6 Best Fertilizer For Onions And Garlic | How & When To Use

Few things are as satisfying as growing your own food. And it’s hard to think of many vegetables that compete with the versatility (and flavor profile!) of onions and garlic.

This self-dividing and multiplying bulbs are members of the Allium genus, so it’s no surprise that they require nearly identical care in the garden. This overlap is especially obvious between common onions (Allium cepa) and cultivated garlic (A. sativum).

The hardest part of growing onions and garlic is, without a doubt, nutrition. It’s not enough to select the right fertilizer for your allium plants. When you choose to apply said fertilizer is equally as important!

Best Onion and Garlic Fertilizer

If you’re in a hurry, here are my top two recommendations, otherwise keep reading to find out how, when, and why you need fertilizer for onions and garlic and how to choose the best product for your crop.

Choosing The Best Fertilizer For Onions And Garlic

Garlic and onions are unique in that the bulbs are usually the most desirable part of the plants. So, a slightly different approach is needed than for vegetables that are grown for their leaves or fruit.

Also, the nutritional needs of alliums shift throughout the growing season. Most notably, the amount of nitrogen you should apply to your onion and garlic plants will be higher in spring versus summer.

When it comes to onions, garlic, and other edible bulbs, emphasis is often placed on organic options. This is understandable since we eat the part of the plant that comes in direct contact with any soil treatments. But rest assured, synthetic fertilizers are equally safe and effective when used properly.

N-P-K Ratio For Alliums

Plants need many vitamins and minerals to survive but the three most important are nitrogen, phosphorus, and potassium. These compounds are represented on fertilizer labels via something called an N-P-K ratio.

An N-P-K ratio is a sequence of three numbers divided by hyphens — e.g., 10-10-10 or 6-8-4. No matter the product, the order is always the same.

N represents the percentage of nitrogen in a given fertilizer. Nitrogen is key for green, healthy foliage, among other plant processes.

P represents the amount of phosphorus. While many cultivated plants don’t require supplemental phosphorus, alliums tend to be an exception.

Finally, K represents the volume of potassium within a fertilizer. Since potassium plays a major role in root development, it’s important to ensure your onions and garlic have ample access to this nutrient.

Bone Meal

All alliums, including onions and garlic, are big fans of bone meal. Bone meal is high in phosphorus and can be used alone or with a balanced fertilizer — ideally, as indicated by a soil test — to boost growth.

Since bone meal releases nutrients more slowly than synthetic fertilizers, it’s best applied at the very start of the season. It offers a wonderful nutritional base for starter onions and garlic bulbs.

While bone meal primarily deposits phosphorus and calcium, it is not 100% nitrogen-free.

Balance Fertilizer

The most obvious examples of balanced fertilizers have N-P-K ratios like 4-4-4 or 10-10-10. But I’d also consider a ratio like 3-2-2 to be fairly balanced and this is great for the start of the alum growing season when extra nitrogen is needed.

In most gardens, a balanced formula will provide everything your allium plants need during the spring growing season. If a soil test indicates insufficient phosphorus or potassium, a dose of the aforementioned bone meal is a wonderful starting point.

Ideal pH For Onions And Garlic

The ideal soil pH for growing onions or garlic is between 6.0 and 7.0. This means that your garden soil should be slightly acidic.

Current soil pH can be determined with a simple test kit. If necessary, use elemental sulfur or aluminum sulfate to boost soil acidity. Apply agricultural lime to make the soil more alkaline.

For the best results, pH-adjusting amendments should always be applied before planting.

According to the North Carolina Cooperative Extension office, excess sulfur will also impact onion flavor. Stay away from products containing sulfur or sulfate once your onion sets are in the ground.

Continual Release Fertilizer

Alliums are heavy feeders, and there’s very little room for error when it comes to applying fertilizer in the spring. A slow- or continual-release fertilizer will ensure there are zero gaps in your onions or garlic’s nutrition.

You have several options to choose from when selecting a slow-release fertilizer. While many granular formulas are advertised for continuous feeding, it’s important to remember that organic fertilizers like aged compost or bone meal also release nutrients over a long period of time.

Granules Vs Liquid Fertilizer

When growing ornamentals, I tend to encourage gardeners to experiment with liquid and granular fertilizers to determine which best meets their needs. But when growing onions and garlic, a granular formula is almost always the best option.

Personally, the only allium-related scenario in which I’d reach for a liquid formula over a granular one would be to quickly address a known nutrient deficiency. Alternatively, you could use a liquid fertilizer throughout the growing season to supplement an initial application of slow-release granules in the early spring.

Liquid Fertilizer

Liquid fertilizer is convenient and mess-free. In many cases, all you need to do is attach a pre-mixed spray bottle to your garden hose and get to work! 

Despite its many benefits, liquid fertilizer has a hard time keeping up with the needs of onions and garlic plants. Liquid fertilizers deliver a near-instant dose of nutrition. However, those nutrients don’t stick around in the soil for long.

Granular Fertilizer

Fertilizer granules are small pellets or powders that are applied by spreading atop the soil’s surface. The granules are activated by water and will slowly leach nutrients into the soil as the fertilizer dissolves.

Granules are my go-to choice for onions and garlic because they stick around in the soil for a very long time. When growing alliums or other heavy feeders, slow-release granular fertilizer is sometimes the only way to keep up with your garden’s appetite while also saving your sanity!

Organic Fertilizer

Organic fertilizer is anything made from natural, organic matter (i.e., plant and animal waste). This is in contrast to synthetic fertilizers which are made from inorganic matter like minerals and gasses.

Contrary to popular belief, many organic fertilizers are processed in factories. But things like backyard compost also fall into this category.

In terms of nutrition, organic and synthetic fertilizers are nearly identical. For example, the quality of nitrogen you’d get from an organic formula is no different than that of a synthetic one. Meanwhile, organic fertilizers usually feature lower N-P-K ratios than synthetic ones. They also tend to release nutrients at a slower rate.

Another way organic and synthetic fertilizers sometimes differ is in the presence of beneficial bacteria and microbes. These materials can improve soil health and may even improve root uptake of certain nutrients. 

6 Best Fertilizers For Onions And Garlic

Compared to other vegetable crops, onions, and garlic require a certain level of trust. There’s no fruit to watch grow and ripen. Instead, you must trust that you’ve given your alliums everything they need to produce large, flavorful bulbs just out of sight under the soil’s surface.

Selecting a high-quality fertilizer that you can feel confident in will go a long way in building that trust:

Pros

• Extremely balanced, organic formula
• Versatile enough to use on most vegetables and ornamentals

Cons

• ‘Organic’ odor 

I really like this Down to Earth granular formula for my alliums and I also use it as an all-purpose vegetable fertilizer. It’s 100% natural and is suitable for use right from the start of the growing season – even for new plantings.

Because the N-P-K in this formula is perfectly balanced I recommend applying it to vegetables that you know have been planted in nutrient-rich soil rather than in an environment that needs amending to better balance soil pH.

I‘ve featured Down to Earth products throughout this review section simply because these people know what it takes to farm and fertilize organically. I know I can trust them when it comes to taking care of my onions and garlic as well as complementing my tomato plant growing and corn fertilizing too.

How To Use: Before planting, mix 3 to 6 pounds of fertilizer per 100 square feet into the top several inches of soil. During the growing season, side-dress monthly until scapes are about to emerge.

Pros

• Excellent source of phosphorus and calcium
• Low nitrogen content minimizes the risk of overfeeding

Cons

• Soil contents should be monitored with long-term use

If you’re looking for an excellent source of phosphorus and calcium with a lower nitrogen ratio, I recommend reaching for this Down to Earth bone meal instead!

This bone meal soil amendment can be applied at the very start of the growing season for slow-release nutrition. And it’s relatively low nitrogen content means it can easily be combined with other fertilizers with minimal risk of overfeeding.

Because this fertilizer contains such a high amount of phosphorus, I would recommend periodic soil testing just to ensure you’re not over-supplementing!

How To Use: At the time of planting, mix 1 tablespoon or less of bone meal per onion or garlic set into the surrounding soil. Alternatively, mix up to 5 pounds of bone meal per 100 square feet to prepare an entire bed for vegetable planting.

Pros

• Contains seven beneficial microbes to support soil health
• Organic formula is safe for people and pets

Cons

• Not a good source of available nitrogen

I really can’t overstate the importance of beneficial microbes for long-term soil health. This organic formula by Dr. Earth is a prime example of a fertilizer packed with bacteria, fungi, and other ingredients that will feed your alliums as well as the native soil.

Since this fertilizer is relatively low in nitrogen, I’d recommend using it later in the season for side-dressing. It can also be (carefully!) combined with a more concentrated source of nitrogen at the start of the growing season.

How To Use: Before planting sets, mix 1.5 cups of fertilizer per 10 square feet into the top 6 to 12 inches of soil. Side-dress by incorporating ¾ cup of fertilizer per 10 square feet in the soil around your alliums.

Pros

• Delivers high levels of phosphorus and calcium
• Made of 100% ground bone meal

Cons

• Risk of overfeeding if mixed with other nitrogen sources

This ground meal from Burpee is an excellent way to give your onion and garlic set a huge boost at the start of the growing season.

While the main highlights of this formula are the high ratios of phosphorus and calcium, it also contains a healthy dose of nitrogen. Just be careful to avoid combining this bone meal with another nitrogen-rich fertilizer as a double-whammy could burn tender plants.

The best time to use this bone meal supplement is at the time of planting. However, it is also suitable to continue using as the growing season progresses as a side-dressing.

How To Use: For new plantings, mix 1 cup of bone meal per 20 square feet or 1 teaspoon per individual set into the soil. For side-dressing, gently work 1 teaspoon per bulb into the surrounding soil.

Pros

• Formulated with 100% vegan ingredients
• The slow-release formula reduces the risk of overfeeding

Cons

• It May have an unpleasant odor

For vegan gardeners, I recommend this organic granular formula from Down to Earth. Even if you don’t need a fertilizer made with vegan ingredients, this mix is worth adding to your routine.

The relatively low yet balanced N-P-K ratio of this fertilizer makes it easy to avoid overfertilizing. Its slow-release formula works great at the time of planting or as a side-dressing throughout the growing season.

How To Use: Incorporate up to 5 pounds per 100 square feet of soil into the top layer of your vegetable garden bed. I recommend side-dressing with around 1 tablespoon per plant during the growing season.

Pros

• Convenient shake-and-feed packaging
• Safe to use on all edible garden plants

Cons

• Very high in potassium

Onions and garlic are usually grown alongside other vegetable varieties. While I wouldn’t choose this Miracle-Gro fertilizer for alliums alone, it’s a wonderful option for the diverse vegetable garden!

Use this granular formula early in the year when your onion and garlic plants’ nitrogen needs are highest. It is safe to use on and around all edible plants.

How To Use: Spread fertilizer at a rate of 1 tablespoon per square foot of the garden bed. Mix into the top layer of soil and water thoroughly to activate.

How To Fertilize Onions And Garlic

Onions and garlic plants require a heavy dose of nitrogen early in the growing season. They will continue consuming large amounts of nitrogen until bulbs begin to form.

Later in the season, you’ll notice that your alliums need less nitrogen and more phosphorus than the other veggies in your garden.

One of the best methods to meet these needs is by supplementing the soil with organic or slow-release fertilizer before spring planting.

How Often Feed Onions And Garlic

After the first spring application, continue feeding every 2 to 4 weeks according to your plant’s nutritional needs and your chosen fertilizer.

Overfertilizing Onions And Garlic

Overfertilizing, typically with nitrogen, can lead to excessive top growth. Similarly, applying nitrogen too late in the season can cause alliums to prioritize leaf growth instead of bulb growth. This is bad news when your goal is to raise onions or garlic with large bulbs.

Extreme cases of overfertilization will damage (or even kill off) onion and garlic plants. 

When To Fertilize Onions And Garlic

Onions and garlic have a very short feeding window. Nearly all fertilizer applications should occur between early and late spring.

While many gardeners have success starting alliums in the fall, you shouldn’t fertilize with nitrogen until the following spring.

When To Stop Fertilizing

When you stop fertilizing onions and garlic is nearly as important as when you start. Fertilizing with nitrogen too late in the season will create onions with a soft bulb and a shortened shelf life. Garlic that is fertilized too late in the year will produce smaller bulbs.

For onions, stop fertilizing when the bulb starts to swell. This usually happens approximately 1 month before the expected harvest.

Garlic should be fertilized until shortly after the bulbs swell but before the scape (flower stalk) emerges. This typically happens in late spring but can vary. 

You might also like to read Choosing Companion Plants for Onions

Verdict: Best Fertilizer For Onions And Garlic

If you have an established veggie patch, growing onions or garlic is an excellent way to fill in gaps between other plants.

I highly recommend Down to Earth Organic Vegetable Garden Fertilizer 4-4-4 for onions, garlic, and all other vegetables grown at home. Its balanced formula can be easily supplemented with bone meal or another soil amendment as needed.

For a higher dose of phosphorus that will have your onion and garlic bulbs thriving, reach for Down to Earth Organic Bone Meal Fertilizer 3-15-0 later in the growing season.

Or to improve overall soil health with the addition of beneficial microbes as the growing season progresses, I recommend you reach for Dr- Earth’s Organic 5 Tomato, Vegetable & Herb Fertilizer.

FAQ’s Fertilizing Onions And Garlic

Is bone meal good for garlic?

While the bone meal is high in calcium and phosphorus, blood meal provides a heavy dose of nitrogen. Bone meal is good for garlic in the springtime when the plants need access to plenty of nitrogen. But be careful not to over-apply nitrogen if using a balanced fertilizer as well.