Synthetic fields, which are typically made of a mix of sand, rubber pellets and plastic fibers that sit on gravel, pose a different challenge. Costing hundreds of thousands of dollars or more, they have been marketed to schools, cities and teams as a way to reduce maintenance costs and increase usage, though the savings, if any, can vary widely based on how frequently they are used. No seeding, fertilizer or mowing is needed, and the fields can be used in many types of weather, but they must be replaced every decade or so.

The number of synthetic fields has grown to more than 12,000 nationally, and about 1,500 fields were installed this year, according to the Synthetic Turf Council.

With proper upkeep, synthetic fields are considered as safe as natural grass, even though they can be used far more often. But routine maintenance like raking of the rubber pellets and tilling of the sand is required to ensure the field does not get too hard. Rubber pellets must be replaced when they wear away from goal creases and other high-use areas of fields.

Despite the increase in synthetic fields, precise data on how they are maintained is spotty. Experts who maintain fields professionally say that many schools and parks departments forgo basic maintenance and do not test their fields to determine whether they have become too hard because they are unaware it is needed or because they want to keep costs down.

“Artificial fields are still sold as maintenance-free — you put it down, you don’t worry about it — but honestly, they’re hardly being maintained at all,” said Dr. John Sorochan, co-director of the Center for Athletic Field Safety at the University of Tennessee. “People put in the fields because they don’t want to budget for it once they’ve got it.”

Few athletic directors and groundskeepers are willing to admit that they do not properly care for their fields because it may open them up to a lawsuit or void their warranty, which is typically eight years, according to industry experts. But Gary Hover, the former director of parks and facilities in Redwood City, Calif., who now owns a turf maintenance company, said that he had seen fields that were rarely maintained.