Parks across Chicago have caused a noticeable stink in the city's efforts to keep the grass green.

The cause of the smell is from the park district's choice of fertilizer — one that's made from human poop.

The organic fertilizer, which is known as biosolids, is made by the Metropolitan Water Reclamation District, which produces 165,000 tons of biosolids for reuse every year.

Biosolids are the material that results from treating "sewage sludge" that can then be recycled, according to the Environmental Protection Agency. Local governments can decide whether to recycle the biosolids as fertilizer or incinerate or bury them in a landfill.

MWRD claims biosolids are both cost-effective and environmentally friendly in comparison to alternative chemical fertilizers. Furthermore, they provide a rich source of nutrients for plants, which is essential for their growth.

In addition to parks, farms, baseball fields and golf courses are among the green spaces that use biosolids as fertilizer.

While the fertilizer is good for the grass and safe to use, the EPA also admits that biosolids "may have their own distinctive odor," which is described as "a slight musty, ammonia odor."

The biosolids may smell a little funny, but they look completely normal. According to MWRD, they are a material that "looks and feels like dark, fine-textured topsoil or 'black dirt.'"

Rather than incinerate or bury the biosolids, it appears Chicago chose the fertilizer option to make the grass in city parks more green. The fertilizer has been used for many years and in many parks across Chicago, including Maggie Daley Park.

Last year, the city announced a new combination of wood chips and biosolids to be used "for maintaining or establishing turf grass in parks, athletic fields and public access areas throughout Cook County."

All 50 states use biosolids as fertilizer, according to the EPA.