Most of us think of fertilizer as a product that’s great for yards, gardens, and croplands. But have you ever considered fertilizer’s impact on the environment and our water supplies?
Fertilizer contains minerals such as nitrogen, phosphorus, and potassium — often in the form of nitrates and phosphates, which are nitrogen and phosphorus combined with oxygen. These nutrients provide plants with the food they need to grow and flourish.
Plants get most of their nutrients from the soil, but not all types of soil contain the nutrients plants need to thrive. Some soils contain only a few nutrients — a plant trying to grow there would be like a human trying to live on bread and water. Fertilizer contains a variety of nutrients to address these soil deficiencies.
But fertilizer doesn’t always stay where we put it. When it runs off into other areas and adds nutrients to soil and water that don’t need them, it can cause numerous issues, from environmental contamination to serious illness.
What Causes Fertilizer Runoff?
Agricultural application is one of the most common uses for fertilizer — farmers need it to ensure that their crops flourish. But sometimes, instead of remaining in the fields to nourish growing plants, fertilizer runs off and contaminates the groundwater and surrounding land.
Common garden applications of fertilizer can lead to unwanted runoff as well. Misapplying fertilizer around your yard and garden can lead to waste and have adverse ecological effects.
What are some of the causes of this runoff?
- Overapplication: Using too much fertilizer almost inevitably leads to runoff. Once the plants’ roots become saturated and can hold no more, the rest of the fertilizer will run off into less-saturated areas.
- Poor timing: Fertilizer applied to frozen or partially thawed ground cannot penetrate the soil as easily as it would otherwise. Instead, it is likely to run over the hard ground until it reaches a more hospitable environment, like a stream, lake, or warmer soil. And if you fertilize while grass and other plants are dormant, their roots will be unable to absorb the fertilizer, and it will wash away in the rain much more easily. Many states have regulations that prohibit fertilizer application between about November 15 and March 1.
- Excessive irrigation: Excessive irrigation or yard watering can also lead to fertilizer runoff. If agricultural land or garden soil becomes saturated with water, that water will run off to drier land that can absorb more moisture.
- Heavy rains: Even if you’re careful with the amount of water you use on your plants, a heavy rainstorm can undo your meticulous work. Like excessive irrigation, heavy rainfall can sweep fertilizer away and contaminate the groundwater and soil with chemicals.
- Improper yard waste disposal: Clippings from your lawn often contain fertilizer residues. If you dump them carelessly — say into a nearby empty lot— those residues become vulnerable to runoff. The next heavy rain could carry them away and cause soil and water contamination elsewhere. If they end up in drains, they could leach fertilizer chemicals into the water supply.
The Effects of Fertilizer Runoff
Even though we generally think of nutrients as beneficial, the nutrients in fertilizer can cause harm when they come into contact with the natural environment. Here are a few of the adverse ecological impacts fertilizer runoff can have:
One of the primary effects of fertilizer runoff is eutrophication — excessive nutrient richness in a body of water such as a lake, pond, spring, stream, or estuary. According to the National Oceanic and Atmospheric Administration (NOAA), 65% of studied estuaries and coastal waters in the contiguous United States have experienced eutrophication.
An abundance of nutrients may sound like a good thing, but eutrophication that results from fertilizer pollution in water upsets the delicate proportions of nutrients and disrupts the balance of plant life. Where different species once existed harmoniously in an aquatic ecosystem, a single species may begin to dominate at the expense of all others.
2. Algae Blooms
One immediate consequence of eutrophication is the proliferation of algal blooms.
Algae thrive on nitrates and phosphates. And the life cycle of algae is exceptionally fast, so they multiply quickly when high concentrations of these nutrients are present.
When fertilizer runs off into freshwater sources like lakes and streams, its nutrients can soon lead to algal overgrowth. Streams, ponds, and lakes may develop thick green or greenish-blue scums or mats on their surfaces. They may give off an unpleasant odor, and they can turn the water different colors — green, brown, or even red.
Algal blooms are well known for their toxic effects. Algal toxins like the microcystin found in cyanobacteria‚ a type of blue-green algae, can have profound impacts on human health, causing symptoms like headaches, skin and respiratory irritation, diarrhea, vomiting, nausea, numbness, dizziness, and impaired liver or kidney function.
People can drink water contaminated with algal toxins, and they can come into contact with these toxins in other ways as well. People who swim, boat, or waterski on affected waters may accidentally ingest contaminated water droplets. Or they may eat contaminated fish or shellfish and ingest the toxins that way.
Algal toxins can also have devastating effects on pets. When dogs swim in algae-infested waters, they can become ill and die. Because dogs are more likely than humans to drink water that contains visible algal scum, they are more likely to experience profound symptoms. And because they cannot communicate their symptoms, owners may not be aware that something is wrong until their pets are in obvious, extreme distress.
3. Seaweed Growth
The nutrients in fertilizer can also promote seaweed growth.
Seaweed growth occurs in response to fertilizer runoff for much the same reason algal growth does. Most seaweed is a large form of algae known as macroalgae. It responds to nutrients like nitrates and phosphates in the same way algae does — by overgrowing its aquatic environment. Though seaweed does not tend to produce toxins the way cyanobacterial algae does, it can still choke out other organisms by consuming the resources they need to thrive.
4. Oxygen Depletion
When fertilizer pollution in water leads to an abundance of nutrients and promotes algal overgrowth, one frequent result is oxygen depletion. Algal blooms consume and use up the available dissolved oxygen, leaving little left for fish and other aquatic organisms. Severe oxygen depletion, known as hypoxia, occurs when oxygen levels in the water fall to less than 2 parts per million.
When oxygen levels in an aquatic environment drop, the consequences can be severe. Fish and other organisms may grow sick and die. If you notice a lake covered with blue-green algae and containing unusual numbers of dead fish, a toxic algal bloom is most likely the culprit.
Hypoxia can sometimes create dead zones in bodies of water. One famous example is the dead zone in the northern Gulf of Mexico, which occurs every summer because of excessive nutrient pollution, algal growth, and oxygen depletion. At its largest, in 2017, it measured 8,776 square miles. This dead zone becomes uninhabitable for the many animals that once thrived there — fish, shrimp, and crabs can sometimes swim out of the area, but other organisms slowly suffocate.
5. Biodiversity Loss
As time goes by, an algal bloom in a lake or stream begins to choke out other species. Lowered oxygen levels mean that fish, aquatic plants, and small animals can no longer live there. The altered balance of species has significant impacts on the food chain and larger ecosystem as well. A shortage of fish may drive away mid-chain consumers like otters or herons and lead even apex predators like ospreys and eagles to leave in search of a more reliable food supply.
6. Ocean Acidification
Scientists have known for a while that climate change is making the world’s oceans more acidic. When carbon dioxide dissolves in the ocean, it reacts with the water to form carbonic acid. Ocean acidification causes many problems, including interfering with the shell-building abilities of crabs and other organisms that require calcium carbonate to build.
Recent evidence has shown that the eutrophication associated with fertilizer runoff also leads directly to coastal ocean acidification and compounds its harmful effects.
7. Nitrate Poisoning
If fertilizer nutrients like nitrates make it into the drinking water supply, they can have particularly severe adverse health impacts on infants. It’s not uncommon for nitrates to contaminate local drinking water. Public water supplies usually receive thorough treatment under federal law, but even so, municipal filters may be older and insufficient for filtering nonbacterial contaminants like nitrates. Babies may be at risk if family members unknowingly use contaminated water to mix their formula.
In infants, especially, nitrates can cause an illness called methemoglobinemia, known colloquially as “blue-baby syndrome.”
Methemoglobinemia occurs when nitrates turn to nitrites in the body and oxidize the iron in our blood’s hemoglobin. They cause some of the hemoglobin to turn into an alternate form known as methemoglobin. Methemoglobin differs from hemoglobin in that it cannot bind oxygen, so people who develop this illness often see precipitous drops in their blood oxygen saturation. Many infants begin to turn blue because of the lack of oxygen in their bodies. Methemoglobinemia can also cause symptoms such as headaches, fatigue, dizziness, confusion, seizures, coma, and even death.
How to Prevent Fertilizer Runoff
At this point, you may be wondering how to prevent fertilizer runoff from your lawn or garden. Fortunately, you can take steps to reduce runoff and minimize your yard and garden’s environmental impact.
To reduce the effects of fertilizer runoff, try applying some of these tips:
1. Leave Lawn Clippings
When you mow your lawn, consider leaving the lawn clippings there as nourishment. As they decay, they return nutrients to the soil, so they can help you cut down on your need for purchased fertilizer. They also help absorb water to prevent runoff. You can take the collection box off your lawnmower to keep the clippings on your lawn, or you can use a mulching mower to spread the clippings.
2. Use Slow-Release Fertilizer
Instead of dumping huge volumes of chemicals into your plants all at once, slow-release fertilizer controls the nutrient release so that it remains low and constant throughout the growing season. This schedule means the fertilizer will release nutrients when the grass is actively growing and can absorb them to prevent fertilizer runoff.
3. Set Your Mower Blades High
Setting your lawnmower blades to 3 inches or higher can help you reduce runoff in your yard. Taller grass absorbs more water, and it also develops a more robust and intricate root system that can hold water in place to impede runoff. A healthy root system withstands drought better, so you’ll need to water your lawn less often and can reduce the chance of runoff even further.
4. Keep Fertilizer Away From Water
When you apply fertilizer to your grounds, be sure to keep it at least 20 feet away from water sources. If you have a stream running through your property or your yard overlooks a lake, keep your fertilizer well away from these areas to reduce the risk of nutrient pollution.
5. Apply Mulch
If you’re applying fertilizer in your garden, use mulch and layering to your advantage. Try spreading a layer of compost, applying fertilizer directly to the plant roots, and then spreading a 2- or 3-inch layer of mulch over the top. The mulch will help hold in soil moisture and keep fertilizer in place even during heavy rainfall.
6. Use the Correct Type of Fertilizer
Because of the harmful effects of eutrophication, some states started banning the use of the mineral phosphorus in fertilizer. Today, many fertilizer companies use phosphorus only in fertilizers intended for short-term use on new lawns and gardens. When you’re purchasing fertilizer, be sure to get a type that contains only the nutrients you need for your soil.
7. Use the Right Amount of Fertilizer
A good rule of thumb for fertilizing a lawn or garden is to apply half a pound to a pound of nitrogen for every 1,000 square feet. You can tell how much of each nutrient you’re getting by looking at the numbers on the bag.
Fertilizer bags typically contain NPK numbers that look something like 10-5-5, where the numbers are the percentages of nitrogen (N), phosphorus (P), and potassium (K) respectively. A 100-pound bag marked 10-20-10 holds 10 pounds of nitrogen, 20 pounds of phosphorus, and 10 pounds of potassium. To apply 1 pound of nitrogen from a bag like this, you’d use 10 pounds of fertilizer.
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