SLATER, Iowa — Nick Helland’s central Iowa farm looks much like every other nearby farm on this chilly March day, with corn stubble stretching from a gravel road up over a low hill to the northern horizon.

But look closely, and you can see patches of muddy ground where a few months ago crews buried low-tech systems called bioreactors and streamside buffers that filter fertilizer-borne nitrates from water as it drains from Helland’s field into nearby Big Creek and eventually the Des Moines River.

The underground devices work. The question is whether one Iowa county’s promising new approach to an old problem can be expanded enough to finally address nitrate pollution that, for years, has endangered drinking water, made more than half the state’s waterways unfit for fish or humans, and fueled a giant dead zone nearly 1,000 miles away in the Gulf of Mexico.

Polk County is doing it by making it painless for farmers — handling all the logistics and arrangements for the systems — and throwing in payments of $1,000 per site. Installations have exploded in the past two years, to 104, after only a handful were installed the eight years before that.

“They paid me and they paid the cost of all the installation,” Helland said. “That’s sort of a no-brainer to me that with very little lift, very little time, I can have this installed on my farm and it will ensure better water quality for everyone else downstream.”

The big challenge now is encouraging counties to launch and fund similar efforts to reduce runoff from Iowa’s 10 million acres of tile-drained farmland and combat the state’s multi-billion dollar problem with nitrogen pollution.

Nitrogen-based fertilizers and manure can lead to excessive nitrates in groundwater that can be toxic to livestock and humans. High levels have plagued waterways in Iowa and throughout the Midwest for decades from chemical fertilizers and animal manure sprayed on fields. Modern tractors let farmers assess their soil and apply only as much fertilizer as needed, but it’s still common to overspray.

It’s easy to see why. Yields of corn — the king crop in these parts, and planted on about 90 million acres nationwide — are at least doubled by fertilizer, and farmers want to be sure their crops have enough nutrients. Adding to the problem are the quick drainage systems that lie beneath so many fields — known as tiles, but actually plastic pipes — that whoosh excess water away and into streams.

Numerous studies have found the low-tech systems remove half the nitrate or more from runoff before it reaches waterways. In bioreactors, the water passes through a buried mound of wood chips that break down much of the nitrate. In the buffers, it moves through a grassy area parallel to a stream.

Too much nitrate and phosphorous in rivers and streams makes great food for algae and other plant growth that cuts oxygen in the water and blocks sunlight. Combined with industrial farming practices that have altered waterways by straightening streams and removing wetlands, that’s bad news for fish that need clear water and slower currents.