Lawn is not a crop but when it comes to fertilizer, it helps to think of it as one. Turfgrass gets “harvested” by mowing often, usually weekly. All that growth you are cutting depletes the nutrients in the soil and can only be sustained by replenishing those nutrients.
So fertilizing your lawn is crucial to its health but there is also the risk of over-fertilizing, if you use the incorrect fertilizer, or too much fertilizer, or fertilize at the wrong time. Over-fertilizing your lawn leads to a host of lawn problems and in the worst case can kill your lawn.
Here’s how over-fertilization can happen and how to avoid it.
How Fertilizer Affects Lawns
Fertilizer contains the three macronutrients nitrogen, phosphorus, and potassium (N-P-K). Because nitrogen is what is most needed for vigorous leaf growth and the rich green color of turfgrass, special lawn fertilizers are higher in nitrogen than in the two other macronutrients. Soil often contains enough phosphorous (P) and potassium (K) so their portion in a fertilizer makeup is proportionately either very low or zero.
Importance of Soil Tests
Although the way a lawn looks can give you some clues about its overall health at this moment, the only way to know for sure which fertilizer it needs and how much to sustain continued growth is to do a soil test. Everything else is mere guesswork and could lead you to over-fertilize your lawn.
A soil test done by a specialized laboratory can give you detailed recommendations about which nutrients you need to add to your lawn. The lab results also indicate whether the pH is within the proper range for turfgrass or needs to be lowered, which is equally essential, as soil pH impacts how nutrients are available in the soil. It is recommended to get your soil tested every two to three years, and sandy soil annually, as it is leaching nutrients much faster.
Ways How Lawn Can Become Over-Fertilized
There are several ways how a lawn can become over-fertilized.
Adding the Incorrect Fertilizer
Lawns need fertilizer that is high in nitrogen (N), which in indicated by N being the highest number and lower numbers for phosphorus (P) and potassium (K). A common N-P-K for lawn fertilizers is 3:1:2 or 4:1:2. A complete or balanced fertilizer which contains the three macronutrients in equal amounts (i.e., 10-10-10) is not suitable for lawns and will cause over-fertilization with phosphorus and potassium and a lack of nitrogen.
Adding Too Much Fertilizer
For lawns, it is recommended to use a slow-release or controlled-release fertilizer (and not an immediate release fertilizer) because it feeds the grass with a small amount of nutrients over an extended period of time. But over-fertilization can still happen if you apply more than the recommended 1 pound of nitrogen per 1,000 square foot of lawn—by not measuring your lawn area and calculating the required amount of fertilizer correctly, or by having the spreader on the incorrect setting. When you fill your spreader, don’t do this on the lawn because you might accidentally spill an excessive amount of fertilizer in a single spot.
You might also be inadvertently over-fertilizing your lawn by doing the right thing and leaving your grass clipping on the lawn, which returns nitrogen to the soil so you need about 25 to 40 percent less fertilizer. Keep this in mind when calculating the fertilizer amount. If in doubt, use less rather than more.
Fertilizing Too Often
Because the recommended lawn fertilizer is slow-release, it is not applied more than twice, maximum three times a year, in the spring and late summer, and sometimes in the fall to winterize the lawn.
In heavy soil with poor drainage, fertilizer can build up. Only a soil test can determine whether that’s the case and you need to adjust the amount of fertilizer to prevent over-fertilization.
Signs of An Over-Fertilized Lawn
Too much fertilizer can manifest itself as a crusty residue on top of the soil, which you can remedy by watering after scratching the soil with a rake.
Other signs of an over-fertilized lawn are more severe and long-lasting.
If you have over-fertilized your lawn, it usually does not take more than a couple of days for it to show fertilizer burn or fertilizer scorch: yellow and brown strips, which depending on the amount of excess fertilizer, turn into patches of dead grass. Excessive fertilizer rapidly increases the level of nitrogen and salt in the soil and literally burns the grass.
Too much nitrogen can also lead to excessive and rapid growth of the grass blades, at a much faster rate than the root system. This is problematic because the ratio between roots and shoots becomes unbalanced; there are not enough roots to take up water and nutrients for the grass, and as a result the grass will wilt under drought stress.
Applying too much nitrogen, especially in the spring, can cause thatching, which in turn makes the lawn vulnerable to pests and diseases.
Consequences of Over-Fertilized Lawn
The harm caused by fertilization goes often beyond what is obvious to the eye.
Lawn fertilizer has also become a major nitrogen pollution issue. The excess nitrogen runs off and seeps into groundwater, eventually ending up in streams and rivers. Nitrogen also feeds fast-growing algae, including toxic red tide algae that out-compete slower-growing beneficial plants. What’s more, the lack of oxygen in water bodies because of dying plants has a devastating effect on fish and other marine life.
Excess nitrogen turn soils more acidic. Turfgrass grows best in pH range between 6.0 and 7.0. An acidified soil affects the availability of nutrients and the activity of soil microbes. At a pH of 5.0 or below, phosphorus, which is crucial for root growth, can become unavailable.
Fixing Over-Fertilized Lawn
When you realize that you over-fertilized your lawn, not all is lost. There are ways to remedy it depending on the severity of the over-fertilization. Sometimes immediate action helps, such as generously watering the lawn to wash away the excess fertilizer. In other instances, you just need to wait it out until the excess fertilizer has been has disappeared. Or, worst case scenario, you need to reseed spots where grass has been killed from fertilizer burn.
Soil Testing: An Essential Tool for Healthy Lawns and Gardens. University of Missouri.
Choosing Fertilizers for Home Lawns. University of Illinois Extension.
Grow Healthy Lawns with Less Nitrogen. University of Vermont, Department of Plant and Soil Science.
Fertilizer Burn. Utah State University Extension.
Turfgrass Fertilization. Texas Cooperative Extension.
How to Control Thatch in Your Lawn. University of Minnesota Extension.
Don't Over Fertilize Your Lawn This Spring. Penn State Extension.
Fertilizers and Soil Microbes. University of California, Agricultural and Natural Resources.
Turfgrass Information. Kansas State University.