What is the Best Fertilizer for Onions? The Best Onion Fertilizer at Greenway Biotech

Written by Amir Tajer


Posted on June 17 2024

From mixing in pastas to creating salsas, using them as toppings for sandwiches, and more, onions are a versatile veggie staple. This plant grows fairly easily, too, when you give it the right nutritional inputs.

So, what is the best fertilizer for onions? We’re here to unveil 5 of the best onion fertilizer options you have at your disposal:

Each of these is available right here at Greenway Biotech, too, where you’ll gain access to the cleanest, most bioavailable plant nutrients online. We’re proud to serve home gardeners and commercial farmers alike with consistent, dependable specialty fertilizer

Beyond showing you the best organic fertilizer for onions, we’ll offer tips on fertilizing onions in general - when to feed them, how to do it, and more to bring out the full potential of your crops.

What is the Best Fertilizer for Onions?

Let’s not waste any time - what is the best fertilizer for onions? Ultimately, it depends on where and how you’re growing. This involves choosing between organic vs synthetic fertilizer, determining whether or not you need a water soluble fertilizer, and more.

Don’t worry, though. We’re here to help. We’ll break down the best onion fertilizer for different scenarios below, and if you still have any questions when it’s all said and done, you can get in touch with our customer service team!

Blood Meal

This is an excellent source of slow-release nitrogen for onion foliage growth which supports the development of large bulbs when it comes time to begin fruiting. 

With a nitrogen content typically around 13%, it enhances the soil’s nutrient profile significantly. This organic option not only boosts growth but also improves the green color of the leaves, indicating a healthy nitrogen level. 

Feather Meal

Feather Meal is another organic option that provides a slower release of nitrogen than blood meal, making it suitable for providing sustained nutrition to onion crops over a longer period. 

With nitrogen levels usually around 12%, feather meal helps in steadily maintaining the growth momentum necessary for onions without the risk of overwhelming them quickly. This makes it perfect for growers looking for a steady growth pace as onions develop their bulbs.


Urea is highly concentrated in nitrogen, usually around 46%, and is one of the most common synthetic nitrogen fertilizers. It’s particularly effective for onions due to their high nitrogen requirements. 

Urea should be applied carefully to avoid excessive nitrogen levels which can lead to lush foliage at the expense of bulb development. It’s best applied before planting and can be top-dressed lightly during the growing season to maintain optimal nitrogen levels.

Ammonium Sulfate

Ammonium sulfate contains around 21% nitrogen and 24% sulfur, which addresses two crucial nutrients for onions. 

The sulfur in this fertilizer helps in enhancing the flavor of the onions, which is a desired trait for many varieties. The nitrogen component supports leaf and bulb growth, making it a dual-benefit choice for onion farmers. This fertilizer is ideal for soils lacking in sulfur.

Calcium Nitrate

Calcium nitrate is an excellent choice for onion cultivation, particularly because it offers both calcium and nitrogen - two nutrients essential for the growth of healthy onions. 

Calcium strengthens the cell walls of onion plants, enhancing their vigor and resistance to diseases and pests. It also helps in preventing disorders like bulb rot which can devastate crops. 

The nitrate form of nitrogen provided by calcium nitrate is readily available to plants, promoting vigorous growth of both roots and foliage.

Why Shop at Greenway Biotech?

Each of these onion fertilizers is available right here at Greenway Biotech, your trusted source for all your organic fertilizer needs. Whether you’re seeking the best fertilizer for onions or the best fertilizer for fruit trees, you can rest assured we have the potent, pure solution you need.

All our products are professionally manufactured in California and sourced from sustainable raw materials, making this an ethical place to source your plant nutrients. 

Many of our fertilizers are water soluble, too, allowing for rapid absorption of key nutrients for your onion crop. They can be used as a soil drench or sprayed on the foliage, whichever approach you prefer.

You’ll also enjoy free shipping on orders above $35, and we get your order out the door within 1 business day so you can start giving your crop the nutrition it deserves ASAP.

What Fertilizer for Onions is Right for You?

Each of these fertilizers above has its place, but you don’t need all of them to feed your onions. So, what fertilizer for onions is best for your specific needs?

Organic options like blood meal and feather meal are excellent for enriching the soil naturally, while synthetic options like urea and ammonium sulfate offer more controlled and immediate nutrient release. Meanwhile, calcium nitrate provides a balance of essential nutrients that support overall plant health and bulb quality. 

Assessing your soil condition through testing and understanding the nutrient dynamics will guide you in choosing the best fertilizer for your onions. 

Better yet, send us your soil test and we’ll make recommendations based on the existing content of your soil. Our commercial customers can even get a specially crafted formulation to fill in the gaps in their soil for the healthiest, happiest plants possible.

When and How Often to Apply Onion Fertilizer

Once you have your hands on a quality onion fertilizer, it’s important to carefully follow application guidelines. Your crop needs specific nutrients at specific stages of its life. So, here’s some advice on when to fertilize onions.

When to Fertilize Onions for the First Time

We suggest you fertilize your soil before planting onions. This initial feed should incorporate a balanced mix of nutrients into the soil, setting a strong foundation for growth. It should be more nitrogen-heavy to support strong vegetative growth, as this will support stronger fruiting later on.

Integrating a slow-release nitrogen source for your onion plants ensures that they have a steady supply of nutrients as they establish. But, you’ll need to reapply down the road - so let’s talk about how often to fertilize onions.

How Often to Fertilize Onions After the Initial Application

Once your onions have established and begun to form bulbs, a second application of fertilizer is beneficial. This typically occurs about 3 to 4 weeks after planting. 

At this stage, a fertilizer higher in phosphorus and potassium, like Bone Meal, Monopotassium Phosphate or Potassium Sulfate, can promote robust bulb development.

Depending on the growth rate and soil fertility, additional light feeding may be required every 4 to 6 weeks. Use a nitrogen-rich fertilizer like Ammonium Sulfate sparingly to avoid excessive leaf growth over bulb development.

More Advice on Fertilizing Onions

Fertilizing onions can be overwhelming, but we have tips below to set you up for success:

  • Balanced Nutrition: While nitrogen is essential early on for leaf and root growth, over-application can lead to lush foliage at the expense of the bulbs. Ensure your feeding schedule includes a good balance of potassium and phosphorus to encourage strong, healthy bulbs.
  • Micronutrients Matter: Onions also benefit from micronutrients such as boron, magnesium, and calcium. These help in overall plant vigor and disease resistance. Using a comprehensive micronutrient fertilizer or incorporating organic matter like compost can provide these essential elements.
  • Soil Testing: To avoid the guesswork in fertilization, conduct a soil test before planting and after each growing season. This test will guide you on the exact nutrient needs of your soil, helping you tailor your fertilization practices more precisely.
  • Watering with Fertilization: Onions require consistent moisture to absorb nutrients effectively. Water plants thoroughly after each fertilizer application to help dissolve nutrients and aid in their uptake.

Other Ways to Get Onions to Grow Bigger

Growing bigger onions is always the goal - but fertilizer alone won’t get it done. Nutrition is just one piece of the puzzle. Here are some more ways to encourage massive onion bulbs:

  • Choose the Right Variety: For larger bulbs, select varieties known for their size, such as 'Ailsa Craig' or 'Exhibition'. These types are genetically predisposed to develop larger bulbs under optimal conditions.
  • Optimal Planting Time: Plant onion sets or seeds at the right time for your climate zone. In cooler areas, early spring planting is essential, as onions need a long growth period before the heat of summer. In warmer regions, planting in the fall allows onions to utilize the cooler months for development.
  • Spacing Matters: Crowded conditions can stunt growth, resulting in smaller bulbs. Typically, setting onions about 4 to 6 inches apart allows ample room for each bulb to mature fully.
  • Consistent Watering: Water regularly to keep the soil moist but not waterlogged. A drip irrigation system can be particularly effective in maintaining the ideal soil moisture level without overwatering.
  • Mulching: Mulch helps retain soil moisture, suppress weeds, and keep soil temperatures stable. Organic materials like straw or shredded leaves are excellent choices, as they also contribute nutrients to the soil as they decompose.
  • Weed Control: Weeds compete with onions for nutrients, water, and light, significantly hindering growth. Regular weeding ensures that all resources go directly to your growing onions.
  • Disease and Pest Management: Use appropriate organic methods to control common onion problems like onion maggots and thrips. Regular inspections and preventative measures such as crop rotation and using clean, disease-free sets can minimize risks.

These tips, coupled with the best onion fertilizer at Greenway Biotech, will have you well on your way to cultivating your tastiest, heaviest onions yet. So, what are you waiting for? The only thing left to do now is get the best fertilizer for onions at Greenway Biotech!

Discover the Best Organic Fertilizer for Onions at Greenway Biotech Today!

We hope this conversation on the best organic fertilizer for onions has left you with a clear understanding of all the options you have at your disposal. 

From blood meal to feather meal, urea, ammonium sulfate, and calcium nitrate, each ensures balanced nutrition to foster robust growth and substantial yields. 

We specialize in eco-friendly products that nurture both your plants and the environment, ensuring your gardening is both productive and sustainable. There’s no better place to source your onion fertilizer! Be sure to apply the other advice we’ve shared as well as far as when to fertilize onions and how often to fertilize onions. 

Don’t forget that Greenway Biotech also has all the other traditional and hydroponic nutrients you could need for any crop, including the best strawberry fertilizer, the best tomato fertilizer, the best organic fertilizer for vegetables, the best nitrogen fertilizer, and everything in between.

Your crop deserves nothing less than the best, so don’t settle. Enhance the garden's productivity with our scientifically formulated fertilizers - perfect for every stage of your onion's growth!



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