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Why Does Fertilizer Burn Grass? The Science Explained

Why Does Fertilizer Burn Grass? The Science Explained

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Paul Brown

If you’ve ever applied fertilizer to your lawn only to see it turn brown and patchy, you may be wondering what went wrong. This common problem has baffled many gardeners, but the science behind it is pretty simple.

Fertilizer burns grass when applied in a higher concentration than the grass can handle. This can happen if you use too much fertilizer, apply it too often, or don’t water the grass properly after applying fertilizer. High concentrations of fertilizer chemicals can damage plant tissue.

While it may be tempting to go heavy-handed with the fertilizer to get your grass looking green and luscious as quickly as possible, this will only end up harming your grass in the long run. Read on to learn more about why fertilizer burns the grass and how to avoid it in the future.

The Science Behind Fertilizer Burns

Fertilizer is made up of various chemicals, including nitrogen, phosphorus, and potassium. These chemicals are essential to plant growth, but they can damage plants in too high of a concentration. 

Nitrogen promotes leaf growth by stimulating chlorophyll production. Lack of sufficient nitrogen results in stunted growth and yellowing leaves, causing your lawn grass to look pale and unhealthy (source).

Phosphorus is necessary for root development, and potassium helps grasses overcome heat, cold, and drought stresses. When you apply fertilizer to your lawn, the chemicals are absorbed through the grass leaves and roots, where they begin to work their magic (source). 

However, if you apply too much fertilizer, the chemicals and salts draw water away from the roots of the grass, causing the grass to dry out and turn brown.

This results in brown or yellow patches on your lawn that are often referred to as “fertilizer burns.” Lack of adequate watering and quick release or “hot” fertilizers can exacerbate this problem (source). 

How Fertilizer Burns Appear on the Grass

Fertilizer burns often appear as brown or yellow patches on your lawn that distinctly differ from the healthy, green grass surrounding them. The patches may be small at first, affecting only a small portion of your lawn, but when not addressed quickly, the patches can spread and eventually kill your grass entirely.

If caught early enough, fertilizer burns are often reversible, and your grass will eventually recover. However, the damage may be irreversible in severe cases, and you may need to reseed or replant your lawn entirely.

Fertilizer burns are common in spring when the grass is actively growing and absorbing nutrients more quickly. However, they can also happen during the summer if you use too much fertilizer or don’t water your lawn properly after applying it. The high concentration of soluble salts in the fertilizer can cause the grass to scorch in hot weather.

Causes of Fertilizer Burns on the Grass

While fertilizer is necessary for healthy grass growth, too much can be harmful. The last thing you want is for your lawn to turn brown and patchy after applying fertilizer.

Here are the top causes of fertilizer burns on grass:

1. Applying Too Much Fertilizer

One of the most common causes of fertilizer burns is applying too much fertilizer at one time. Fertilizer contains high concentrations of salts and chemicals that can damage your grass if not used properly. 

Applying too much of it causes a drying effect by drawing water away from the roots of your grass. The grass will then wilt and begin to turn brown and yellow as it dries out.

2. Not Watering Your Lawn After Applying Fertilizer

Water is essential for healthy grass growth, and this is especially true after you’ve applied fertilizer. The high concentration of salts in fertilizer absorbs water from the grassroots and surrounding soil which can cause the grass to dry out and turn brown

This stress is compounded in hot weather and can result in scorched patches on your lawn. Watering your lawn regularly dissolves the fertilizer salts and prevents them from damaging your grass. The excess nutrients leach out of the root zone and are flushed away before they can cause harm.

3. Applying Quick Release Fertilizers

Quick-release–or “hot” fertilizers–contain high concentrations of nitrogen that are immediately available to the plants. Professional landscapers and golf courses often use these types of fertilizers because they provide a quick growth boost. However, they can also be more harmful to your grass if not used properly.

Hot fertilizers can cause fertilizer burns if applied in a high concentration or not watered incorrectly. It’s best to use slow-release fertilizers that release nutrients over a longer period to avoid damaging your grass.

4. Using Incompatible Fertilizers

Different types of fertilizer contain different nutrients that can react when mixed. For example, combining an ammonia-based fertilizer with a nitrogen-based fertilizer can create a chemical reaction that releases heat. This can cause the grass to scorch and turn brown.

It’s essential to read the labels on your fertilizer products and follow the manufacturer’s instructions to avoid damaging your lawn.

How To Prevent Fertilizer Burns

Lush, vigorous, and healthy grass is the goal of every lawn enthusiast. The striped, uniform look of a well-manicured lawn evokes satisfaction and pride. However, the road to a beautiful lawn is not always smooth sailing. 

Fertilizer burns can plague your lawn and turn your hard work into an eyesore. Luckily, there are several ways you can prevent fertilizer burns and keep your lawn in top shape:

1. Use a Slow Release Fertilizer

Unlike quick-release fertilizers that release nutrients all at once, slow-release fertilizers provide a steadier supply of nutrients over a longer period of time. 

This gradual release prevents fertilizer burn because the grass has a chance to absorb the nutrients before they become concentrated. A slow-release fertilizer has all the same benefits as a quick-release fertilizer without the risk of damaging your lawn (source).

2. Follow the Label Instructions

Like all products, it’s essential to read the label on your fertilizer and follow the manufacturer’s instructions. The label will tell you the type of fertilizer, the recommended application rate, and when it’s best to apply the fertilizer. 

Be sure to follow these instructions to avoid over-or under-fertilizing your lawn which can lead to fertilizer burn. If you’re unsure how to use the fertilizer, ask a professional at your local garden center for guidance.

3. Water Regularly

Fertilizer contains a high concentration of nutrients that can harm your grass if left unchecked. Watering your lawn regularly dissolves the fertilizer salts and prevents them from damaging your grass. The excess nutrients leach out of the root zone and are flushed away before they can cause harm. 

It’s best to water your grass 1 to 2 times a week during spring when fertilization season starts. During summer, you can water up to 3 to 4 times a week to prevent the fertilizer from burning your lawn. 

If you have used a liquid fertilizer, wait about 2-4 hours before watering to allow the fertilizer to soak into the soil. With granular fertilizer, you’ll need to water immediately after applying it to prevent it from damaging your grass.

4. Have Your Soil Tested

Soil testing is a critical process in maintaining a vigorous and healthy lawn. A soil test will help you determine the nutrient levels in your soil and which nutrients need to be added. This will help you pick the right fertilizer for your grass and avoid using too much of any nutrient. 

Over-fertilizing your lawn can lead to fertilizer burn, so getting your soil tested is vital before you start fertilizing. You can purchase a soil test kit like the Luster Leaf Rapitest Test Kit (link to Amazon) for a DIY option, or you can have your soil tested by a professional.

5. Use a Lawn Spreader

Applying fertilizer by hand can lead to uneven distribution, which can lead to fertilizer burn. A lawn spreader ensures an even fertilizer application, so you don’t have to worry about damaging your lawn. There are two main types of lawn spreaders: broadcast and drop.

Broadcast Spreaders

Broadcast spreaders such as the Agri-Fab Tow Broadcast Spreader (link to Amazon) are suited for large lawns as they can cover a large area quickly. The spreader sits on the lawn and distributes fertilizer evenly as you push it along.

Drop Spreaders

Drop spreaders like the Scotts Turf Builder Classic Drop Spreader (link to Amazon) apply fertilizer in a more targeted and precise way. They are best suited for small lawns or areas where you need to be careful not to over-fertilize. 

The spreader sits behind the lawn and drops fertilizer only where the spreader wheels touch the ground. This prevents you from accidentally applying too much fertilizer in one spot and damaging your lawn.

The best spreader for you depends on the size of your lawn and your personal preferences. Read the fertilizer label to determine the recommended spreader settings.

There is one other option that I’ve really taken to over the last couple of years, and that’s a liquid fertilizer. I have an extensive hands-on review article on the product I use if you are interested.

6. Use Organic Fertilizer

Organic fertilizers are an eco-friendly alternative to chemical fertilizers. They are made from natural materials such as manure, compost, and bone meal. These materials release nutrients slowly, so they are less likely to cause fertilizer burn (source). 

Organic fertilizers also improve the quality of your soil over time as they add organic matter to the soil. This helps to improve drainage, aeration, and water retention. However, organic fertilizers are not as concentrated as chemical fertilizers, so you may need to use more to achieve the same results.

See Organic vs. Inorganic Fertilizer: Which Is Really Better?

7. Mow High

With many grass types, you shouldn’t mow your lawn too low to the ground as this can make your grass more susceptible to disease. Keeping your grass too short can attract pests and make fertilizer burn more likely as the grass doesn’t have enough leaves to absorb the nutrients. 

When mowing, set the blade to a higher setting so you leave a grass height of 2-3 inches. This ensures the grass has enough leaves to absorb nutrients without being damaged by the fertilizer.

Another approach is to ensure that when you are mowing, don’t take off more than 1/3 of the blade’s height at a time.

How To Treat Fertilizer Burn

If you accidentally over-fertilize your lawn, don’t panic. When caught early, fertilizer burn is easy to treat with a little TLC (tender loving care).

1. Water the Lawn

The first step is to water the lawn thoroughly. This will help dilute the fertilizer and prevent further damage to the grass. Be sure to water deeply, around 1-inch of water, so the roots are reached. 

Repeat once a day for 3-4 days or until the symptoms of fertilizer burn have disappeared. A sprinkler system will make this process easier, but you can also water by hand using a hose or watering can.

See our guide on creating an EASY DIY Automatic Watering System.

2. Avoid Mowing the Lawn

Allow the grass to grow taller than usual until it has recovered from the fertilizer burn. This will give the grass time to absorb more water and nutrients. Once the grass has recovered, you can return to your regular mowing schedule.

3. Apply a Top Dressing

Applying a layer of finished compost over the lawn can help replenish nutrients and improve soil quality. This is especially helpful if you accidentally over-fertilize with a chemical fertilizer, as it can help to detoxify the soil.

4. Reseed Affected Areas

If the fertilizer burn is severe, you may need to re-seed the affected areas. Use the same grass seed you used initially, and be sure to follow the seeding instructions on the package. Properly care for the new grass seedlings by watering, fertilizing, and mowing as needed.

Be careful not to overdo it on the fertilizer, or you’ll end up right back where you started!

Final Thoughts

Fertilizer burns grass when too much is applied at once. Since fertilizer is high in chemicals and salts, it can damage the grass if it’s not diluted or applied correctly. On the grass, fertilizer burn appears as brown or yellow patches but the grass may die in severe fertilizer burn cases. 

Avoid excessive fertilizer application and regularly water your lawn to prevent fertilizer burn. If you accidentally over-fertilize, take steps to treat the damage and prevent further harm to your lawn. With a little time and care, your lawn will return to its green self in no time.

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