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7 Best Onion Fertilizers for 2022 | How and When to Use

Onions are one of the more popular crops to establish at home so understanding the ins and outs of fertilizing onions is also an essential component to ensuring healthy and tasty yields.

With a massive portion of growers moving to more organic methods of growing crops, it can be a little overwhelming when trying to choose the best onion fertilizer for your plants. Here is our full guide on how to fertilize onions along with a few helpful growing tips.  

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Best Onion Fertilizers How and When to Use

Product


Price

1. Dr. Earth Organic 5 Tomato, Vegetable and Onion Fertilizer

Dr. Earth Organic 5 Onion Fertilizer

2. Miracle-Gro Shake ‘n Feed All Purpose Continuous Release Fertilizer for Onions

Miracle-Gro Shake ‘n Feed All Purpose Fertilizer for Onions

3. Down to Earth Organic Bio-Fish 7-7-2 Onion Fertilizer Mix

Down to Earth Organic 7-7-2 Onion Fertilizer Mix

4. SoilMoist NitroGreen 100% Organic High Nitrogen Fertilizer for Onions

SoilMoist NitroGreen 100% Organic Fertilizer for Onions

5. Fox Farm 6-4-4 Grow Big Liquid Concentrate Onion Fertilizer

Fox Farm 6-4-4 Grow Big Onion Fertilizer

6. Osmocote Smart-Release Plant Food Flower & Vegetable

Osmocote Smart-Release Plant Onion Fertilizer

7. Jobe’s Tomato and Onion Fertilizer Spikes

Jobe’s Onion Fertilizer Spikes
A kid fertilizing onions

Onions are easy to grow, meaning they are accessible to all growers regardless of experience. Being a key ingredient in so many common culinary masterpieces in households and commercial kitchens across the globe, it’s no wonder the humble onion plants have become so widely distributed.

Most varieties are harvested between May and July, but this will mean you need to supply your crop with the necessary nutrients at the right time to ensure a bountiful yield. 

A Short Introduction to Onions

Part of the family of Allium plants alongside shallots and garlic, most varieties of onions are native to the middle to western regions of Asia.

They contain many vitamins, essential minerals and some antioxidants. Furthermore, they can help in treating colds and coughs and can also help enforce regular digestion in the body. 

When growing and fertilizing onions, it is important to know that they are known as heavy feeders and they thrive in very fertile soils, meaning you will need to know when to feed them and which fertilizer for onions are best for your plants.

A little extra care with your onion crop will ensure you grow high-quality onions that also taste wonderful. 


Most Popular Onion Varieties to Grow at Home

Spring Onion

Spring Onion are delicious and can complement almost any dish

These bunching alliums are delicious and can complement almost any dish. They feature a tender yet fresh taste and texture, taking only about 2 months to grow and be ready for harvest. 

Red Burgundy Onion

Red Burgundy Onion has a mild and sweet flavor

This fresh onion variety will take longer to reach full maturity but it produces large 3-to-4-inch white bulbs with red rings and red skin. This variety has a mild and sweet flavor, meaning you can enjoy it fresh or as a cooked-in addition to dishes. 

Yellow Sweet Spanish Onion

Yellow Sweet Spanish Onion grows to massive bulbs in about 120 days

This is a sweeter cultivar of onion that grows to massive bulbs in about 120 days. Offering a sweet and mild flavor profile, these onions also grow excellently in our endemic climates. 

Walla Walla Onion

Walla Walla Onion is a very popular variety known to produce large vegetables

This is a very popular variety known to produce large vegetables. They have a very pleasant sweet and mild taste with a higher water content than other varieties. They can be cured and stored in a cool and dark environment for about only a month. 

Candy Onion

Candy Onion is a hybrid cultivar perfect for storage

This hybrid cultivar is perfect for storage. With a slightly crisp and tangy flavor, these onions have become widely distributed. They take about 200 days to reach full maturity and are delicious, enjoyed fresh or cooked.

Check out our guide and get to know more about the different types of onions and their growing habits.


What Nutrients do Onions Need to Grow Best?

These undemanding, hardy vegetables still need certain nutrients and growing conditions to thrive and grow best. Their shallow root systems mean that they need to get most of their growing requirements, including nutrients, from the top surface of the soil.

This is where you come in. By supplying your crop with the right balance of nutrients at the right time, you will give yourself the best chances to yield fruitful crops. 

Here are some of the nutrient requirements when fertilizing onions:

  • They tend to prefer moderate to high amounts of nitrogen and higher levels of potash and phosphate as well. 
  • Soil should be well-drained and slightly acidic with a pH level of between 6.0 and 6.8. 
  • Soil can easily be boosted by adding some lime, either in powdered or pellet form. 
  • Fertilizing onions can make or break a yield. Too many of the wrong nutrients can quickly sway the odds against you. You’ll want to feed your onions with fertilizers that offer a total package of nutritional elements so your veggies can benefit as much as possible.  
  • The first application should be about 3 to 4 weeks after planting and then thereafter, fertilizing onions every 2 to 3 weeks is recommended.

How to Choose the Right Onion Fertilizer

Ideal fertilizer mixes for onions will contain high levels of nitrogen (the N in N-P-K) as while the onions develop, they will pull most of the nitrogen out of the soil, leaving the ground less nutritious overall.

These fertilizers will also allow you to grow bigger onions so they are certainly worth paying attention to. The best application tends to be that of a slow-release dosage versus something more fast-acting.

However, onions still need both phosphorus and potassium to grow and develop properly. So, fertilizers should still be well-balanced overall. 

A man adding onion fertilizer

Types of Onion Fertilizers

Fertilizers can be in a liquid, dry or spiked form. Liquids tend to be quickly absorbed and are oftentimes a little pricier. Dry, granular fertilizers are the most popular types due to their affordability and slow-releasing nature.

Fertilizer spikes are a more recent feeding method where spikes filled with fertilizer are placed near plants to provide a consistent slow dosage over time. Organic fertilizers tend to be the best picks when growing vegetables for consumption at home. 

How Much Fertilizer Should You Use?

For nitrogen-only fertilizers, you’ll want to spread about a cup over a 20-foot row of onions. Repeat this every 2 to 3 weeks for the best results. By using potassium and phosphorus fertilizers like a 0-10-10 NPK ratio, you can supplementarily add nitrogen as needed without worrying about poisoning your crop.

Ideally, you’ll want a high nitrogen vegetable fertilizer that still has potassium and phosphorus. Be sure to follow the recommended feeding instructions depending on the product purchased. 


How to Test Your Soil Before Fertilizing

It’s always a good idea to test your soil first before applying fertilizer. This will ensure you’re not overfeeding your plants and will ensure the soil is in optimal condition to grow and yield the best onions possible. 

The easiest and most affordable way to test your soil levels is by performing a simple analysis test with a soil test kit. Test your soil before preparing your planting beds and test afterwards as well. 

Perform multiple tests around different areas of your beds to get an entire picture of the condition of your ground before planting and establishing your onion crop. These quick soil tests are easy to use and give you results in a matter of minutes. 


Best Onion Fertilizer Reviews for 2022

1. Dr. Earth Organic 5 Tomato, Vegetable and Onion Fertilizer

Dr. Earth Organic 5 Tomato, Vegetable and Onion Fertilizer

Source: amazon.com

Made with 100% organic and natural ingredients, this fertilizer allows for nutritious and tasty vegetable yields and will encourage more abundant crops.

 

It features a complete combination of probiotic strains that are beneficial soil microbes. It will feed your onions for several months and is easy to use. This is our top pick. 

2. Miracle-Gro Shake ‘n Feed All Purpose Continuous Release Fertilizer for Onions

Miracle-Gro Shake ‘n Feed All Purpose Continuous Release Fertilizer for Onions

Source: amazon.com

With a 1lb, 4.5lb and 8lb pack to choose from, you’re bound to find something that fits your needs and budget with Miracle-Gro. It is packed full of essential vitamins and nutrients to feed and nourish your veggie crop above and below the surface.


It also contains vital micronutrients meaning your crop's overall health will be fully accounted for. Being easy to use and easy to spread, this fertilizer is our top pick for great value. 

3. Down to Earth Organic Bio-Fish 7-7-2 Onion Fertilizer Mix 

Down to Earth Organic Bio-Fish 7-7-2 Onion Fertilizer Mix

Source: amazon.com

Coming in a 5lb box, this fertilizer is loaded with only the finest marine-based ingredients to help improve your soil while growing outdoor plants and vegetables.


It is perfect for heavy feeders like onions and can be used throughout the entire growing season, helping to ensure bountiful yields. It has the ideal combination of nitrogen and phosphorus for fertilizing onions and is one of our premium choices. 

4. SoilMoist NitroGreen 100% Organic High Nitrogen Fertilizer for Onions

SoilMoist NitroGreen 100% Organic High Nitrogen Fertilizer for Onions

Source: amazon.com

With a 13-0-0 NPK ratio, this fertilizer is perfect for those who would prefer to use nitrogen-only fertilizers. Be sure to stick to the recommended feeding schedule in order not to overfeed your onions with nitrogen.


This fertilizer is 100% water soluble so you can closely control your dosage each time making it one of our top picks for great value. 

5. Fox Farm 6-4-4 Grow Big Liquid Concentrate Onion Fertilizer 

Fox Farm 6-4-4 Grow Big Liquid Concentrate Onion Fertilizer

Source: amazon.com

A premium choice and premium product, this fertilizer contains the ideal NPK ratio for fertilizing onions and is an excellent organic option.


With high-quality ingredients like earthworm castings, Norwegian kelp and many essential micronutrients, this special brew of fertilizer will allow for more abundant yields and healthy root development. 

6. Osmocote Smart-Release Plant Food Flower & Vegetable

Osmocote Smart-Release Plant Food Flower & Vegetable

Source: amazon.com

This fertilizer will promote vigorous top-growth and strong root development for your crop. It is perfect for vegetable gardens and is designed to promote flavorful veggies.


It can feed for up to 4 full months so it’s certainly one of our top picks for great value. 

7. Jobe’s Tomato and Onion Fertilizer Spikes

Jobe’s Tomato and Onion Fertilizer Spikes

Source: amazon.com

Coming with 50 spikes in its resealable bag, this fertilizer is also organic making it one of our top picks. It is a time-release fertilizer that has been pre-measured to provide your crop with the perfect balance of nourishment at its roots.


This fertilizer will improve your soil, help to resist disease and deter pests. The spikes are easy to use and won’t create clutter in your beds. 

Fertilizing Onions – Quick How-to Guide

Getting rid of weeds

It is very important to make sure that your onion beds are free from weeds before applying any fertilizer. If you feed your onions while there are weeds around, you’ll also be stimulating the growth of the weeds which can quickly affect the health and robustness of your onions.

Be sure to remove any weeds from the surrounding areas before applying fertilizer. See our product review on the best weed eaters for 2022.

Using a gardening hoe and other similar tool, gently maneuver between your onions to remove the weeds covering the surface and the ones just below the surface. Be sure to take the weeds off-site after removal. 

Buy Your Preferred Onion Fertilizer

Remember to feed your onions with healthy amounts of nitrogen, potassium and phosphorus in the right proportions for optimal growth.

Nitrogen will help with leaf growth and overall plant growth, potassium will give your onion plants strength and vigor, and phosphorus will ensure healthy root development. 

When to Fertilize Onions

Onions will need nutrients throughout their growing season. It is recommended that the first application should be about 3 to 4 weeks after planting and then thereafter, fertilizing onions every 2 to 3 weeks is best.  

Measure Out and Apply the Correct Amount of Onion Fertilizer

Most of the time, it is better to follow the recommended instructions for the specific onion fertilizer you choose to buy and use. This will ensure you’re not overfeeding your plant or damaging it. 

You may need to sprinkle and spread more granular-based fertilizers, whereas liquid fertilizers may need to be diluted into water and then evenly distributed across your onions.

Spread your fertilizer evenly by using a lawn spreader and as recommended evenly along the lines of your onion crops at the recommended dosage and intervals. 

Watering Onion Fertilizer In

Using your watering can gently water the fertilizer in after each application. Be sure to flip your watering cans rose or cap upside down as this will ensure the water first flows up and then falls onto your onions.

This will help to mimic rainfall which will prevent your fertilizer from being washed away by quick, hard blasts of water. Apply only just enough water to allow your spread fertilizer to be dampened and lightly watered into the soil.

Be sure to avoid overwatering as onions hate sitting with wet feet.


Planting & Growing Tips for Onions

A woman using the best onion fertilizer
  • Prepare your soil so that it is loose, well-draining, high in organic materials and slightly acidic. The looser your soil, the bigger the bulbs of your onions can grow. 
  • Dig some organic compost into your soil before planting and make sure it is well broken down and evenly distributed.
  • Be sure to position your crop in an area that gets as much full sunlight as possible. 
  • Avoid planting your onions on ground that has grown other alliums within the last two years. This will ensure your crop avoids any potential fungal diseases. 
  • Onions can easily be grown from seeds that have been started about 10 to 12 weeks before you intend on transplanting them outside. In colder regions, start your seeds inside in February or plant them directly into the ground in regions with warmer winters. 
  • For single plants, space with 5 inches each way as a minimum. For rows, space each plant 5 inches each way with a row gap of 9 inches at a minimum. 
  • Onions are mature and almost ready to harvest when their tops have yellowed and begin to fall over. Harvest bulbs only once the tops have turned brown.

Fertilizing Onions Frequently Asked Questions

Is tomato fertilizer good for onions?

The Old Farmer’s Almanac Organic Tomato and Vegetable Fertilizer has a good NPK ratio of 8-4-8 that will have all the nutrients your onions need to thrive.

When should you stop fertilizing onions?

It is best to stop fertilizing onions when they start to bulb. 

Is bone meal good for onions?

Yes, bone meal can act as an excellent fertilizer for onions because it is a great source of phosphorus, which is ideal for root vegetables such as onions.

Interested in establishing other fast-growing vegetables in your garden? Be sure to check out our other helpful guide below:


Feast on Tasty & Healthy Onion Yields by Fertilizing Onions the Right Way

Now that you know everything you need for fertilizing onions and which onion fertilizers are best for your crop, you can look forward to yielding tasty and complementary additions to your culinary endeavors at home.

Whether you choose to use granular, liquid or spiked fertilizers, you can be rest assured that you will be providing your plants with everything they need to grow and thrive best.

With our guide on the top picks, premium choices and best-value fertilizers, fertilizing onions doesn’t have to cause you any more confusion. 

About the Author Pat Moreno

Pat is our gardening tool expert here at Sumo Gardener. Working for many years as a private and commercial landscaper, Pat has used almost every type of gardening tool there is. Along with a vast knowledge for types of plants and putting together an amazing looking and maintainable garden, Pat developed a passion for gardening tools as he found that using the right tools vastly improved the ease and outcome of any landscaping job he undertook. When spending hours, days or years using a particular tool, you want to make sure you’ve got the best one for the job, and Pat is the right guy to guide us to the best gardening tools.

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