Skip to Content

How to Fertilize Your Onions for the Best Harvest Ever (Step-by-Step Instructions)

Ways to fertilize onion plant.

Homegrown onions are a prize for their versatility, flavor, and ease of growing. They are an indispensable addition to any kitchen, whether you’re sautéing them with other vegetables or using them as a topping on your favorite dishes. But did you know that onions are not only good to eat, but also good for you, too?

Onions are a rich source of antioxidants and are linked to a reduced risk of cancer, heart disease, and diabetes. They’re also a good source of fiber and vitamins C and B6. So go ahead and add some onions to your next meal – your body will thank you for it!

Fertilizing onions helps them grow better and also gives them optimal nutrients. Read on to learn the best fertilizing schedule and types of fertilizer to use with your onions.

1. Start with Organic Soil

Organic soil in the garden with onion plant.

Giving your onions organic soil to start with can be the best thing for your onions. Also, you can add organic amendments to help your plants. Grass clippings commonly make their way into the soil mix. Also, try adding other byproducts of yard waste. This can be shredded leaves or even vegetable waste. Or, you can mix in fresh manure.

Using this method, you would have to wait several months before all of it decomposes. If you don’t want to wait that long, you can add compost to your onions.

2. Mix the compost in with the soil before planting.

Want to make your compost? Here is a step-by-step guide on how to do that.

Step-by-step how to make your compost pile:

1. Choose an area in your yard that gets at least six hours of sun and is away from any water sources.

2. You will need a bin or container to hold your compost materials as they decompose. If you don’t want to purchase one, you can build your own out of wood pallets or wire fencing.

3. Once you have your bin, fill it with a layer of brown material such as dead leaves, twigs, or even small branches.

4. On top of the brown layer, add a layer of green material like grass clippings, vegetable scraps, or fruit peels.

5. Repeat layers until the bin is complete, then cover it with a lid or tarp.

6. Now it’s time to let nature do its thing! Stir your compost pile every few weeks, and add water if it starts to look dry. In six months to a year, you should have rich, crumbly compost to use in your garden.

3. Fertilize Onions throughout the Season

Farmer with gloves fertilizing onion plant.

Onions are heavy feeders and will need to be fertilized several times throughout the season to ensure they have all the nutrients to grow big and strong. The best way to fertilize onions is with a fertilizer high in nitrogen. Nitrogen is essential for onion growth and helps promote leafy green growth.

A general rule of thumb is to apply 1/4 pound of fertilizer per 10 square feet of garden space. Be sure to read the fertilizer label carefully and follow the directions.

It’s also good to do a soil test before fertilizing your onions to see what nutrients are already present in the soil and what might need to be added. A soil test kit can be purchased at your local nursery or garden center, or you can send your soil sample to your local cooperative extension office for testing.

4. Watering Onions

Proper watering is essential for growing healthy onions. Onions need 1-2 inches of water per week, so give them a deep watering every few days. Water early in the day so the leaves have time to dry out before nightfall to minimize disease risk.

If you’re not sure whether your onions are getting enough water, check the soil around the plants. It should be moist but not soggy. If the soil is dry, give your onions a good watering.

5. Using Compost Tea on Onions

Compost tea is a great way to give your onions (and other plants) a boost of nutrients. It’s easy to make at home with just a few simple ingredients:

-1 shovelful of compost

-5 gallons of water

-A 5-gallon bucket or pail

-An airtight lid

Simply mix the compost and water in the bucket and seal it with the lid to make the tea. Let it sit for 3-5 days, then strain the solid materials. The liquid that remains is your compost tea. Apply it to your onion plants every 2-4 weeks throughout the season.

6. Mulching Onions

A farmer with gloves putting mulch in the onion plants.

Mulching around onions helps keep the soil moist and cool and prevents weeds from competing with the onions for moisture and nutrients. Organic mulch like straw or shredded leaves works well as a natural fertilizer. Apply a thick layer (3-4 inches) and replenish it as needed throughout the season.

7. Fertilize when Planting

The best time to fertilize your onions is when you plant them. You can either side-dress them or top-dress them. To side-dress, make a trench about 6 inches to the side of the row of plants. Place 1/2 cup of fertilizer on the bottom of the trench and then cover it up with soil. Water the area well.

To top-dress, simply sprinkle 1/2 cup of fertilizer over the soil around each plant. Be sure to rake it in and then water the area well.

8. Fertilize Regularly throughout the Season

It’s important to fertilize your onions regularly throughout the season for them to reach their most enormous size.

Onions’ roots start to grow about three weeks after planting. From this point, be sure to fertilize them with a nitrogen-rich fertilizer at least every six weeks. If you notice your plants starting to yellow, that is a sign that they are not getting enough nitrogen, and you should fertilize more frequently.

9. Stop Fertilizing in Late Summer

In late summer, around August or September, stop fertilizing your onions. This will help them bulb up and get ready for harvest. If you continue to fertilize them at this point, they will keep growing leaves and won’t form bulbs. them to your compost bin

10. Using Onions in the Garden

A farmer holding fresh harvest onion.

There are many different ways that you can use onions in the garden. Here are a few ideas:

-As a natural insecticide: chop up an onion and mix it with water. Spray it on your plants to keep pests away.

-As a weed killer: Spray full-strength vinegar on weeds in your garden. The acetic acid in the vinegar will kill them.

-As a fertilizer: Add chopped onions to your compost bin. They will add nitrogen to the compost and help speed up the decomposition process.

-As a companion plant: Onions make a good companion plant for many different vegetables, including carrots, tomatoes, and potatoes. Planting them together can help deter pests and improve yields.

Fertilizer Brands Best for Onions

Make fertilizing onions simplified with these premium blends and formulas. They are specially made for the best onion and vegetable growth. Plus, many of them are affordable as well as easy to find.

1. Organic Onion Fertilizer, Fertilizers – 3 Pounds

Organic fertilizer for onions.

Source: ufseeds

This  3-4-3 (NPK) blend is designed for organically grown onions. Use 1 pound per 100 feet of row when transplanting, 1/2 pound per 100 feet of row during bulb formation, and 1/4 pound per 100 feet of row after harvest.

This organic fertilizer is made from fish meal, kelp meal, bone meal, and other natural ingredients. It’s a complete fertilizer that will provide your onion plants with the nutrition they need to produce big, healthy bulbs. Apply it at planting time and then every 6 weeks throughout the season.

2. Jobes Organics Fertilizer Spikes for All Purposes

Jobe's 06028 Fertilizer Spikes Vegetable and Tomato, 50, Brown

These fertilizer spikes are made from organic ingredients like kelp meal and bone meal. They provide a slow, steady release of nutrients to your onion plants over 2-3 months. Push them into the soil around the base of each plant at the beginning of the season.

3. Miracle-Gro Performance Organics Shake ‘n Feed Tomato, Fruits & Veggies Plant Food

Miracle-Gro Performance Organics All Purpose Plant Nutrition Granules - 2.5 lb., Organic, All-Purpose Plant Food for Vegetables, Flowers and Herbs, Feeds up to 240 sq. ft.

This organic fertilizer is made from a blend of natural ingredients like kelp meal, bone meal, and fish meal. It’s designed to provide your onions with the nutrition they need for big, healthy bulbs. Apply it at planting time and then every six weeks throughout the season.

4. Hoss Tools Ammonium Sulfate Fertilizer for Onions

Hoss Ammonium Sulfate.

Source: gardenertofarmer

This fertilizer is best used in the spring or fall.

For onions, we recommend using 1/2 to 1 pound of ammonium sulfate per 100 square feet. This fertilizer can be applied either before planting or after the plants have emerged.

If you are applying it before planting, simply spread the fertilizer over the area where you will be planting and till it into the soil.

If you are applying it after the plants have emerged, be sure to apply it around the base of each plant so that the roots can take up the nutrients. Water the area well after applying the fertilizer.

Onion Fertilizer Tips

1. Use a fertilizer with a high nitrogen content. Nitrogen is essential for healthy leaf growth and will help your onion plants to produce big, healthy bulbs.

2. Apply fertilizer at planting time and then every six weeks throughout the season.

3. Stop fertilizing in late summer to allow the plants to start bulbing up.

4. Use composted onions as a natural fertilizer in your garden. They will add nitrogen to the soil and help improve yields.

5. Follow the package directions when using any fertilizer product. Over-fertilizing can damage plants and reduce yields.

Sources: