Fertilizers are necessary to support plant growth, but knowing what nutrients are essential for your plant’s growth is equally critical. Lawns, gardens, and woody plants all require different nutrients, and these vary depending on the season.
Good fertilizers for lawns in spring have NPK numbers of 15-0-6 or 20-0-5, while fertilizers for summer and fall should be 6-0-0. Trees and shrubs do well with NPK numbers of 16-8-8 for all seasons. Gardens need anything from 12-55-6 to 16-20-0 in spring to 10-5-10 in summer and 24-4-12 in fall.
In this article, I’ll explain the letters and numbers used for fertilizers in more detail and explain why you need different fertilizers for different types of plants. I’ll also explain why you need various fertilizers for different seasons, so let’s get into it!
What Are Fertilizer Numbers and Letters?
When you pick up a commercial fertilizer, you might notice a series of letters and numbers on the label. This standardized labeling refers to the three major plant nutrients needed for growing plants.
Fertilizer letters and numbers indicate the ratio of nitrogen, phosphorus, and potassium in a fertilizer. The letters are the chemical symbols for these essential macronutrients. The numbers on the fertilizer indicate the percentage of nutrients in the fertilizer by weight.
The letters and numbers are always written out as NPK or N-P-K, referring to Nitrogen, Phosphorus, and Potassium, in that order. The nitrogen value is always written first, followed by phosphorus, and potassium is at the end. This labeling is standardized worldwide and is known as the fertilizer grade.
Tip: I’ve written in-depth articles about why plants need nitrogen, phosphorus and potassium. Check them out!
A holistic, balanced fertilizer that supports plants at all stages of growth is 20-20-20. This label indicates that the fertilizer has 20% nitrogen, 20% phosphorus, and 20% potassium by weight. So if you buy a 10-pound bag of 20-20-20 fertilizer, it’ll have a total of two pounds (0.91 kg) of each of the nutrients distributed throughout the entire product.
The fertilizer grade is helpful because it allows you to purchase what you need for your plants instead of indiscriminately adding improperly balanced fertilizer to your soil.
Too much fertilizer can cause fertilizer burn (I’ve written an in-depth article all about fertilizer burn and what to do about it), unsustainable growth, and increased salt concentration in the soil, which can hurt your plants. So, it is crucial to ensure that you’re adding the necessary fertilizers to support plant growth and no more than that.
The Importance of N-P-K
Nitrogen is responsible for high yields in your plants. It helps plants develop the proteins necessary to form healthy, strong foliage. Another role of nitrogen is serving as a component of chlorophyll, which gives your plants a healthy, vibrant green hue.
Phosphorus is essential for root development and seed production.
On the other hand, potassium indirectly supports plant growth by supporting reactions that control yield and water intake.
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The Best Fertilizer Ratios For Your Lawn
Healthy lawns play many essential roles in the environment, and they’re also a great space for play, rest, and relaxation. Maintaining your lawn’s health can have beneficial effects on your health as well, so here are a few tips for fertilizing your property effectively.
Before you decide to fertilize, you’ll need to consider where you live and what kind of grass you have. You would grow warm-season grass like bermudagrass or buffalograss in states like Texas. People grow cool-season grasses like Kentucky bluegrass and perennial ryegrass in states such as Colorado.
Like most plants, you will need to fertilize your grass right before the growing season. The growing season is different for warm and cool-season grasses. Still, overall, you should never add fertilizers during their dormant season, which is typically in the winter.
You should also be aware of your state’s laws regarding fertilizer applications. States like Maryland prohibit lawn fertilizers between November and March.
Here, I’ll list out the best fertilizer numbers for lawns in the different seasons.
What Are the Best Numbers for Lawn Fertilizers in Spring?
You can fertilize both warm and cool-season grass in the spring.
Cool-season grasses grow when the weather is chilly, as the name suggests. You should fertilize them in early spring after the winter thaws when the weather is between 60º F and 75º F (15.56º C and 23.89° C).
Warm-season grasses grow best as the weather grows warmer, so you should fertilize them in the latter half of spring as the weather goes warmer and leads into summer. The ideal temperature for fertilizing warm-season grasses is 80º to 95º F (26.67º C to 35° C).
Most lawns need fertilizers with a higher percentage of nitrogen than any other nutrient. While excess nitrogen can have harmful effects on your grass, applying a fertilizer with a higher nitrogen percentage in the growing season can support your lawn’s growth and resilience.
Typically, lawn fertilizers will have little or no phosphorus.
The best numbers for lawn fertilizers in spring would be about 15-0-6, 20-0-5, 30-0-3, or 35-0-5 – all of which have a higher percentage of nitrogen, some potassium, and no phosphorus.
If you suspect that your lawn needs phosphorus, getting a soil test done would be advisable before buying specialty fertilizers that contain phosphorus for your grass.
What Are the Best Numbers for Lawn Fertilizers in Summer?
You should not fertilize Cool-season grasses in the summer if you can avoid it, as a high percentage of nitrogen-based fertilizers can lead to excessive growth. While this sounds good, it leaves your lawn vulnerable to diseases.
However, warm-season grasses should be fertilized in the summer, especially if it’s been newly planted or you’ve just taken over the care and maintenance of the lawn. The best fertilizer for this weather is slow-release fertilizer which supports your lawn’s growth without forcing it to grow too quickly.
The best numbers for lawn fertilizers in summer are 6-0-0, 10-0-4, or 15-0-8. Warm-season grasses typically need a lower percentage of nitrogen than cool-season grasses.
What Are the Best Numbers for Lawn Fertilizers in Fall?
You should not fertilize lawns in late fall. However, you can fertilize cool-season grasses in early fall.
Using a fertilizer with a low percentage of nitrogen can support your plant’s resistance to winter pests and opportunistic weeds and help it recover better in the spring once the weather warms up.
Keeping the percentages low is essential because excess nitrogen in the soil in the winter can force your lawn to grow, which will kill it faster as the plants will not be able to survive the cold weather.
The best numbers for lawn fertilizers in fall are 6-0-0. Regardless of your fertilizer’s numbers, you should ensure that you don’t add more than 1lb of nitrogen for about 100 sq. ft (9.29 sq. m) of soil.
Applying Fertilizer Correctly to Lawns
Apart from picking up the correct numbers of fertilizers, it’s essential to ensure that you’re fertilizing your lawns correctly.
Always use a fertilizer spreader for even distribution rather than trying to fertilize your lawn by hand. Rotary fertilizers are useful for covering large, open areas. However, if you have a lot of trees on your property or other obstructions, you might want to use a drop-type fertilizer that’ll let you work around any structures on your lawn.
The Best Fertilizer Ratios for the Garden
Gardens need fertilizers different from those used on grass.
As I mentioned, lawn fertilizers tend to have a higher percentage of nitrogen with little or no phosphorus. In contrast, gardens typically need a higher rate of phosphorus than the other two macronutrients.
Nitrogen-heavy fertilizers are better for mature plants and trees. Too much nitrogen can delay the maturity of your garden plants and reduce the plant’s capacity to develop flowers and fruits.
A good garden fertilizer would have a grade of 10-20-10 or 12-24-12, which provides all the nutrients to plants and supports root development and higher yields. These fertilizers also have a sufficient percentage of potassium which is especially necessary for plants like tomatoes and potatoes.
One of the best organic sources of garden fertilizer is compost, which will not only have the trifecta of macronutrients but traces of minerals and micronutrients required by your garden plants.
Before you pick a fertilizer, it’s critical to know what type of soil you have. Clay soils need less fertilizer than sandy soils. If your garden is primarily in pots, you might be using potting mixes which already have fertilizer mixed in, so you’ll want to be careful to ensure that you’re not overfertilizing your plants.
What Are the Best Numbers for Garden Fertilizers in Spring?
Spring is the growing season for most garden plants. Flowering plants are best fertilized right before spring, as soon as the ground is workable, and about a week after the last frost to ensure that it was the last one of the season.
If you have a vegetable garden, then you can add fertilizer to your soil just as spring begins so that your plant has sufficient nutrients to draw on when it grows its fruits. Doing so will ensure a high yield.
The best numbers for garden fertilizers in spring are 12-55-6, 15-15-15, or 16-20-0 for flowering plants. For vegetable gardens, you could stick to a balanced fertilizer grade like 20-20-20, and that ought to meet the majority of your plant’s needs.
However, if you’re just starting your vegetable garden, then numbers like 3-4-4 can help you ensure that you’re not overfertilizing your fragile seedlings.
These numbers deliver the required amount of nitrogen to the soil while ensuring that your plant has enough phosphorus to root well and produce flowers, fruits, or veggies.
What Are the Best Numbers for Garden Fertilizers in Summer?
With summer comes heat stress and drought, and depending on the types of plants you have, you may or may not need to fertilize your garden in the summer.
On average, you will need to fertilize your garden right before the summer rains begin. That’s because rain will allow the soil to absorb the nutrients completely. In most locations, the rain starts in May or early June. Fertilizing at this time is effective as it supports your plants at the height of the growing season.
However, you’ll want to hold off on fertilizing your garden if forecasters predict heavy rain within the week, as this will result in your fertilizer leaching out of the soil and into groundwater.
Fertilizing in summer is especially important for nightshades like eggplants and tomatoes and long-growing plants like sweetcorn, as the growth of these plants is nutrient intensive. Cruciferous vegetables like cabbage, cauliflower, and broccoli could also benefit from a summer fertilizer application.
Your plants could benefit from a higher percentage of nitrogen and potassium rather than phosphorus at this stage, as the roots should be well-developed by summer.
The best numbers for garden fertilizers in summer are 10-5-10 for plants you have recently transplanted. For growing plants, 20-5-20 or 40-10-40 are the best fertilizer grades to use in summer.
What Are the Best Numbers for Garden Fertilizers in Fall?
As with lawns, fertilizing your garden in the fall will help your gardens survive the chill of winter and recover faster in the spring.
Earlier, we discussed that when your plants are just beginning to grow in spring, they need support for root development and need heavy phosphorus fertilizers and very little nitrogen.
In contrast, the fertilizers you use for your gardens in fall need to have higher nitrogen percentages to help your garden survive the winter.
However, the overall quantity of fertilizers needs to be lower so that you don’t force your plants to grow in weather that will kill them. The fertilization rate needs to be about half of your fertilization rate in spring.
The best number for garden fertilizers in fall is 24-4-12. This ratio gives the plant sufficient nitrogen for nutrition, some phosphorus to support the roots, and potassium to help the plant draw up whatever water is available in the soil during this season.
Another good number to look for in fall fertilizers is 5-2-2 if you’re fertilizing in late fall or early winter.
The Best Fertilizer Numbers for Trees and Shrubs
Trees, shrubs, and woody plants generally don’t need fertilizer or need very little fertilizer throughout their lives. However, the quality of soil is essential.
Urban areas, especially those with extensive construction, are detrimental to trees and shrubs. These areas are likely to have compacted soils or restricted root zones, which prevent them from accessing the nutrients they need. In such cases, fertilizer will help support the tree’s growth.
Young trees need regular fertilization — about once a year, but most other kinds of trees only need to be fertilized once every few years.
Fruiting trees might need a more frequent fertilization schedule to improve yields and the quality of the fruit. Some varieties of palms need a regular fertilization program to stay healthy.
For the most part, trees, shrubs, and woody plants only need fertilizer if they show signs of deficiency. Nitrogen deficiencies are the most common. Usually, you can tell if a tree requires more nitrogen if its leaves have yellowed.
An average fertilizer you can use for trees at the peak of the growing season is the number 21-4-4. This ratio will ensure that the trees receive the nitrogen they need.
Of course, you should always consider the specific needs of your trees and shrubs and what deficiencies they may have before applying any fertilizer. A soil test will go a long way in helping you determine which nutrient your trees and shrubs need.
What Are the Best Numbers for Tree and Shrub Fertilizers in Spring?
The growing period for trees and shrubs in spring and early summer is an excellent time to apply quick-release fertilizers. These fertilizers are soluble, making it easy for the plants to take up the nutrients and use them for growth.
Rapid growth isn’t ideal in trees and shrubs as the foliage will grow at the expense of the root system, resulting in a large tree with a relatively small root system. However, fertilizing trees and shrubs in early spring with the right kind of fertilizer can help the growth of your trees and shrubs in a balanced way.
The best number for tree and shrub fertilizers in spring is 10-10-10 for overall fertilization. If you suspect a nitrogen deficiency or simply want to support growth and yields, you could consider a 16-6-12, 16-8-8, 20-10-5. Also, 8-3-9 is a good set of fertilizer numbers for fruit trees.
For younger trees and shrubs that need support with root development and growth, you can consider using a 12-24-12, which will encourage root growth with a higher percentage of phosphorus.
What Are the Best Numbers for Tree and Shrub Fertilizers in Summer?
You should only fertilize trees and shrubs in early summer. Late summer fertilization can result in new growth that won’t harden in time for winter and will die when the winter frost comes in.
To prevent any development that will not survive the winter, you should avoid fertilizing your trees and shrubs between July to early August, when the first leaves have dropped from the trees in fall.
The best numbers for tree and shrub fertilizers in summer are 16-8-8 or 20-10-5. Summer is the growing period for trees and shrubs, so you can use the same amount and fertilizer ratios that you would use in spring.
What Are the Best Numbers for Tree and Shrub Fertilizers in Fall?
Fall is an excellent time to apply fertilizers for trees and shrubs, especially if you’re using slow-release fertilizers, as these won’t lead to fast, unsustainable growth.
You should apply fertilizers after the leaves have turned color and started falling off to ensure that the nutrients support your tree’s survival over winter instead of encouraging new growth.
The best numbers for tree and shrub fertilizers in fall are 12-6-4 and 18-6-12. These supply enough nitrogen to help trees and shrubs sustain the colder, more barren winter months.
The Numbers for Fertilizers by Season and Plant Type
| General | Spring | Summer | Fall | |
| Lawns | 15-0-15 | 15-0-6; 20-0-5; 30-0-3; 35-0-5 | 6-0-0; 10-0-4; 15-0-8 | 6-0-0 |
| Gardens | 10-20-10; 12-24-12 | 12-55-6; 15-15-15; 16-20-0; 3-4-4 | 10-5-10; 20-5-20; 40-10-40 | 24-4-12; 5-2-2 |
| Trees & Shrubs | 21-4-4 | 10-10-10; 16-6-12; 8-3-9;12-24-12 | 16-8-8; 20-10-5 | 12-6-4; 18-6-12 |