Fertilizer 101 for container gardens

Gardening is almost never “one and done” or “plant it and forget it.” If you want to have a great harvest, you need to learn about the three essential nutrients and the best fertilizer for container gardens at various growing stages.

With container gardens, you have the benefit of using potting soil or potting mix. Commercially produced brands usually include slow-release fertilizer, so you mainly need to know how to supplement.

If you’re making your own potting soil – or you’re reusing some from last season – you definitely want to read up on how to supply the best balance of nutrients.

No matter what type of potting soil or potting mix you use, it’s also important to know what mistakes to avoid!

While fertilizer lingo might seem confusing at first, it’s actually pretty simple. Once you understand the basics, you’ll be set for an awesome growing season.

If you’re wondering how often to fertilize plants, how much fertilizer to use, and how to understand NPK ratio … you’re in the right place!

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Why fertilizer is important for container plants

Plants in containers have access to a limited supply of nutrients because they can’t send roots out farther into soil to search for food.

At the same time, frequently watering a relatively small amount of potting soil leaches nutrients more quickly than in-ground gardens. This counts double for smaller pots!

At Patio Garden Life, we focus on growing edible plants in containers, specifically herbs and vegetables. Those plants especially need the right nutrients to ensure healthy, plentiful production.

NPK ratio

If you’ve ever shopped for fertilizer, you’ve probably seen numbers on the packaging like 10-10-10 or 1-1-2.

Those numbers are the NPK ratio! They tell you how much of each essential nutrient the fertilizer will deliver to your plants. The numbers represent the percentage of each nutrient by weight.

Let’s break it down:

What does NPK stand for?

The NPK ratio will tell you the proportion of nitrogen (N), phosphorus (P), and potassium (K) in each fertilizer blend. The order is standardized, hence the “NPK” designation with the letters always in that order.

For example, if the numbers say 10-10-10, that means the bag of fertilizer contains 10% nitrogen, 10% phosphorus and 10% potassium.

So in a 100-pound bag of 10-10-10 fertilizer there will be at least 10 pounds each of the three nutrients, with the remaining 70 pounds as filler (often sand or limestone).

The composition of the fertilizer can make a big difference in how it affects the plant.

• Nitrogen (N) encourages foliage growth
• Phosphorus (P) promotes root development and helps grow flowers
• Potassium (K) boosts overall plant health

Nitrogen gives plants their green, so if you have very leafy plant varieties, they need a lot of nitrogen. This is why lawn fertilizer will have a higher N number on the label – grass is green! For vegetable gardens, nitrogen helps grow foliage but can hinder fruit production, so don’t over-fertilize container plants with nitrogen.

Phosphorus promotes root development to anchor your plants and can also increase blooms (and fruit!), which is why tomatoes love phosphorus.

Potassium (sometimes referred to as potash) helps plants stay vigorous, which means they can withstand extreme temperatures and avoid disease. Think of it like your own immune boosters. Most soil already contains potassium, so this is often the lowest ratio number.

Plants also need trace minerals like iron, calcium, and magnesium, but the NPK nutrients are the most critical for growth and get used up fast.

Note: One great way to boost those trace vitamins and minerals is with seaweed or kelp fertilizer, which has upwards of 60 beneficial elements. It can be worked into soil (even without decomposing) or applied in liquid form. Use seaweed fertilizer as a supplement to your main NPK fertilizers, not the main course. It has a little bit of potassium but no nitrogen or phosphorus.

Why is it important to understand NPK?

Knowing what the ratios mean will help you select the proper fertilizer for the type of plants in your containers. A complete fertilizer has all three of your NPK elements, and incomplete fertilizers may be lacking in one area if you know you don’t need that element.

It can be helpful to test your soil first before adding soil amendments. Just like you would get a test of your vitamin D levels before choosing how to supplement, you don’t want to tax your plants with the incorrect nutrient levels.

However, if you’re using commercially prepared potting soil or potting mix, the NPK ratio should be on the package. In that case, it’s most important to understand when you should supplement, and with which nutrients.

Also pay attention to what your plants are doing that week. When vegetables get to the flower producing stage, for example, you’ll want to back off on nitrogen so you can encourage flowers and fruit, rather than supporting the leaves.

Types of fertilizer

Type	Details	When to use (follow package directions)
Granular	Slow-release pellets usually mixed into the potting medium. Releases nutrients when you water the soil.	Mix into potting soil when planting.
Liquid	Quick-release, often high in nitrogen for fast green growth. Ex: fish emulsion, liquid kelp. Good way to boost lackluster young plants.	Apply every 2-4 weeks early in the season. Switch to a lower-nitrogen formula before flowering.
Water-soluble	Quick-release dry granules meant to be reconstituted in water.	Apply every 2-4 weeks, alternating with liquid fertilizer. Like with liquid fertilizer, high nitrogen is good early on. Switch to low-nitrogen formula just before flowering.
Foliar	Liquid fertilizer applied directly to leaves rather than in the soil. Plants can absorb essential nutrients this way. Note: not a substitute for well-nourished soil!	Apply liquid fertilizer this way once/month or when your plant looks weak despite adequate water.

How often to fertilize container plants

Now that you understand the basic components of fertilizer and what they’re used for, it’s important to learn about timing.

Feeding plants the right nutrients at the right stage of growth can make the difference between a weak plant and a bumper crop!

Seedling stage

Work granular fertilizer into the potting soil before planting your edible plant seedlings. Watch for good phosphorus levels to get a good root base. You do not want to encourage the plants to grow too tall, too fast.

When growing foliage

Make sure you have good nitrogen levels to ensure green growth. Occasional foliar feeding at this stage can be helpful as well.

When plants are stressed or slow-growing

Cut off dead foliage, unhealthy fruit or damaged stems. Then add liquid fertilizer to the soil or directly to healthy leaves.

Best fertilizer for container plants

You have a lot of options when buying fertilizer for container gardens. Here are some of my favorites, based on my experience and expert opinions:

Granular fertilizer recommendations

• Jobe’s Organics All Purpose Fertilizer – Certified organic, well-balanced 4-4-4 that includes a unique blend of healthy micro-organisms to help break it down in the soil.
• Jobe’s Organics Vegetable & Tomato Granular Plant Food – Certified organic, 2-5-3 fertilizer designed to help edible plants bear more fruit.
• Dr. Earth Organic 5 Tomato, Vegetable & Herb Fertilizer – Certified organic, 4-6-3 fertilizer perfect for flowering and fruit production.

Liquid fertilizer recommendations 

• Fox Farm Liquid Nutrient Trio – Awesome combo pack with their Big Bloom, Grow Big and Tiger Bloom liquid fertilizers. Each formula is targeted for a specific stage of growth – a great way to have everything you need for the season (and they come with super-specific weekly instructions!).
• GS Plant Foods Organic Liquid Fish Fertilizer – This year my heavy feeders (pumpkins, zucchini and cucumbers especially) have been LOVING this stuff! Also great for foliar application. Highly recommend. Note: Fish emulsion is high in nitrogen, so switch to something with a lower nitrogen percentage the minute you see flowers or fruit about to form.
• GS Plant Foods Organic Kelp Fertilizer – Kelp or seaweed fertilizer is an excellent supplement to other fertilizers. It contains dozens of trace minerals and vitamins. It also has unique hormones that stimulate root growth and boost overall plant health. It’s awesome for seedlings and throughout the growing season.

DIY fertilizer

You can easily add your own DIY fertilizer to your garden plants. Even if you aren’t up for full composting or don’t have room for it, you can see benefit from straight ingredients.

• Coffee grounds: save your used coffee grounds as they can add acidity
• Eggshells: wash and crush to add lime (calcium carbonate)
• Banana peels: we eat them for their potassium, and some gardeners believe burying a peel in your garden or watering with banana water (or another fertilizer “tea”) does the same thing
• Grass clippings: as long as you know what is on your lawn (avoid this if you use chemicals on your grass that you don’t want on edible plants), you can collect the clippings to mulch your containers and give a nitrogen boost
• Aquarium water: if you have a fish tank, the water you swap out on cleaning day is full of nitrogen
• Epsom salt: dissolve in water for plants that love magnesium and sulfate

Should you add fertilizer to potting soil?

Most commercially produced potting soil and potting mix contains a small amount of slow-release fertilizer.

That can last a couple of months in most cases, but it’s a good idea to incorporate well-balanced slow-release fertilizer granules in your potting soil when you are preparing the pots for the season.

This allows a ready stock of nutrients for your plants as they grow throughout the season.

I especially recommend supplementing your potting soil for heavy feeders like celery, cucumbers, okra, pumpkins, squash and tomatoes. They use up nutrients in the soil so fast!

Remember to keep the amount of phosphorus in mind to promote root growth to start! The Jobe’s options above are 4-4-4 and 2-5-3, which are great ratios for long-term feeding because they aren’t too high in nitrogen.

How to recharge potting soil

Your plants will use up the nutrients in the soil through the growing season, so when fall arrives the soil is depleted. You can revive it for use in the spring with my potting soil tips.

If you want to get very specific with reviving your soil, perform a pH test. As mentioned above, a soil test will also help you achieve the perfect NPK balance.

If you only do one thing and want the “easy button,” then simply make sure and pick up a high-quality bagged compost and a slow-release, well-balanced granular fertilizer to add in to your potting soil at the start of each season.

Put your newfound fertilizer expertise to good use! Try these ideas in your container garden:

• How to grow cucumbers in pots
• Growing pumpkins in containers
• Growing zucchini in containers
• Growing okra in pots
• How to grow catnip in pots
• How to grow parsley in a pot
• How to grow chamomile in a pot
• How to grow green beans in a pot

Growing your own herbs and veggies can be very fulfilling. Understanding fertilizer for container gardens can help you make the most of your growing season.

You don’t have to be a scientist to understand the basics like the NPK ratio and how often to fertilize plants. And if you’re ready to dig in further, it only takes a few simple tests and learning how to read fertilizer bags to achieve success.

I hope these tips help you take your container gardening to the next level and you won’t feel so overwhelmed with the fertilizer jargon. Share comments and questions in the comments, and let us know how it goes!