of a major three-year study on which the agency was going to base its recommendations. The Chamber’s 84-page report says its own analysis of the agency’s research concluded that a sulfate standard is unnecessary.

But John Pastor, a University of Minnesota Duluth biologist who was one of the lead researchers on the study commissioned by the agency said the current standard or something close is “about right.”

The Chamber and other supporters of iron and copper-nickel mining have argued that the standard is obsolete, while the state’s American Indian bands fear any weakening of it could imperil a food source they consider sacred and central to their cultural identity.

A 40-year-old state law, based on research from the 1940s, limits discharges of sulfates into waters that produce wild rice to 10 milligrams per liter. The new research suggests that even low levels of sulfates can hurt wild rice, under the right conditions, when the sulfates turn into toxic hydrogen sulfide in the mucky sediment of northern Minnesota’s lakes and streams.

Pastor said the MPCA has a difficult task in deciding whether to change the standard.

“It’s really hard to say what’s a single number that works for all conditions. Unfortunately, that’s the way the regulatory laws work,” he said. “This is a very, very complicated problem.”