Search for lawn fertilizers, and you’ll find a million options with different numbers and words in the description. It’s hard to figure out which one to buy if you don’t know what it all means. This guide explains in detail how to choose the right lawn fertilizer, from understanding the NPK ratio to choosing which application method you prefer.
Want to make your fertilizer shopping process easier? See LawnStarter’s top picks for the best lawn fertilizers of the year. We’ve also ranked the best fertilizers for Bermudagrass and the best fertilizers for St. Augustinegrass.
- How to Read Lawn Fertilizer Numbers
- Granular vs. Liquid Lawn Fertilizer
- Synthetic vs. Organic Lawn Fertilizer
- Specialty Lawn Fertilizers
- FAQ About Lawn Fertilizers
- When and How to Fertilize Your Lawn
How to Read Lawn Fertilizer Numbers
On any lawn fertilizer label, you’ll find a combination of three numbers. They might be 10-10-10, 32-0-10, 16-4-8, or any other combination. Those numbers are called the NPK ratio. The NPK ratio is the most important factor in choosing the right fertilizer for your lawn.
NPK stands for nitrogen (N), phosphorus (P), and potassium (K). These are the three most important nutrients for your grass. The numbers in the ratio tell you what percentage of each nutrient the fertilizer contains. For example, a fertilizer with an NPK of 10-10-10 is 10% nitrogen, 10% phosphorus, and 10% potassium.
Multiply the percentages by the amount of fertilizer in the package to figure out how many pounds of nitrogen, phosphorus, or potassium the fertilizer contains. For example, if you buy a 14-pound bag of fertilizer with an NPK of 16-4-8, you would use these calculations:
16% (0.16) nitrogen x 14 pounds of fertilizer = 2.24 pounds of nitrogen
4% (0.04) phosphorus x 14 pounds of fertilizer = 0.56 pounds of phosphorus
8% (0.08) potassium x 14 pounds of fertilizer = 1.12 pounds of potassium
Nitrogen, phosphorus, and potassium all serve a different purpose:
- Nitrogen boosts fast growth and dark green color.
- Phosphorus encourages strong root growth.
- Potassium improves disease resistance and overall plant health.
So, if your main goal is to make your lawn grow faster and greener, you’ll want a high-nitrogen fertilizer.
A soil test can help you figure out what NPK to look for in your fertilizer because the results tell you which nutrients your soil contains and which ones it’s lacking.
If you find out your soil doesn’t have enough phosphorus, you’ll want a fertilizer with plenty of phosphorus. The same goes for potassium. Your soil test results will also tell you how many pounds of nitrogen you should apply to your lawn. Depending on which lab you use, the results might even tell you the best NPK ratio to look for.
You should be able to get a soil test from your state university’s local county extension office. Find your university extension using this directory.
Granular vs. Liquid Lawn Fertilizer
Should you use liquid or granular fertilizer? That’s a matter of personal preference. Here are the pros and cons of each to help you decide.
Pros and Cons of Granular Lawn Fertilizer
Granular fertilizers come in the form of tiny particles (aka “granules”) in a bag. You apply them to your lawn using a fertilizer spreader, without diluting or mixing with water first. Most granular options are “slow-release fertilizers,” which means they add nutrients to the soil gradually over time instead of all at once.
|Pros of granular fertilizer||Cons of granular fertilizer|
|✓ More even coverage than liquid|
✓ Slow-release formula means longer-lasting results
|✗ More complicated application process |
✗ Takes longer to break down into the soil and green-up the grass
Pros and Cons of Liquid Lawn Fertilizer
Liquid fertilizers come in a few different forms. Some come in ready-to-spray bottles that you simply hook up to your garden hose, some come in a concentrated liquid form that you dilute with water before spraying, and others come in water-soluble particles that become liquid when you mix them with water.
|Pros of liquid fertilizer||Cons of liquid fertilizer|
|✓ Faster green-up|
✓ Easier application
|✗ Usually contains immediate-release fertilizer, which means results don’t last as long |
✗ Often results in patchy coverage
✗ More likely to burn the grass than granules
Synthetic vs. Organic Lawn Fertilizer
If you’re interested in environmentally-friendly lawn care or you have outdoor pets to worry about, you should consider using an organic lawn fertilizer. Here are the benefits and drawbacks.
Pros and Cons of Synthetic Lawn Fertilizer
Synthetic lawn fertilizers use man-made chemicals to add nutrients to your lawn.
|Pros of synthetic fertilizer||Cons of synthetic fertilizer|
|✓ Cheaper than organic |
✓ Faster results than organic
|✗ Pollutes water sources with chemicals as a result of stormwater runoff|
✗ Harms beneficial organisms in your soil such as earthworms and plant-friendly bacteria
✗ Toxic for pets
Pros and Cons of Organic Lawn Fertilizer
Organic lawn fertilizers use natural materials such as manure, compost, cottonseed meal, bone meal, or seaweed to add nutrients to your soil.
|Pros of organic fertilizer||Cons of organic fertilizer|
|✓ Doesn’t pollute the environment with unnatural materials |
✓ Improves soil structure
✓ Boosts the number of beneficial microorganisms such as earthworms and plant-friendly bacteria in your soil
✓ Safe for pets, even immediately after application
|✗ More expensive than synthetic|
✗ May take a few applications to see significant improvement in your lawn
Specialty Lawn Fertilizers
Some lawns have needs that run-of-the-mill fertilizers can’t satisfy. In that case, these specialty fertilizers can do the trick.
Fertilizers for New Lawns
If you recently planted new grass seed or laid new sod, you’ll want to use a special fertilizer called a lawn starter fertilizer. Starter fertilizers are specially formulated to benefit new grass. They contain a high amount of phosphorus for healthy root development and a low amount of nitrogen to prevent burning.
Pre-emergent weed-and-feed fertilizers applied in early spring prevent common summer weeds such as crabgrass.
Post-emergents kill weeds that have already sprouted in your lawn.
Seasonal Lawn Fertilizers
Some fertilizers are designed to use at specific times of year. If you choose to use seasonal fertilizers, you’ll need to purchase a different product for each application throughout the year. You shouldn’t use a summer fertilizer in fall or vice versa.
FAQ About Lawn Fertilizers
Phosphate-free fertilizer is fertilizer that doesn’t contain any phosphorus.
Even though phosphorus is an essential plant nutrient, it disrupts aquatic ecosystems by causing algal blooms.
Many states ban or limit the use of fertilizers with phosphorus.
In spring, you want a fertilizer that’s low in nitrogen to prevent burning the grass as it wakes up from winter dormancy. The first number in the NPK ratio should be 10 or less.
Spring lawn fertilizers should also contain some phosphorus (if allowed in your state) and potassium.
In summer, your grass needs lots of nitrogen to help it stay thick and green through the punishing heat. The first number in the NPK ratio should be 20 or more.
Phosphorus and potassium aren’t as important in summer.
Note: Don’t fertilize cool-season grasses in summer. Cool-season grasses go dormant in summer, so fertilizer at this time of year – especially high-nitrogen fertilizer – would burn the grass.
In fall, your grass needs help preparing to survive the winter. This is especially true if you live in an area that gets snow. You need low nitrogen and as much phosphorus (if allowed where you live) and potassium as possible.
The first number in the NPK ratio should be 10 or less, and the second and third numbers should be higher than the first.
When and How to Fertilize Your Lawn
Once you’ve chosen the right lawn fertilizer, you have to apply it. When is the best time to fertilize your lawn? That depends on your grass type. See our guide on When to Fertilize Your Lawn for lawn fertilization schedules for cool-season and warm-season grasses.
For a step-by-step explanation of how to apply liquid and granular fertilizers, see our guide on How to Fertilize Your Lawn.
When you’re done fertilizing the lawn, it will grow faster than ever. That means your lawn willl need mowing more often, but you don’t have to kiss your free time on the weekends goodbye. Weekly or bi-weekly lawn mowing from LawnStarter’s local lawn care pros is only a click away.
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