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Top 5 Best Fertilizers For Onions (2023 Review)

By: Katharine Oden

best fertilizer for onions

Onions weren’t my top choice for my first vegetable gardens, but since I’ve learned to cook with fresh harvests a little better, I started wanting beautiful, home-grown onions. You know the type, braided and hanging to dry in a kitchen or by a hearth? Goal!

Fertilizers are important for all vegetables, but especially onions, which need certain nutrients to stay moist and flavorful and combat pests.

We’ll get into that in detail in a bit.

You might think you can just use a good fertilizer for root development, but onions are a little different in this regard.

I’ll also discuss how to choose the right fertilizer for your onions, complete with a list of my top 5 favorite fertilizers for onions, including budget-friendly options and some choices for short- and long-day onions, for cool and warm climates, respectively. 

This year I’m actually growing shallots for the first time.

Shallots are a close botanical relative of onions, although there are differences. Shallots grow like garlic and have a sweeter taste.

Hot fact: Until 2010, shallots were considered a separate species, but that was ultimately nixed and shallots and onions are now grouped together.

Basically you can get as fancy as you want, with onions, or keep it basic. Keep scrolling to read my deep-dive into the best fertilizer for whichever onions you choose.

Alaska Fish Emulsion Fertilizer
  • Robust nitrogen for starting onions
  • Promotes strong growth
  • Water-soluble and scented
  • Robust nitrogen for starting onions
  • Promotes strong growth
  • Water-soluble and scented
View on Amazon
Jobe's Organics Bone Meal
  • Slow-release nutrients
  • Infrequent application
  • Improves soil structure
  • Slow-release nutrients
  • Infrequent application
  • Improves soil structure
View on Amazon
Espoma Organic Garden-Tone
  • Can be used for all plants
  • Long-lasting nutrients
  • Organic formula for better soil
  • Can be used for all plants
  • Long-lasting nutrients
  • Organic formula for better soil
View on Amazon
Osmacote Smart-Release Plant Food Plus
  • Won't burn plants
  • Powerful nutrient formula
  • Can be used everywhere
  • Won't burn plants
  • Powerful nutrient formula
  • Can be used everywhere
View on Amazon
Muriate of Potash Fertilizer
  • Packed with potassium
  • Easy to water in
  • A little goes a long way
  • Packed with potassium
  • Easy to water in
  • A little goes a long way
View on Amazon

My Overall Top #1 Pick: Alaska Fish Emulsion Fertilizer

Fish emulsion is an excellent choice for starting onion seedlings. Some of you may have tried fish emulsion before and witnessed the difference it can make in growing vegetables in your home garden. Although you can see much less of the onion plant, you can trust that Alaska Fish Emulsion Fertilizer is working wonders beneath the soil.

This is a natural and organic fertilizer derived from fish byproducts, making it rich in the essential three nutrients of nitrogen, phosphorus, and potassium.

These nutrients are vital for the healthy development of onions, getting your plants off to an excellent start. It’s especially the strong dose of nitrogen in this 5-1-1- formula that jump-starts vigorous growth.

Fish emulsion is also great for its micronutrients like calcium, magnesium, and trace elements that contribute to strong and vigorous seedlings.

Alaska Fish Emulsion Fertilizer touts a gentle formula that won’t burn and, an added plus, the scent of wintergreen to mask unpleasant fishy odors. Anyone who’s suffered the stench or avoided using fish fertilizer indoors will appreciate this!

Another benefit to its gentle formulation and relatively pleasant scent is that Alaska Fish Emulsion Fertilizer can be applied either by foliar spraying or watering the seedlings at their base.

And if you’re looking for value, Alaska Fish Emulsion Fertilizer has it with this concentrated formula. Follow directions you’ll be using 2 tablespoons per gallon of water for every 25 square feet of soil, and this is a 3.7-liter bottle — that means you’re buying 125 applications, a real deal that could carry you through several seasons of a large garden.

Top 5 Fertilizers for Onions

Need top picks now? No worries! Here are my top 5 fertilizers for onions:

  1. Alaska Fish Emulsion Fertilizer (Best Overall Pick)
  2. Jobe’s Organics Bone Meal (Best Budget Pick)
  3. Espoma Organic Garden-Tone (Best Pick for Short-Season Onions)
  4. Osmacote Smart-Release Plant Food Plus (Best Pick for Hot-Climate Onions)
  5. Muriate of Potash Fertilizer (Best Pick for Onions Just Before Harvest)

Why Do You Need a High-Quality Fertilizer For Onions?

Onions seem like a pretty basic vegetable — no showy flowers or above-ground fruits. Do they really need fertilizer at all?

The answer is yes. There is always something you can do to improve soil nutrient levels and soil structure to grow bigger, juicier onions that’ll resist diseases and pests. You wait all season to reveal your underground harvest — why not take steps to make sure it pays off?

A little like growing a nice big pumpkin, you want your onion plants to get the right nutrients they need to pump into bulb formation, and not waste overly much ‘growing energy’ on foliage.

This means specifically balancing nitrogen application at the outset with phosphorous and potassium closer to harvest time.

Onion Diseases

It’s important to do what you can to encourage good growth and disease resistance in onions so that you can rest assured your underground harvest is thriving. Here are some common ailments and what you can do with fertilizer to prevent or remedy them.

Fusarium Basal Rot is caused by the Fusarium fungi that attack the base of onion plants, causing yellowing and wilting. The bulbs may develop soft rot, and a pinkish-white fungus may be visible.

To prevent fusarium rot, improve soil drainage with organic fertilizers to avoid the conditions that breed it.

Botrytis Leaf Blight (Gray Mold) is another fungal disease that leads to grayish lesions on leaves, which later turn brown and papery. It thrives in damp conditions and can spread rapidly, especially during cool, wet weather.

To remedy, remove infected plant material to promote good airflow to control the spread of this disease. Apply a fish fertilizer that will provide a rapid nutrient boost.

Onion White Rot is a soil-borne disease caused by a fungus that affects the roots and bulbs of onion plants. Infected plants show stunted growth, yellowing, and ultimately bulb decay.

To halt onion white rot, rotate your crops every season/year so rot microbes don’t have a chance to build up in the soil. Avoid introducing contaminated soil with infected transplants.

Onion Pests

There are also certain specific pests that attack onions and that you can stop in their tracks by cultivating healthy onions from the get-go.

Onion thrips are tiny, slender insects that feed on onion leaves. What you’ll see is a series of silver streaks on the leaves. Severe infestations will adversely affect plant growth and onion yield.

To combat thrips, start with a nitrogen-rich fertilizer for robust early growth but guard against overfertilizing, which can weaken plants as much as underfertilizing.

Monitoring for thrips is key, and then spraying with insecticidal soap or neem oil as soon as you find signs of them. Introducing beneficial insects like ladybugs can also help control thrips.

Onion Maggots

The larvae of onion flies, known as onion maggots, tunnel into the onion bulbs, causing rot and decay and devastating your careful crop in a matter of days. What you’ll see above ground is yellowing and wilting of the leaves.

With onion maggots, prevention is key. Start fertilizing within the first few weeks of your seedlings’ growth so they get off to a healthy start. Consider using floating row covers, crop rotation, and sticky traps to help prevent infestations.

Giving your onion starts adequate nutrition with a balanced or nitrogen-rich fertilizer is always the best way to prevent disease and pests, and you can see onions have their fair share of ills to fight off.

If things get bad in the onion patch, you might have to remove and destroy infected plant material, use organic pesticides, and even spray plants preemptively (known as “integrated pest management”).

I personally think regularly supplementing with a good fertilizer and keeping an eye on plants as they develop is the best way. Onions can be tricky because below-ground, but if you rely on any one of these top fertilizers in your garden you’ll be off to a running start.

This video gives a more in-depth look at onion thrips and how to avoid these little nasties from ruining your harvest.

Fertilizing your onions is the best preventative medicine against diseases and pests. Think of it as good nutrition!

And if you consider coffee nutrition, onions will love your leftover coffee grounds for a hit of nitrogen. As a bonus, coffee grounds can repel voracious pests like slugs.

What to Consider When Choosing The Right Fertilizer For Onions

Growing Conditions for Great Onions

Onions need nitrogen when they’re first starting and then plenty of phosphorous (P, on fertilizer labels) and potassium (K) later on for good bulb development.

A good rule of thumb is to start ramping up phosphorous and potassium about halfway to the harvest time stated on the seed packet, or immediately for onion sets, which have already begun to bulb.

You can fertilize all you want, but if you have the wrong onion for your climate, you’re not going to get much in the way of harvest.

Onions are generally a cool-weather crop, but you can grow them just about anywhere, even in a plastic pot on your hot fire-escape — with the right nutrients and watering conditions.

Alternatively, if you live with winters, you can actually over-winter onions planted in the fall to get an early start on next summer’s crop. There’s nothing like watching those green shoots first emerge in the chilly spring air!

You’ll need to pick the right onion variety for your climate. Onions actually exhibit something called photoperiodism, which means they’re sensitive to the amount of light and dark they receive over the day.

Different onions therefore have different demands for light and dark. Look for short day, long day, and intermediate onion varieties and buy according to your climate and season.

For cold climates, for example, you’ll need to select long-day onions. These will thrive under your seasonal conditions and with the top picks for fertilizers for onions I’ve gathered here.

If you’re starting from seed, sow them directly into the ground or start indoors in containers as much as 12 weeks before the last frost date (!!!). Sow them densely — you’ll be separating them and thinning them later into the garden.

Starting onions like this prevents them from drying out and also stops them from bolting, if you’ve got hot conditions. Bolting means going to seed early and affects the taste of onions and all vegetables.

You can also find smaller onion varieties that are well adapted to a cold climate. As with any onion, however, you need to make sure they have enough room to develop a healthy bulb and fertilize with a nitrogen-rich solution/granules to get them going on the right track.

Using Onion Sets

Using onion sets, or small dried bulbs, is a good way to get a head start on a season. Buying onions at this stage saves you months of growing time, not to mention time and money fertilizing.

You can find onion sets at nurseries and big-box stores like Home Depot. They can be put in the ground around the last frost date, even a week or two before it. Cover your onion seedlings with a sheet if a hard frost is forecast.

That said, they’ll tolerate cold pretty well and come back. Even young onions can tolerate some brief freezing temperatures. Fertilize your onions regularly up until harvest.

If you’re starting onion sets in the fall to overwinter, stop fertilizing around the first frost date. They’ll go dormant and reward you in the spring with fresh growth.

Soil Preparation for Onions

Since the onion you want to eat grows underground, it pays to have the best soil conditions possible, soil that’s rich but airy, drains well, and is free from pathogens and pests. A good fertilizer will help these conditions instead of inhibit them.

Particularly fertilizers with organic macro- and micronutrients can improve the soil structure.

Organic fertilizers don’t disrupt the natural ecosystem of the soil and don’t leave behind any unnatural residue once the plant absorbs what it needs. They’re also less likely to burn your plant since they’re all-natural. However, they can be more expensive.

Tilling the soil can loosen it and aerate it, but most farmers and researchers now advise against tilling overly much because it can damage soil and lead to erosion.

I personally have tilled my garden with a rented rototiller once over the past three years and don’t plan on doing it again for many more. I only tilled it because the weeds were out of control and I wanted to get a fresh start and work in a truckload of aged manure.

Take care of your soil by treating it gently and adding good-quality fertilizers and you’ll develop optimal growing conditions for onions.

My Reviews of the Best Fertilizers for Pothos

1. Alaska Fish Emulsion Fertilizer (Best Overall Pick)

This water-soluble formula is my top choice for many reasons. It’s easy to use and has a 5-1-1 NPK ratio, with high nitrogen to really get onions growing well from the start.

Its naturally-scented formula is a great plus for anyone who’s not a fan of that distinctive old fish smell (…I have not met one fan of that smell).

Fish fertilizer, made with byproducts of the fishing industry, is full of macro- and micronutrients like calcium, magnesium, and trace elements key to seedling growth and development.

PROS (+) CONS (-)
+ Great for starting onions
+ High nitrogen for strong growth
+ Water-soluble and scented
– Still not a smell you want to live in

2. Jobe’s Organics Bone Meal (Best Budget Pick)

This is a classic granular formula for more slow-release nutrients, espcially phosphorous key for onion bulb growth.

Bone meal is one of those organic supplements we mentioned above that will help soil structure.

Another plus is it doesn’t have to be applied as often. This is particularly handy if you’re having a wet summer and don’t want to be watering in fertilizer every week or two.

Talk about wet summers, this year in New Hampshire we’ve had flood conditions almost weekly since the last frost date. I do not want to be watering my plants for fear of rotting my plants!

PROS (+)CONS (-)
+ Slow-release nutrients last in soil
+ Infrequent application
+ Will actually improve soil structure
– Costly for large gardens
– Hey, some gardeners don’t want to put bones in their garden, we get it

3. Espoma Organic Garden-Tone (Best Pick for Short-Season Onions)

This is my pick for you gardeners in cool climates with a short growing season and “long-day” onions. (Just remember “mullet” or short-long when choosing onion varieties.)

It’s another organic pick that will aid your soil conditions in the long run.

It has a 3-4-4 NPK ratio to blast-off great bulb growth with phosphorous and potassium and get you in onions before the first frost date hits.

That said, it is another of the pricier options that’s really better for small onion patches. A bag will cover just 50 square feet, which we enthusiastic gardeners use up faster than you can say “heirloom.”

PROS (+) CONS (-)
+ Can be used for all plants
+ Long-lasting nutrients
+ Organic formula improves soil
– Pricey for gardeners with large plots

4. Osmacote Smart-Release Plant Food Plus (Best Pick for Hot-Climate Onions)

Osmacote is one of those innovative fertilizer brands that gets rave reviews. Most people who have used it swear by it and won’t buy anything else.

At an NPK ratio of 15-19-12, this formula has that extra kick of phosphorous that vegetables like onions just love, especially if you’re coaxing along short-day onions in a steamy climate. Drought resistance, anyone? Osmacote has you covered.

If it’s too pricey for you, consider that you only need apply once a season or 6 months. This formula packs a punch.

PROS (+)CONS (-)
+ Won’t burn plants
+ Contains a powerful nutrient formula
+ You’ll use it everywhere
– More time-consuming hand-application, which could also mean uneven results

5. Muriate of Potash Fertilizer (Best Pick for Onions Just Before Harvest)

This is pure potassium in granular form and will jet-propel your onions to harvest.

Despite the granular forum, you’ll get a fast-acting nutrient delivered to maturing onions when they need it. Be careful not to overdo it and burn plants, however — this is concentrated stuff (NPK 0-0-60!).

It’s water soluble and for those of you trying to “buy local,” this company prides itself in its made-in-the-USA products. It’s also worth noting that 11 states ban potassium fertilizers because of their usage in home-made explosives. Stay within the law, kids.

PROS (+)CONS (-)
+ Packed full of potassium for onion bulbs
+ Concentrated formula easily water soluble
+ A little goes a long way
– Not available in many states.

My Top Pick: Alaska Fish Emulsion Fertilizer

Onions benefit from a range of different fertilizers, as you can see here. Choose any one and you’ll be improving your onion harvest, there’s no doubt.

That said, Alaska Fish Emulsion Fertilizer has my heart, if not my nose. No, despite the odor (or, in this case, the masked odor), I love using nutrients that would go to waste otherwise, like the fish not fit for the dinner table.

It’s satisfying to think of all those elements from the sea going into my soil and bathing my onions in stinky goodness.

It’s also an easy-to-use and economical choice. You’ll find yourself learning to love this thick, icky stuff for all its benefits for your onions, your vegetable garden as a whole, and, heck, landscape plants, to boot.

Final Thoughts

No matter what fertilizer you go with from this list, rest assured, you’ll be plucking some nice, juicy onions from the ground before you know it.

Be sure to dry them carefully to preserve that harvest — Iowa State University has some hot tips for drying onions you might want to check out.

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