TOLEDO, Ohio — Ohio’s agriculture leaders say thousands of farmers have completed training that will be required for putting fertilizer on fields, but many more face a September deadline to finish the program aimed at combating the toxic algae fouling Lake Erie.

The first of its kind requirement is one of several steps Ohio has taken to reduce the farm runoff that feeds algae in the state’s lakes and rivers.

State lawmakers put the measure in place in 2014, just months before algae in Lake Erie contaminated the drinking water for more than 400,000 people in northwestern Ohio and southeastern Michigan.

They gave farmers three years to be certified for commercial fertilizer use on more than 50 acres of farmland. So far, about 12,000 farmers and workers who apply commercial fertilizer — mainly phosphorus and nitrogen — have been certified, said Dave Daniels, director of the state’s agriculture department.

An estimated 6,000 to 10,000, he said, still need to finish the three-hour course — how many exactly is difficult to pin down because some farmers hire contractors to apply their fertilizer.

Ohio State University Extension says it will offer about 200 training sessions this year in nearly every county.

Daniels, who attended one of the first sessions a few years ago, said he found almost no one wanted to be there. But surveys of participants show that almost all say they learned something that will help them save money and improve the environment, he said.

Research has shown that about one-third of farmland doesn’t need additional fertilizer and that farmers could save thousands if they realized this, Daniels said.