Best Onion Fertilizers: Ratings, Reviews, and Top Picks

Onions are among the most popular vegetables you can plant at home. Onions are relatively easy to grow since they don’t require constant maintenance, like eggplants or artichoke. As long as you pick an onion variety that can thrive in your locale, growing onions should be a breeze.

When it comes to growing onions, one aspect may make you stumble—what sort of fertilizer you should use. Choosing the right fertilizer can be a challenge. If you add too much, the effects could be worse than if you didn’t add any at all.

In this guide, we’ll break down the best way to choose a fertilizer for your plants. We’ll include a quick diagnostic framework so you can choose the right amount of fertilizer for the problems your garden is facing. After that, we’ll jump into our favorite formulas for onions.

At A Glance: Top Onion Fertilizers

Fertilizer Balance

Generally, fertilizer has an “NPK” number on the front of the container. The NPK number consists of three numbers, with each representing the concentration of nitrogen, phosphorus, and potassium, hence the name.

For the right formula for onions, you want to choose a mix that’s high in nitrogen. As onions grow, they pull all the nitrogen out of the soil, leaving it less nutritious. According to studies, if you use the optimum nitrogen fertilizer concentration, your plants will yield more and larger onions.

Fertilizer Types

Traditionally there are three different types of fertilizers you can use. You can use liquid, dry, or spikes. Liquid is popular due to how quickly it’s absorbed. But it can also be a little more expensive. Dry, granular fertilizers are budget-friendly, and are most common. More recently, spikes have become popular, which is a continuous feeding method of putting fertilizer spikes in your soil that fertilize over time.

How Much Fertilizer?

If you use a nitrogen-only fertilizer, like ammonium nitrate, you’ll have to follow a schedule. For the best onions, you want to spread a cup over a 20-foot row of onions. You’ll have to spread the nitrogen fertilizer every 2 to 3 weeks.

For a potassium and phosphorus fertilizer (without any nitrogen), the best concentration to purchase is a 0-10-10 NPK ratio. That way, you can add as much nitrogen as you like without worrying about poisoning the plant.

Calculating Quantities

Following any method that uses solid fertilizer, you should end up using 160-80-140 pounds of fertilizer per acre of onions. The first step is to divide 160 (the nitrogen value) by 43,560, which is the square footage of an acre. We’re left with 0.0037.

Let’s break those numbers down. If you plant a 20-foot row that’s 3 feet wide, that means you have 60 square feet of farming area. Multiply the 60 square feet by 0.0037 to find out how much nitrogen you need in the particular area. For 60 square feet, you’ll need 0.22 pounds of nitrogen.

But your fertilizer doesn’t equal the exact amount of nitrogen you need. You have to divide the 0.22 pounds by the concentration of nitrogen in your mix. For this example, we’ll use a concentration of 21%. The math looks like this:

• 160 (pounds of nitrogen)  ÷ 43.560 (square footage of an acre) = 0.0037
• 60 (square feet of garden) x 0.0037 = 0.22 (pounds of nitrogen needed)
• 0.22 ÷ 0.21 (fertilizer nitrogen concentration) = 1.05 (pounds of fertilizer)

So, for a nitrogen concentration of 21%, we would need to use 1.05 pounds of fertilizer over a 60 square foot area for maximum utility. You should fertilize your onions every 2 to 3 weeks. Since onions take 3 months to grow, that means each time you fertilize, you should use roughly 25% of your allotted fertilizer. You don’t want to add any fertilizer too close to harvest.

As for phosphorus and potassium, we can use the ratio to figure out how much we need. For 60 square feet, you’ll need 0.193 pounds of potassium and 0.11 pounds of phosphorus.

Top Fertilizers For Onions

Next up, we take a deeper look and review our favorite onion fertilizer formulas. Many of these formulas make great all-purpose formulas that can be used for many different vegetable types. Onion fertilizers can be reused for asparagus, eggplant fertilizer, and plenty of other plants. While you can get certain types that are plant specific, generally speaking, a high quality organic mix is best, followed by more commercially available (and less expensive) formulas. Let’s take a look at our top picks!

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One of the best onion fertilizers is Miracle-Gro Shake ’n Feed. Miracle-Gro is one of the most well-known names in gardening, with products ranging from soils to seeds. Although Miracle-Gro makes all types of products, their most famous line is their All-Purpose fertilizers.

With an NPK of 12-4-8, Shake ’n Feed All Purpose has an almost perfect concentration of each nutrient. The high amount of nitrogen helps keep the onion bulb healthy during growth, while the phosphorus and potassium ensure your plant’s energy systems are in working order.

To use Shake ’n Feed, apply the formula from above to find out how much you need. In this case, we would need a total of 1.83 pounds of Shake ’n Feed spread over a 60 square foot area. For each feeding, you should use 0.46 pounds of fertilizer.

Shake ’n Feed is among the easiest formulas to use. Measure the correct amount and spread it evenly around your crop. As you water your plants, the fertilizer will dissolve and your plants will absorb the nutrients.

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Osmocote Plus is another fertilizer with a great NPK ratio for onions. This formula uses a ratio of 15-9-12, which leaves your plants with an abundant source of nitrogen. One of the main differences between Osmocote products and other fertilizers is their time-release formula.

Osmocote only recommends fertilizing your plants once during the growing season. Since their formula is time-released, every time you water your plants more fertilizer should be released. With a time-release formula, you won’t have a strict schedule.

With time-release fertilizers, the amount you use is different from normal formulas. According to Osmocote, you should spread 1 pound in a 50 square foot area. Work the fertilizer about 1 to 3 inches into the soil.  You will only have to spread it once per growing season.

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This organic formula from ArgoThrive a is a great option for gardeners looking for an organic fertilizer that’s also liquid. It’s fast acting, and can generate visible results within a week of being used. The NPK ratio is 3-3-2, which means it’s balanced, and can be used for onions safely.

ArgoThrive has many different formulas, but the all-purpose formula will allow you to be a little more flexible in your garden. The cost is fairly reasonable when considering other organic competitors, and it comes in a variety of different sizes to also give you some choice when it comes to how much you want to keep in storage.

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The Old Farmer’s Almanac is good for more than just predicting seasonal weather! Aside from their legendary yearly book, the Old Farmer’s Almanac also produces bug repellents, garden art, and yes, fertilizer.

With an NPK ratio of 8-4-8, the Old Farmer’s Almanac Organic Tomato and Vegetable Fertilizer have all the nutrients your onions need to thrive. The nitrogen helps the plant during growth, the phosphorus helps the onion bulb grow, and the potassium helps build strong roots.

The Organic Tomato and Vegetable Fertilizer are easy to use too. During your growing season, you’ll need to use a total of 2.75 pounds over a 60 square foot area. For each feeding, you should use 0.69 pounds.

Simply spread the fertilizer on the soil around your sprouting onions. You can then work it into the soil, but don’t worry about mixing it totally. As water soaks the fertilizer, it will dissolve, leaving the nutrients to your onions.

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MIracle-Gro Plant Food Vegetables and Herbs is one of the most popular fertilizers for gardeners no matter what they’re growing. The water-soluble formula is best utilized when it dissolves in water and you spray it on your plants’ soil. You can do this with either a hose attachment or a watering can.

The NPK ratio for Miracle-Gro Vegetables and Herbs is 18-18-21. Although the formula has less nitrogen than other brands, it still works well when growing onions. The amount of potassium is slightly high, so you’ll have to use less than the recommended dose.

To mix the fertilizer, take a tablespoon and pour it into a watering can. Next, take a gallon of water and pour it into the watering can. Mix well. For 60 square feet, you should use a total of 6 gallons of fertilizer mixture, or 1 per every 10 square feet.

Once you’ve mixed it with water, you can apply it to your soil. You should pour the water directly on the base of your plant. Repeat this process every 3 weeks for the best results. The recommended dose is once every 2 weeks, but if you follow those directions you could poison your onions, causing them to die. 

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Although they call it a citrus fertilizer, Down to Earth Organic Citrus Fertilizer has an amazing NPK ratio of 6-3-3 for growing onions. Since there’s twice as much nitrogen as there is phosphorus or potassium, you won’t have to worry about giving too many non-nitrogen nutrients to your onions.

Down to Earth’s formula is made with mostly natural materials, like bone meal, alfalfa, and basalt. Although these are naturally occurring materials, you don’t want to handle them too much. Fertilizers can cause health problems if you handle them often.

To use this formula, start by determining how much you’ll need. For 60 square feet, you’ll need a total of 3.66 pounds of fertilizer, which equates to 0.92 pounds for every 3-week feeding. Spread it around the base of your onions, mix it into the soil, and water well.

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Espoma Holly-Tone fertilizer is another great choice. It has a gentle formula, with an NPK ratio of 4-3-4. As a solid fertilizer, you’ll have to spread it around your plants and work it into the soil.

Holly-Tone’s available nitrogen gives a boost to your onion’s growth, while the other nutrients help the root system thrive. This formula is also slightly acidic, which is a plus since onions enjoy slightly acidic soil.

For a 60 square foot garden, you’ll need to use a total of 5.5 pounds. For each feeding, you should use 1.38 pounds of Holly-Tone. Spread the fertilizer on the soil around your onions and carefully work it into the soil. Water well so that the nutrients can leach into the ground.

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Lilly Miller Citrus and Avocado fertilizer is a solid choice for veggies. With an NPK ratio of 10-6-4, it has enough nitrogen, but too much phosphorus. If you’re not careful when feeding your plants, you could cause them to die.

Onions can be a finicky food to grow, and having too much phosphorus can be another challenge. For the Lilly Miller fertilizer, that means using less than the others on this list.

So, if this were a normal fertilizer, we would want to use a total of 2.2 pounds for 60 square feet. Since the phosphorus concentration would be too high at that level, we want to use three-quarters of that amount. In this case, that would amount to a total of 1.65 pounds.

As with other fertilizers, you want to feed your onions every three weeks. That means each feeding, you should use 0.41 pounds of Lilly Miller fertilizer. At these levels, your onions will have the best chance to thrive.

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Frequently Asked Questions

Here are some of the most common questions we get asked about onions and the best fertilizer for growing onions.

Final Thoughts

So, as you can see, the type of fertilizer you use can affect the outcome of your plants. If you’re looking for the best onion fertilizer, you want something that has an abundance of nitrogen with moderate levels of phosphorus and potassium.

These three elements are the main nutrients for any plant. Without them, your plants are essentially on their own. By giving them a boost with the right formula, you can ensure your onions are the envy of your whole neighborhood.

So, whether you choose Miracle-Gro Shake ’n Feed, Espoma Holly-Tone, or the Old Farmer’s Almanac fertilizer, you can be sure you’re choosing from among the best fertilizers for onions. Don’t waste any more of your garden’s precious time and plant your onions and get yourself the best onion fertilizer you can!