Unfertilized lawns are bigger contributors to runoff pollution than fertilized surfaces
By Mike Jiggens
Runoff is pollution all by itself, without adding extra chemicals to it, lawn care professionals were told in March at the fourth annual Nutrite professional seminar in Guelph.
Lakehead University’s Christopher Murray spoke about the risk of runoff water quality with regard to the fertilization of turfgrass. He is currently involved in a joint study between the university and Landscape Ontario to look at the influence of turfgrass fertilization on the quality of runoff.
“What’s most important to understand is what actually dominates and causes water pollution,” he said.
Runoff is a natural phenomenon which would exist even without lawns and buildings. Residential homes and other buildings, driveways and roads create areas which are essentially waterproof, thereby increasing the velocity and volume of water concentrated in small areas. This causes erosion which is at the root of pollution, Murray said.
“As runoff increases, pollution increases.”
As areas become developed with waterproof roofs, driveways and roads, water is collected and concentrated into a fast-moving stream. Yet lawns are one area where a difference can be made to reduce runoff.
“It’s one last place where water can make its way down into the aquifer.”
Whatever enters a storm drain typically makes its way directly into rivers, lakes and streams. There is no wastewater treatment plant in between.
Murray said municipalities can play a key role in educating the public as to the difference between stormwater and wastewater. Water from sinks and toilets goes through wastewater treatment plants where it is cleaned up before entering rivers, lakes and streams.
“A storm drain might have nothing between that drain where you have dog feces, grass clippings, runoff from driveways and roads, all going into a pipe that goes straight down into the lake where you might be drawing your drinking water from.”
That’s what makes it so important for the public to pay particular attention to what goes into storm drains, and perhaps even more so than what is flushed down a toilet, Murray said.
From a stormwater management point of view, he said we are increasingly being told to expect more severe droughts and to consider more drought-resistant grasses while restricting summer water use. At the same time, the strength of the storms being experienced is getting increasingly stronger. One of the obvious effects of the climate change in North America is how much more erratic the weather is getting, he said.
Although climate change models suggest things will be good for North America for the short term, in the form of longer growing seasons, it doesn’t account for the increasing erratic trend. The question, Murray said, is how we are to account for these longer dry spells as well as more intense rain when it comes.
“Stormwater management is an industry that has basically popped up over the last 50 years to reduce the impact of these increasingly erratic storms.”
The idea, he said, is to “smooth out” that rainfall to something similar to what would exist if an area was undeveloped.
Murray said clean water is actually pollution which, he added, might sound counter-intuitive. The reason why is that if it rains particularly hard—after many years of pollution-laden sediment catches at the bottom—the water coming in is 100 per cent pure, but it grabs the sediment, resuspends it and sends it downstream.
It’s an example where clean water, if there is enough of it, can present a problem.
“Too much water, even if it’s crystal clean and pure, is a problem from a stormwater point of view.”
In some older municipal systems, there are combinations of stormwater and wastewater systems. When stormwater goes up, it can spill over into the wastewater where it is called combined sewer overflow and is intentionally there to deal with overflows. If it gets high enough, after it is mixed with sewage, it will begin to pop up through manhole covers. It’s then no longer stormwater but “black water” because it has mixed with sewage and has become a significant environmental biohazard which needs millions of dollars to clean up.
“That’s another reason why we have to try to reduce how much water is running off developed areas.”
For lawn care professionals, the nutrients going onto a lawn are most concerning, especially in their relationship with runoff. Nitrogen and phosphorus are the key considerations.
“There is no evidence that potassium is really doing damage when it comes to water systems.”
Nitrogen is necessary for plant health, and lawn care professionals are able to provide what is needed. Phosphorus isn’t as vital as nitrogen, but it’s still important and is responsible for energy transfer. It is obtained from mined minerals and is provided to the plant in an easy-to-access form when added into fertilizer.
Algal blooms and nutrification are the main concerns with phosphorus getting into runoff.
“When you have excess amounts of this nutrient going into runoff, you have algae that gets out of control, and it starts to suck up all kinds of things that algae shouldn’t be sucking up.”
Murray showed a number of photographs in his presentation, one of which depicted too much fertilizer in the runoff. There are several living things at the bottom of the food chain in aquatic ecosystems which require those nutrients, just as they are needed in land-based ecosystems. Plants and phytoplankton photosynthesize and form the bottom of the food chain, but water needs to be somewhat clear to allow for sunlight penetration and make everything work.
When things become too unbalanced and there is excessive nitrogen and phosphorus, especially in sediment, problems arise such as cloudy water to block photosynthesis, bacteria growing at the surface which is using all the dissolved oxygen, fish gills getting clogged from abundant sediment and not enough oxygen for them to breathe.
“Everything gets thrown out of whack because you’ve disturbed that ecosystem too much.”
The lawn care industry aims to increase the amount of infiltration of rainwater into the soil and decrease the amount going into runoff because of the sediment that comes from surface water that doesn’t come from infiltration.
When there is new development, a couple of problems arise: the vegetation that holds onto the sediment is taken away, and the area is exposed to more rain so that more can make its way into the stormwater.
Sediment comes in all sizes, from large gravel pieces to sand or silt or clay, and most is of the same underlying material—a mineral based on silicon oxide. Sediment is of foremost interest to anyone concerned with stormwater because if sediment can be stopped, so can most other pollution, Murray said. A small amount of sediment in water can be responsible for most of the phosphorus and nitrogen the water is carrying. It will vary from one place to another with some having 90 per cent of their nutrients bound to the sediment. Some places have none, depending on the type of soil being “churned up” when it rains.
Finer particles are more problematic, he said, because they are carried further by the rain. With a greater surface area, more pollution is carried.
“The finer, the more dangerous, from our point of view.”
It can be difficult to stop finer particles with a filter or to give them time to settle. Large storms release more sediment.
The project Murray is overseeing came about from two concerns put forward by Landscape Ontario. One was the feeling that source water protection agencies such as conservation authorities were moving towards the same kinds of restrictions on fertilizers that have been applied to pesticides, with the perception that something might be mandated imminently. Landscape Ontario was also aware of a number of studies which suggested the viewpoint of conservation authorities contradicted research data that found where fertility had stopped, increases in the amount of phosphorus and nitrogen showing up in runoff were documented.
“That almost seems counter-intuitive. Why would Iâ€ˆsee more fertilizer in the runoff when I stopped applying fertilizer to the lawn? But that’s what the studies show.”
Some may ask how much a lawn should be fertilized to obtain the best water quality. Water quality can be reduced by excessive fertilization.
“You cannot indiscriminately fertilize and expect no impact on the runoff. It’s going to make its way into the runoff.”
Yet water quality can still be improved with fertility, Murray said, adding if that is the case, then imposing a ban or restriction will damage water quality.
“If that kind of regulation was put into place, we could in principle get the exact opposite effect of that regulation. Aiming to improve water quality, we instead hurt it.”
Research on the subject is looking into the true state of scientific information.
“Is there really a consensus in the scientific community? Are people agreeing one way or the other whether turfgrass can possibly help water quality? Does that consensus support a ban on fertilizer?”
Murray said every piece of information researchers could find was studied. Researchers looked at the quality of runoff as a function of fertilization—both good and bad—and put everything together. When thinking about these studies, consideration must be given to whether they’re performed in experimental or realistic situations, he said.
“You need to measure both quantity and quality. You need to make an apples to apples comparison because none of these studies is the same.”
Murray said it’s harder to measure a small amount of water running off the lawn in a light rainfall vs. that of a major storm. Large storms disturb the most sediments, so what is easiest to measure is also the dirtiest.
“So if you just measure the large storms, because that’s what you’re capable of measuring, you’re always going to see more polluted water than is really represented.”
There are generally two extremes of tests performed. There are well-controlled experimental tests in which the amounts of fertilizer and rain are constant. Everything that comes in is measured as well as everything coming out.
“It’s, of course, totally unrealistic,” Murray said.
He said if he was to set up a turfgrass experiment in the lab, he would be sure to measure the worst-case scenario whereby he would fertilize and then hose down the lawn immediately afterward to probe the extremes.
The other test, which is more realistic, would be to recruit a number of homeowners and study their lawns.
“It’s very realistic, but you lose control.”
Murray said the rainfall is realistic and the amount of traffic on the lawn is realistic, but a “happy medium” would ideally have to be found between the controlled and realistic forms of testing.
How can adding fertilizer help from a water quality point of view? Turfgrass requires maintenance to thrive and will not, in general, outcompete indigenous plants as weeds which are better suited to harsh conditions. Weeds are not suited to human scale runoff we furnish through waterproof surfaces. Healthier turfgrass reduces runoff and increases the amount of water which makes it way through infiltration, thereby decreasing the fraction that goes into surface runoff.
“Runoff, in fact, can be completely eliminated by turfgrass. Lawns are really the only band-aid we put in a developed area to account for all the development we’ve put in place.”
The problem with runoff is higher in larger cities where the amount of greenspace is generally lower. Murray pondered how the reduction of fertilizer use would make the problem worse and how stopping fertilizer would increase the amount of nitrogen and phosphorus in the runoff.
A small effect would come from the plant as it becomes less healthy, he said. When fertilizing is stopped, grass begins to decay which leads to more nutrients coming out than are going in.
“The biggest effect is that the turfgrass holds water effectively.”
Less healthy turfgrass holds water less effectively. Filtering the water is not enough. Turfgrass does that a little bit through its roots as water percolates through the soil, but the biggest job it does is control the amount.
“So it’s more important as a quantity tool than a quality tool, and that’s really the major role turfgrass plays from a water quality point of view.”
Murray said turfgrass is more than just cosmetic, adding a healthier turfgrass system is going to improve surface water quality, both through filtration as well as through absorption, thereby reducing the runoff in general.
“If you stop the runoff, there’s no pollution to worry about.”
Most of the joint study was conducted in 2012 in which the impact of fertilization on water quality was researched. Some time was spent looking at alternative ground cover, but the focus of the study was on turfgrass.
Questions posed were:
• What is the effect on how much nitrogen and phosphorus get into the runoff as a function of the amount of fertilizer used?
• What kind of recommendations should be made if there is an agreement within the scientific community?
Murray said a study was conducted about 10 years ago which showed there was more phosphorus in the runoff in a fertilized lawn. But the study didn’t measure how much runoff was collected which he said was important to note because the best turf (that which was fertilized) had the least runoff.
After fertilizing had stopped for two years in the older study, the amount of runoff had increased as well as the amount of nitrogen and phosphorus. When lawns were fertilized, there was more infiltration and less runoff. When ground was frozen, more problems occurred because the surface had become impermeable. Researchers were getting more nutrients coming off in the controlled study (without fertilizer) than with the lawns getting fertilizer.
The most runoff, in general, came from the least healthy lawns in the study.
Nutrient concentrations are higher in the runoff when fertilizer is applied, but the amount of runoff is lower where fertilizer is used.
“In general, the total nutrient loss is decreased by fertilizer. So you have more pollution in the runoff when you get it, but you get it less frequently. So, in general, you’re stopping more pollution from making its way into the runoff by catching that runoff.”
With regard to frozen ground, Murray said it is best to avoid fertilizing if there is any chance the ground will not allow it to seep down.
Most alternative ground covers are non-native, ornamental plant species, and turfgrass reduces runoff better than native grasses.
The worst time to fertilizer is just before a rain event, Murray said, suggesting it also be avoided if the surface is about to freeze or has already become frozen. Winter melt will take the fertilizer away into the runoff before it can make it way to the plants.
Frequent, smaller applications or slow release fertilizer are better than infrequent, heavier applications, even though there are some municipalities which limit the amount of applications, leaving little choice but to increase the amount. Watering in fertilizer is recommended as long as it’s not too high an amount that would create runoff.
“From a water pollution point of view, people see less pollution if you do more frequent, smaller waterings than big soakings.”
Murray admitted it sounds counter-intuitive when the practice of infrequent, deeper watering to promote root growth for a healthier lawn is encouraged.
“Would it make more sense to build up that turfgrass and risk runoff by making a healthier turfgrass, or should I just eliminate all risk and never water enough to create runoff?”
Murray said there isn’t sufficient information available to best answer the question.
He said another question pertains as to how much watering and maintenance grasses should be given from a water quality point of view. One might also wonder if he should ignore summer watering restrictions and continue to water his lawn during periods of drought so that it’s ready to take the runoff when it rains.
“Which is going to be better in the long run from a pollution point of view?”
Murray said it’s a good idea to test soil each year for its phosphorus content and resist using any phosphorus-based fertilizer until it’s assured the soil needs it. It’s most useful to apply phosphorus during establishment when the grass is least capable of holding water, which can create a problem.
He warned, however, that simply testing soil for nutrient quantity is not enough and won’t indicate if water will run off or not. The soil’s level of compaction and density are considerations which must also be made before fertilizing.
Murray said some marketing needs to be done to drive the message of the relationship between fertilizer and water quality because it is not in keeping with public opinion.