10 Best Fertilizer for Onions of 2023 (Reviews)

Discover the best fertilizers for growing healthy onions with this complete buying guide.

By
Sean Stratton | Updated January 3, 2023

Onions are heavy feeders, so they demand extra nutrients to grow healthy and big. Fertilizers pack all of the nutritional elements your veggies need to grow.

Best Fertilizer for Onions

You should first fertilize your onions when you plant them and then reapply the fertilizer every 2 to 3 weeks.

onion plants in a row

What Type of Fertilizers Are the Best for Onions?

As heavy feeders, onions rely on fertilizers for proper root and bulb formation. You should look for fertilizer high in nitrogen. When looking for the NPK ratio, choose a fertilizer with a higher N value.

The following macronutrients are essential components of an onion fertilizer:

  • Nitrogen (N): It boosts the quality of bulbs with fast growth. 
  • Phosphorus (P): When it comes to rooting and forming bulbs, phosphorus acts as a health promoter. 
  • Potassium (K): Potassium is a vital nutrient for cell division and proper bulb production, adding taste.

Top 10 Best Fertilizers for Onions Reviews

We review our top 10 onion fertilizers below.

1. Jobe’s Organics 09026, Vegetable and Tomato Plant Food

Jobe’s Organics Granular Plant Food is the best fertilizer for onions. It contains microorganisms, archaea, mycorrhizal fungi & healthy bacteria. This blend improves the long-term quality of your garden soil while helping onions resist disease and repel insects.

Jobe’s Organics is certified 100% organic. It is a fast-acting fertilizer that helps your vegetables grow even in less-than-ideal conditions.

Pros:

  • Safe for use around kids and pets.
  • No synthetic chemicals are used.
  • Improves soil conditions.

Cons:

  • The scent can be intense.

2. Cesco Solutions Ammonium Sulfate Fertilizer 5lb Bag

If your goal is to lower the pH level of your soil to give your plants a little bit of acidic and healthy soil, this ammonium sulfate fertilizer is the thing for you. It has 24% Sulfur and 21% nitrogen, making this 21-0-0 fertilizer an excellent choice for your onions, tomatoes, garlic, potatoes, and other vegetables.

The ammonium sulfate in the fertilizer works excellent for lowering the pH level of the soil and making it acidic. As a result, besides onions or other vegetables, it’s an excellent choice for acid-loving flowers like azaleas, daffodils, rhododendrons, etc. You can even use this fertilizer for your lawn.

This fertilizer is water soluble, so it’s sufficient to put only 1 to 3 tablespoons per gallon of water. If you want to apply it to your lawn, 5 pounds of fertilizer will cover a 1,000 sq feet area. Simply pour in water, let it dissolve, and spray to feed your plants; it’s that easy.

Pros:

  • Best for acid-loving vegetable plants.
  • Not costly, reasonable price.
  • Easy to apply, water-soluble.
  • Resealable packaging

Cons:

  • Directions on the packaging aren’t very clear

Next Read: Top 6 St. Augustine Fertilizers

3. Espoma Organic Garden-tone 3-4-4 Organic Fertilizer

Espoma Organic Garden tone is the ideal organic option for onions. It can be used to grow both cool and warm-season vegetables and herbs. It’s specially formulated to promote lush and healthy growth for onions. Plus, it doesn’t contain any nitrates, nitrites, or toxic ingredients harmful to humans or other animals.

It is a premium, organic blend of essential nutrients for the onions in your garden. It is combined with 4% phosphate, 5% calcium, and 3% nitrogen which are essential nutrients for the better growth of onion plants.

It helps your plants thrive in any season and grow an abundant harvest of delicious onions. This organic blend is derived from plant material grown without artificial pesticides or fertilizers, which is safe for humans and the environment. Espoma Organic Garden Tone doesn’t need mixing as it is a ready-to-use fertilizer.

Pros:

  • 100% natural and contains organic elements.
  • Mixing is not required.
  • Helps the plant to grow faster and healthier
  • No synthetic additives are present.

Cons:

  • Excessive use may burn the plant.

4. Earthborn Elements Ammonium Sulfate, Soil Additive

The soil additive from Earthborn Elements allows you to use more than one type of fertilizer in your soil. It can be added to the soil directly before planting. After spreading it in the soil, when ammonium is in contact with water, it breaks down. This helps increase the yield of your onions.

No additives or fillers have been used, so it is 100% safe for your plants and garden. Also, it is water-soluble and safe to use in all types of soil, including clay and heavy soils. You can mix the Ammonium Sulfate with water and spray the solution directly on plants. It lowers the soil’s pH level, which helps prevent soil-borne pathogens that attack plants.

Ammonium Sulfate is a natural nitrogen source, so this fertilizer’s outcome is fresh and healthy onions.

Pros:

  • Fastens the plants’ root development.
  • Improves soil condition.
  • Stimulates early plant maturity.
  • Good source of nitrogen.

Cons:

  • No clear application instructions are included.
  • The lid can be hard to open.
  • Must use in a well-ventilated area.

5. Down to Earth Organic Bone Meal Fertilizer

Down to Earth organic bone meal fertilizer is an excellent organic fertilizer sourced from bone meal and is ideal for early crop production for onions that have been ground up to ensure a high level of nutrients. This fertilizer contains 15% phosphorus, 18% calcium, and 3% nitrogen.

It also improves the growth of young seedlings and encourages stronger root development for more vigorous plants that can withstand the stresses of growing in poor soil. It is a non-invasive organic fertilizer that is 100% natural and non-toxic.

Bone meal has been used as a natural fertilizer for centuries. This organic fertilizer enhances root development, and early season growth, and boosts fruiting in your plants. It is excellent for your garden or farm and will help you grow the best onion plants.

Pros:

  • Rich in calcium and phosphorus.
  • Improves the soil quality.
  • Increases crop production significantly.
  • Helps to absorb more nutrients and water.

Cons:

  • The smell may attract unwanted animals.

6. Worm Castings Organic Fertilizer, Wiggle Worm Soil Builder

The Worm Castings Organic Fertilizer contains pure earthworm castings, which are the end product of worm digestion. These nutrient-rich castings are very good for your soil and the health of your indoor and outdoor plants.

This odor-free fertilizer is excellent for your soil because it improves soil structure and water retention. It’s beneficial for growing onions because it protects your onion plants from certain root diseases. It also improves seed germination and onion production.

Pros:

  • Total organic.
  • Odorless and free of toxins.
  • Acts as a soil binder.
  • Improves aeration in plants.
  • Adds high concentrations of eco-friendly microbes.

Cons:

  • Packaging is not great for storing after use.

7. Lilly Miller Morcrop Vegetable Food 5-10-10

Lilly Miller Morcrop Plant Food is a slow-release fertilizer that feeds your onions steadily for up to six weeks after just one application.

Its ready-to-use formula contains natural ingredients fortified with a blend of essential nutrients. It does not need to be mixed with water, so you can apply it directly to your soil. Water your onions after fertilizing.

Pros:

  • Blend of all-natural elements.
  • NPK ratio is 5-10-10.
  • Enhances the taste of crops.
  • Improves soil fertility.
  • Adds extra trace of minerals.

Cons:

  • Its use is banned in some states due to its high phosphorus content.

8. Burpee Bone Meal Fertilizer for Bulbs

The Burpee bone meal fertilizer is an excellent food for onions. You can use it to provide organic nitrogen, phosphorus, and calcium. It is ground finely and blends in with the soil slowly but effectively.

Bone meal breaks down slowly and releases nitrogen that enhances the rich growth of the plants. It provides all the vital nutrients for onions, which is an integral part of the nutrient cycle. The phosphorus helps your plants to grow strong roots that will produce delicious vegetables.

The best use of this fertilizer is to add it to the soil before harvesting. Also, the Burpee fertilizer is OMRI Listed for Organic Gardening. Instead of onions, you can also use these for other vegetables like tomatoes, peppers, etc.

Pros:

  • Breaks down into the soil easily.
  • Helps the plant to flourish in harsh weather conditions.
  • Fastens the growth of the plant enormously.

Cons:

  • The smell may attract animals.

9. Miracle-Gro Water Soluble All Purpose Plant Food

Miracle-Gro Water Soluble All Purpose Plant Food is great if your onions need instant nutrition. This fertilizer has a high percentage of nitrogen which will help your onion bulbs grow bigger and faster. Since it feeds your onions a lot of nutrients quickly, you will need to reapply every 7 to 14 days.

It is simple to apply. Mix a tablespoon of this plant food for every gallon of water and then pour over your onions every 1 to 2 weeks.

Pros:

  • Granular and soluble in water.
  • Does not cause plants to burn.
  • Instant feed for active growth.
  • Feeds about 60 square feet.
  • NPK ratio is 24-8-16.

Cons:

  • Overfeeding may damage plants.
  • Need to reapply every 7 -14 days.

10. Osmocote Smart-Release Plant Food

Osmocote Smart-Release is specially formulated to give onion plants an early season start, with strong root development and top-growth. It has resin that is semi-permeable and covered with nutrition granules, and the coating helps water to dissolve all the nutrients into the root zone of the plant.

It is formulated to give strong root development, which helps plants get off to a strong start during their first season. It contains 14% nitrogen and 14% phosphorus for the healthy growth of onions which are the most necessary nutrients.

Osmocote plant food starts to show results in just a few weeks. Once used, it will keep flourishing for the next four months. Its dissolving process is slower to help the soil collect the nutrients for plants for a long time.

Pros:

  • No risk of burning the plants.
  • Strengthens root development.
  • Keeps the soil nutrient-rich for an extended period.

Cons:

  • May need to reapply for long-seasoned plants.

Frequently Asked Questions

A good onion fertilizer can play an essential role in achieving the healthiest and largest onions, but the best fertilizer for your onions will depend on a few key factors.

Pick a Fertilizer According to Soil Quality: Check your soil quality before purchasing an onion fertilizer. A soil test provides sufficient information about what nutrients your onions currently lack to help you find a fertilizer with the nutrients your soil needs.

Fertilizer Balance (NPK): When buying onion fertilizer, the NPK ratio is one of the most important things to consider. NPK represents the amount of Nitrogen, Phosphorus, and Potassium in a fertilizer. It’s listed on the front of a fertilizer bag or container. Take a look before you buy.

Onions are heavy feeders, which means they need constant nourishment. Nitrogen is especially important for your onions when producing big bulbs. If you want to grow your onions in the fastest and healthiest way, consider applying the optimum nitrogen fertilizer for onions.

Make sure that you are not overloading your onions with fertilizer.

Fertilizer Types: Generally, there are three types of fertilizers – liquid, spikes, and dry that you can apply to your onions.

  • Liquid fertilizers are famous for their quickly absorbing quality. It’s very easy to apply. You can save both time and water.  But it can be more expensive than others.
  • Dry granular fertilizers are the most common use, and they continuously provide nutrients to your onion plants. They tend to be more affordable.
  • Fertilizer spikes are small spikes composed of fertilizer that you stick in your soil. A spike releases nutrients into your soil over time.

Choose one of them accordingly to your need and budget.

Check the Ingredients: Bone meal, blood meal, chicken manure, and earthworm castings can help your onions grow fast. They are the key components of organic fertilizers. To make the right purchase, check the ingredients.

You should first fertilize your onions with nitrogen fertilizer when prepping your soil for planting. You will then need to reapply fertilizer 20 to 30 days after planting. After the first month, the fertilization schedule will depend on which fertilizer you use. Some fertilizers will need to be reapplied every 1 to 2 weeks, while others can last longer between applications.

Take care to read the specific instructions for your chosen fertilizer. Also, remember to water your onions regularly.

The specific process of fertilizing onions will depend on the product you choose, but here are the general steps.

Step 1: Measuring

The first step is to measure the amount of fertilizer you’re about to use.

Step 2: Addition of Water

  • If fertilizer is water-soluble: Add water to your measured fertilizer and prepare a solution.
  • If granular but not water-soluble: Needs no addition of water. 

Step 3: Applying

  • Liquid fertilizer: Pour the water-mixed solution you prepared earlier around the base. But avoid the leafy parts.
  • Granular fertilizer: Spread the granules evenly along the plant bed.

Step 4: Watering

As your mulch mixing is complete, get your water cans, and start watering. Onions need about 1 inch of water every week.

Yes, Epsom salt can be good for onions’ growth if the soil lacks sulfur. It can also help keep certain pests away from your onions, such as slugs.

Yes, potash is beneficial in the growth of onions. Potash is an impure combination of Potassium carbonate and Potassium salt. Potash helps in the development of stronger roots and healthier onion bulbs.

Yes. Chicken manure is one of the best chemical-free fertilizers for gardening. It contains macronutrients nitrogen, phosphorus, and potassium for the healthiest plant growth.

Chicken manure has numerous benefits in gardening. It helps to improve your onion’s number of shoot tips, bulb weight, height, and yield and improves the soil quality of your planting area.

Consider applying organic chicken manure to your garden.  Whenever you apply manure, remember to wear gloves.

Yes, definitely. It’s rich in micronutrients like calcium, magnesium, zinc, and iron, and is an excellent source of phosphorus. It improves the overall growing quality of your onion plants, especially helping healthy root development.

Bone meal is a slow-release fertilizer that nourishes your onions over a long period. This fertilizer is an excellent option for organic gardening.

Bone meal is not suitable for all soil pH levels. Make sure to test your gardening soil pH before applying it. The pH level should be 7 or lower. Excess uses of bone meal can burn plants, so avoid over-fertilizing.

Yes, blood meal helps onion plants grow faster and greener. It also performs as a pest controller and improves soil quality. It’s tremendously rich in nitrogen, which is good for heavy feeders like onions. Blood meal is also a slow-release fertilizer like bone meal.

Overuse can cause damage to or burn your plants because of high nitrogen concentrations.

Yes, 10-10-10 fertilizer is suitable for onions because it provides a balanced formula of the leading 3 macronutrients (nitrogen, phosphorus, and potassium) for your soil.

Written by Sean Stratton

Sean Stratton

Hi, I'm Sean, the senior editor here at Fertilizer Pick. I grew up on a farm in North Carolina and have grown fruits, vegetables, and trees since childhood. While I no longer live on a farm today, I still enjoy spending time on my garden and sharing my knowledge with friends and fellow garden enthusiasts.