DES MOINES, Iowa (AP) -

Federal regulators have told Iowa that the state's plan to stem runoff pollution lacks an adequate system to measure results.

The Des Moines Register reports that the Environmental Protection Agency told state officials that they shouldn't have any doubts about setting numeric standards for nitrogen and phosphorous, which are key fertilizer components.

Under the Iowa plan released in November, wastewater treatment and industrial plants would be asked to make costly upgrades to cut pollution while farmers would do so voluntarily in an effort to reduce harmful nutrients in Iowa waterways and the Gulf of Mexico.

In 2008 the EPA ordered Iowa and 11 other states along the Mississippi River to develop strategies to prevent nitrates and phosphorous from reaching the gulf, which experiences a seasonal dead zone.