How to Choose the Right Fertilizer for Onions: Big Onions on a Budget!

fertilizer for onions post
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Last Updated on 3 months by Michelle

This blog post will discuss how to choose the best fertilizer for onions and provide you with information on what fertilizers onions need during the different growing stages.

I will also tell you how much fertilizer to use, and how often you should fertilize your onions for a high yield.

You’ll learn how much the onion seeds, sets, and fertilizer for onions will cost so you can see for yourself how affordable growing your own onions can be.

So read on for all you need to know about choosing the right organic fertilizer for onions! If you’d rather listen along, check out the YouTube summary of the blog post!

Onion Fertilizer Needs at Different Stages of Growth

Most fertilizers come in either a solid or liquid fertilizer form and are made up of three main ingredients- nitrogen (N), phosphorus (P), and potassium (K). These three nutrients are important for plant growth and health, and each one is responsible for different aspects of plant development.

Nitrogen helps plants produce green leaves and stems, phosphorus helps produce flowers and fruits, and potassium helps plants resist disease and pests.

Fertilizers also contain other ingredients that help the plant absorb the nutrients more easily.

When you fertilize your onion plants, these extra ingredients help to put the nutrients right where they need to be, which is in the soil AND in the onion plants.

Onions need different kinds of fertilizer at different stages of their growth. They need a balanced fertilizer with an even NPK ratio at planting time and they need a high nitrogen onion fertilizer the rest of the time.

Ways to Grow Onions

There are a couple of main ways that people grow onions. One way is to grow onion sets and the other way is to grow them from seeds.

Purchase Onion Plants

I purchase onion plants from Hoss Tools or Dixondale Farms if I am planting in the spring. There are about 50-70 onion bulbs in a set and are best planted in the spring. (You can also plant short day onions in the fall in southern climates to overwinter and harvest during early spring, but that is a *little* more in-depth than this blog post will go)

Dixondale Farms Onion Sets

Growing Plants from Onion Seed

I plant my own onion seeds if I’ll be planting onion transplants in the fall or winter. Growing onion plants from seed is the most economical way to grow them. You’ll need to start early if you want to grow large onions, but it can definitely be done by growing from seed.

Your cost to grow onions can be as low as 6 cents per onion when you grow from seed.

Have the Fertilizer You are Going to Use Before you Plant the Onions

Why? Well, there are two reasons.

The first reason is that you will be fertilizing them every 2 weeks for 3 months, so it’s just more convenient to have all your high- Nitrogen fertilizer from the beginning.

The second reason is that you may run into fertilizer sourcing issues so you want to go ahead and get what you need in case you have issues getting it later.

Phase I: Amend Your Soil Prior to Planting

Amending your soil just prior to planting onions will lay a great foundation for your onion transplants, especially for their Phosphorus and Potassium needs.

I wrote an extensive article about amending your soil if you are interested in more detail on the topic of amending garden soil.

For this first stage of fertilizing, I use one of 2 Espoma products:

These products are both organic granular fertilizers and have a mix of quick-release and slow-release nutrients along with mycorrhizal fungi and other essential nutrients to prompt vigorous root growth. 

Fertilizer for Onions

The quick-release will fertilize immediately, and the slow-release fertilizer will reach its full capacity at around one month. A balanced fertilizer like one of these is perfect for amending the soil for your onion crop .

Once the sets are planted, you’ll add a quick dose of high-nitrogen fertilizer every 2 weeks to turbo-charge the green growth to support bulb formation that is needed to grow larger onions.

This high-nitrogen fertilizer is the one you’ll want to have on-hand as soon as possible just so you aren’t scrambling to get fertilizer on your plants during later feedings. 

Phase 2: Green Leaf Growth & Feeding Schedule

This is the feeding schedule you’ll need to follow in order to get the biggest onions possible. By growing the largest onions possible, you save a maximum amount of money while eating onions that are insanely delicious. Onions are heavy feeders, so it’s important to provide enough nutrients.

You’ll side-dress the onions every 2 weeks with your high-nitrogen granular fertilizer.

Make a furrow 2-3 inches deep between your onion rows, add the prescribed amount of onion fertilizer and work into the soil.

planting onion transplants

It will seem like your fertilizer is too far away from your onions, but don’t worry. You will be fine as long as there are around 1.5 feet between your onion rows and your fertilizer furrow is halfway between these rows.

How Much to Feed Every 2 Weeks

Your plants need about 1/2 cup of high Nitrogen fertilizer for every 10 feet of garden row.

Let’s work through an example of the onion patch in my garden so you can see how this works:

  • In my garden, rows are 18 feet long, so I’ll need .5 cup X 1.8 = .9 cups of fertilizer per row every time I feed them.
  • I have 4 rows of onions, so I need .9 cups X 4 rows= 3.6 cups total per feeding

For 6 total feedings, I’ll need around 15 cups of fertilizer for this season’s onions.

Using Blood Meal as High Nitrogen Fertilizer

Some folks get nervous when blood meal is mentioned and they ask, “Is blood meal good for onions?” I can say yes, it absolutely is good for onions.

I use blood meal as my high Nitrogen organic fertilizer, and I weighed a cup of it. A cup weighs 1/4 pound, so 15 cups weigh almost 4 pounds.

This bag of Down to Earth blood meal (click here for current pricing) will be enough Nitrogen-rich fertilizer for all the onions I grow this year.

The 2 Phases of Onion Growth

There are 2 phases of onion growth, and understanding what happens during each phase will help you grow massive onions.

Green Growth Phase

This is where all the action happens in the beginning, and that action is concentrated above ground. The onion leaves act like power plants or solar panels for the onion and the more green growth you have, the bigger your bulb will be.

Your objective during this phase is to have as much green growth as you can as quickly as possible. Those days are getting longer and longer, and you need to have big green leaves to support the massive bulbs we want in the bulbing phase.

Bulbing Phase

The next phase is the bulbing phase. You can’t control when the bulbing phase happens. Bulbing is dictated by what kind of onion you have, how long the day length is where you are, and your temperatures.

There are 3 kinds of onions: Short Day, Intermediate Day, & Long Day onions.

Short Day Onions

Short Day onions only need short days of 10-12 hours of sunshine to trigger the bulbing phase. These onions are best suited to the southern-most areas of the United States.

If you grow long-day onions in this region, they WILL NOT ever bulb no matter how much fertilizer, water, and love you grow them with.

Have you ever grown onions that would never bulb, no matter what you did? You convince yourself that you can’t grow onions and you never try again?

Trying to grow the wrong type of onion for your geographical region could be the reason they wouldn’t grow. 

Intermediate Day Onions

Intermediate Day onions are in the middle. They will begin bulbing when the day length reaches 12-14 hours.

I live in an intermediate zone, and all the onion sets that I plant in spring are intermediate-day onions.

I plant these onions in the first or second week of March, when the day length is over 12 hours. Day length reaches just over 14 hours by the end of May.

That means that for the two months from when I plant the sets until around May 6th or so, I’ll fertilize the onions every two weeks until they begin to bulb. Then, once I see that start to happen, I’ll stop fertilizing.

best fertilizer for onions

Long Day Onions

Long day onions need 14-16 hours of sunshine to form a bulb, so they are perfect for spring planting in the northern regions of the US.

If you want to grow short day or intermediate day onions in northern regions, you can; you have to plant them at different times of the year.

Things to Remember Every Time You Fertilize

  • Onions have shallow roots, so don’t re-apply the fertilizer too close to the plants or the fertilizer will burn them.
  • Put your fertilizer in a row or trench between two rows of onions so that the fertilizer doesn’t burn the roots.
  • Nitrogen is very important during the first phase of growth because it helps establish vigorous green leafy growth. The leaves on an onion plant are the power plant for the plant, and the more green growth you have, the bigger the bulb can become once the bulbing phase begins.
  • Weed every time you fertilize, and make sure you water with an inch of water per week if you have any dry spells. Having enough water is crucial for successful onion growing.
onion crop

Growing Onions Will Save Serious Cash

At the time I’m writing this (March 6th, 2022), this 5-pound bag of high nitrogen blood meal is $22.46.

Fertilizer prices have changed a bit, though, so click here to check the current pricing

After the sales tax, etc is added, let’s say this fertilizer is $25 for 5 pounds or $5 per pound.

I need 4 pounds of this fertilizer for 540 onions. That will cost $20.

Therefore, my cost to fertilize each onion is .037 cents per onion.

So to fertilize these onions with great fertilizer on what is a rather intense fertilizing schedule only costs .037 cents per onion.

I went through this exercise so you can see for yourself that it’s cheaper to grow your own onions.

You may have heard that growing your own food is too expensive, but many times that’s just not the case, and this example proves it.

I paid 10 cents for the onion itself, so we’ll add those together to get the total cost of the onion.

My total cost to grow the biggest, most delicious, easiest to digest onions my family and friends have ever had is .14 cents per onion.

These onions cost .14 per onion, and they always weigh AT LEAST 1 pound and often 1.5 pounds EACH.

Grocery store onions are between $1.00-$1.25 per pound.

Fourteen cents to grow my own versus $1.00 at the grocery store is a no-brainer for me. I will ALWAYS grow onions. When you plant onions, you get the best results for much less money.

best onion fertilizer

High Nitrogen Organic Fertilizers I Recommend

I mentioned high nitrogen fertilizers above, and here are a couple of fertilizers that are high in nitrogen that I have used and that you can use during the green growth phase of your onions:

Down to Earth Blood Meal

I use this blood meal fertilizer as an organic source of nitrogen, and it has given great results.

Greenway Feather Meal Nitrogen Fertilizer

This organic feather meal nitrogen fertilizer has the same percentage of Nitrogen and roughly the same cost as the blood meal, so if you’d rather use this, you’ll still come in at a fertilizer cost of around 4 cents per onion.

The fertilizers above are the organic options I recommend. There are other high nitrogen fertilizers like Urea and Ammonium sulfate, but I haven’t used them so I can’t recommend them.

Coop Poop Pelletized Chicken Manure

Coop Poop is a pelletized composted chicken manure product that will work for your high-nitrogen fertilizer, you’ll just have to use more of it since the % Nitrogen is only 2% vs 12% for the blood meal. Just take the blood meal calculation I did above and multiply the # of cups by 6 to arrive at the amount you’ll need for fertilizing onions.

The upside to using Coop Poop is that you can use this single fertilizer for all your onion feedings since it is a balanced fertilizer that is a bit cheaper than the Espoma products.


Understanding fertilizer is a crucial step in growing the biggest, most delicious onions you’ve ever tasted. If you want to grow them at home and save money while doing so, follow the guidelines above for massive onion harvests.

Are you motivated to grow onions at home now? Let me know in the comments below and be sure to let me know what varieties you’re growing!

"How Much Money Can You Save By Growing Your Own Onions" Calculator

I wanted to make these calculations easy, so I made this handy calculator.

Figure out how much money you’ll save by growing your own onions!



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