(BanksPhotos/iStock)

Americans love their lawns — a little too much, perhaps. And like all loves, this one too has its chemistry — in this case, rooted in nitrogen, phosphorous and potassium, the key components of lawn fertilizer. Sure, it makes the grass grow. But that’s not where the effects end if people over-fertilize or fertilize carelessly, and these nutrients end up getting into our lakes, rivers, or water supplies.

According to the U.S. EPA’s New England Regional Laboratory, 40 to 60 percent of the nitrogen that people put on their lawns through fertilizer winds up in “surface and groundwater.”  Take the case of the Chesapeake Bay: A significant part of the nitrogen and phosphorous that is keeping the Bay polluted comes from urban sources, according to the EPA.

Indeed, careless lawn fertilization can have such negative consequences that some states have passed laws to restrict phosphorus in fertilizer. As the New York Department of Environmental Conservation puts it, “Phosphorus going into the State’s water has been linked to: reductions in oxygen in waterbodies necessary for fish to breathe; algae that turn water bodies green; and algae and algae by-products that degrade drinking water.” Scotts Miracle-Gro, a leading maker of fertilizers and lawn products, itself announced the removal of phosphorus from a key line of fertilizer products, TurfBuilder, in 2013, saying this was “a partial solution to nutrient runoff that can lead to excessive algae growth in waterways.”

In general, the concern is that excess nutrients in water can lead to a process of “eutrophication,” in which algal blooms fed by nitrogen, phosphorous and other nutrients change the chemistry of water bodies, choke off sunlight (potentially killing off plants below the surface) and foster the growth of harmful cyanobacteria. It has been estimated that the damage from eutrophication to U.S. water supplies costs $ 2.2 billion a year to address.

“Nitrogen and phosphorus pollution has the potential to become one of the costliest, most difficult environmental problems we face in the 21st century,” concluded the EPA in 2009.

This isn’t your typical environmental issue, though — the cause is not simply some single big polluter. While the eutrophication problem arises from many sources — much of it can be traced to agriculture and wastewater treatment plants — another key contributor is the large number of Americans busily tending to some 37.5 million acres of turf-grass covered residential lawns, and sometimes making mistakes that across our population, can really add up.

For some reason, these Americans feel that their lawns have to look a certain way — super green, with turf grass standing tall, as opposed to an alternative, less environmentally impactful lawn approach — and that fertilizer is the way to get that to happen. So what underlies the belief?

Keeping up with the Jones’s fertilizer

The short answer seems to be: Our neighbors. A growing amount of research suggests that people fertilize and over-fertilize their lawns in significant part out of a sense of what those who live around them expect their lawns to look like.

In a paper published in 2012, for instance, Amanda Carrico of Vanderbilt and two colleagues found “strong social pressures surrounding lawn maintenance” in the Nashville, Tenn., area. Some 48 percent of people told the researchers that they used fertilizer on their lawns, and the study found that social pressures were a  leading key predictor of their use of fertilizer. As the researchers concluded, “maintaining a lawn is an avenue for engaging with one’s neighbors, for fulfilling expectations of what it means to be a positive member of a community, and to communicate a willingness to cooperate in creating and maintaining a shared space.”

Now, a more recent paper has found much the same result. The study, published in the February 2015 issue of Environment & Behavior by four University of Minnesota researchers, looked at two Minneapolis-St. Paul area neighborhoods, and once again found that people’s beliefs about what their neighbors think had a major effect on their fertilization behaviors. More specifically, in a sample of 942 homes, the research found that among those who perceived that their neighbors thought they should fertilize, there was an 85 percent chance of actually doing so. In contrast, among those who perceived their neighbors felt they should not fertilize, there was a much lower 52 percent chance of engaging in the practice.

“Neighbors are talking to other neighbors about lawns, and what they say matters,” says the University of Minnesota’s Kristen Nelson, one of the study’s authors.

How much is too much?

The study also looked at a particular group of individuals dubbed “high fertilizers” — those who said they apply fertilizer to their lawns four or more different times per year. The focus on this group is key — after all, moderate, tactful lawn fertilization isn’t the problem here. “All lawns will benefit from some fertilizer,” write several experts from the Purdue University Turfgrass Science program. But as they later continue: “more fertilizer is not necessarily better and excess fertilizer will contaminate surface and ground water.”

As the Purdue experts note, the right way to go about fertilizing is to get a soil test to figure out what your lawn actually needs, and then go about having a scientifically informed program for applying it. But where the problem arises is that people can easily apply too much. “Do not apply more than 1.0 pound of actual water-soluble N per 1,000 square feet in any single application,” the Purdue experts note. Moreover, careless practices like getting fertilizer on hard surfaces, like sidewalks or driveways, where it can easily wash away, increases the risk of runoff into water supplies.

“Stop using where it does no good, and it’s not cost effective,” adds Nelson, who says she’s not against lawn fertilization as a whole, just its overuse.

So who are the high fertilizers — those who may create the biggest environmental risk?

In the new study, several key factors were identified. High fertilizers tended to live in the suburbs, rather than urban homes. They tended to employ lawn services rather than managing lawns themselves. But surprisingly, in this group — a smaller number of individuals — the role of neighbors was actually slightly the opposite from what was expected, predicting a slightly decreased (rather than increased) likelihood of being a high fertilizer.

That may just be a quirk of the study, though, notes lead author Nicholas Martini of the University of Minnesota. “We have less confidence in the result because the statistical significance is weaker and the overall effect is small,” he explained by e-mail.

In light of other research, the finding may be just an anomaly. The University of Minnesota’s Kristen Nelson also notes that she conducted another study, more focused on interviewing people about lawn care expectations where they lived. “Everyone discussed the expectations of their neighbors, and discussed the idea of why that was challenging to be able to address, but every single individual could speak to what the expectation was in their neighborhood,” she says.

The finding, overall, underscores what much research has highlighted — humans are social animals and the influences of peers can be extremely powerful. Indeed, the notion that neighbors influence our lawn fertilization behaviors is very much in line with findings that knowing how much electricity our neighbors use influences how much we ourselves use.

As winter slowly fades and lawns start to recover, fertilizer season — its first round, anyway — is about to begin. The latest research suggests that as the grass emerges from snow cover and starts to regrow, people won’t only be eyeing their own lawns for green shoots — they’ll be eyeing those of their neighbors as well.