4 Best Fertilizers for Onions – A Comprehensive Guide

best fertilizers for onions

If you are a fan of homegrown vegetables, there is no better option than growing onions. Very few vegetables match the versatility of onions, and planting them means that you will always have them ready. Onions are low-forming, extremely versatile vegetables that are relatively easy to grow.  You can grow onions from seeds as well as from baby onions.

While the process of growing onions is relatively simple, the vegetable does require some special care. Onions have shallow root systems along with being heavy feeders. Therefore, if you want to yield good quality onions, you will have to take care of how the soil is fertilized. Through this article, you will learn about the kind of fertilization onions require and the best fertilizers for onions available on the market.

Onion Pests and Diseases and How to Prevent Them

Much like any other crop, onions have their share of pests and diseases that they are prone to. You need to be aware of these problems that you might face. Therefore, given below are some tips that you could follow to save your crop.

Onion Maggots

Onion maggots tend to lay their eggs at the base of onions. They thrive on decaying conditions, organic matter, and rainy periods.

To prevent onion maggots from destroying your crops, you could cover fresh onions with a mesh netting. You must seal this netting by molding soil. Avoid using mulch as it contains decaying organic matter. The net will prevent maggots from laying eggs. If you have a dry season, you do not need to take this precaution as the dry season does not attract maggots.


Thrips are insects only as thick as a needle. These insects are usually tan-colored and can be easily spotted if you place onion tops against a dark piece of paper. If thrips are present in your harvest, you must use proper insecticides to prevent them from eating your growing onions.

General Tips to Care of Onions

Besides using fertilizers, there are a few things that you can take care of while growing onions to ensure a good harvest. Some convenient to follow tips are given below.

  • Grow onions as if they are not a root crop but a leaf crop.
  • Fertilize the onions with large amounts of nitrogen until they start bulbing.
  • Once your onions start to the bulb, stop fertilizing them. Do not replenish soil around the onions since the bulbs have to emerge over the soil naturally.
  • If you are aiming to get sweet onions, you can water the plants frequently. However, too much watering can attract pests. One inch of fresh or rainwater per week is sufficient for onions.
  •  If you have a dry season, make sure that you water adequately. This is necessary since bone dryness does not reflect the appearance of onion bulbs.
  • Do not use too many insecticides on your onion foliage because the onions might absorb them. Generally, a biweekly insecticide is a right balance required to have insect-free onion growth. 

Watering Your Homegrown Onions

Onions are high-feeding yet low-forming plants. They require a lot of water throughout their lifetime. However, too much water could also be a cause of issue for most onion crops since too much moisture attracts pests. There needs to be a delicate balance in the amount of water and the regularity in which water is poured into the crops.

How Much Water is Required?

Onions require constant moisture in the soil. However, too much moisture will cause the vegetable to decay rapidly. As mentioned above, you must remember only to irrigate onions till they start bulbing. It would be best if you stopped irrigating after bulbing to let the plants grow without external interference.

Uniform irrigation at a low depth is essential to maintain healthy onion growth. Onions require not more than one or two inches of water per week at a depth of about 4 inches.

You can place some mulch near the bulbs to maintain the soil’s moisture. However, you must remember to remove the much when the onions start bulbing. If you keep the mulch put near the bulbs, you can risk over-moisturization and consequently destroy the crops.

An excess of much after the onions start blooming will also take away the room the bulbs require to push themselves out of the soil.

When to Irrigate Onions?

The best time of the day to irrigate onion is late in the evening. However, you can also irrigate early in the morning. Watering at these hours will help the water penetrate into the soil as the water will not evaporate by the sun’s heat.

How to Irrigate Onions?

You must never succumb to overhead watering because most fungal damages occur due to leaf wetness. Every time the leaves wet, they attract bacterial and fungal spores and ruin the entire foliage.

The best way to irrigate onions is by either using the drip irrigation method. However, this method is expensive to install. Therefore, a soaker hose will work the same way to keep your onion foliage from getting dry and unhealthy.  While using a soaker hose, make sure that you have turned the settings to misting to avoid the uneven water spread.

When and How to Feed Nutrients to your Onions?

Onions are heavy feeders. Therefore, they require a high amount of nutrients to be fed to them if you want a healthy growth of onions. Feeding onions with fertilizers rich in nitrogen is the right way to go. Fish emulsion is also a good fertilizer for onions.

You should conduct the first feeding of fertilizers towards the beginning of the growing season. This feeding will allow the onions to grow large bulbs. The second feeding should be done about a month after the first feeding. You could also use some compost to side-dress the row of plants.

You can continue feeding throughout the time the onions grow. However, the onions will grow bulbs about a month before harvest. You must cut back on both fertilizers and water at the point. Allowing the onions to grow in less fertile and dry soil results in a sweeter and better quality onion in the last month.

Soil Requirements

Even though onions are not generally considered to be difficult crops to grow, they require a lot of fertilization. Other than proper fertilization, there are many other things that onions need to grow big and luscious. Optimum soil fertility is crucial for onions, especially in areas of the land that might have low fertility.

You should get your soil tested before you set to growing onions. Getting a good soil profile will allow you to check the nutrient profile of your soil. This profile will give you an idea of the fertilizers that your soil requires to grow onions.

Given below is a list of nutrients and other soil properties that onions require to grow correctly. Make sure that you provide all these requirements to the growing onions for a golden harvest.

Soil pH

Much like any other plant, onions too have a preferred pH range to thrive. Without the perfect pH level, onions will not thrive in your soil.

The recommended pH level for growing onions is between 6.5 and 8.0. If the pH level is too high or too low, you will notice complications in the way the plants grow. The results of highly acidic soil would read to sour and blackened onions, while a highly alkaline soil will lead to plants’ slow growth.

After you get your soil tested, you will get an idea of the pH level that the soil naturally has. If the pH level is not perfect according to your requirements, you should add alkaline or acidic elements to your fertilizers.

Soil Density

It is necessary for you to provide loose soil for the onions to grow in. No matter how many nutrients you add to the soil, dense and heavy clay makes it impossible for onions to grow. Onions require light and loose soil that allows the bulbs to push out of the soil at ease. 

You can add about two feet of ground leaves onto your soil in the fall season. You can slowly work them into the soil by the time you plant the onions. This will make the soil loose and easy for the onions to grow in.

Organic Matter

While almost all food crops require organic matter to thrive, onions require it on a large scale. So much so that most commercial onions require muck soil to grow. Muck soil is a special kind of soil that is extremely high on organic matter. Therefore, it promotes the rapid and healthy growth of new onion bulbs.

Fertilizers that have fish emulsion or other organic matter in them are perfect for onions. However, you can provide organic matter to the soil by adding surplus vegetables, weeds, manure, and even lawn clippings to the soil.

You can start adding organic materials into the soil about eight months prior to when you are planning to plant onions. This will allow the soil to completely imbibe the organic nutrients to the soil that onions appreciate.

You can add composted manure or compost in the soil immediately before you plan onions. If you had been preparing your soil in advance, you could skip on this step; however, if you want to plant onions immediately, adding some organic material to the soil before plantation is a must.


Fertilizing the soil with high volumes of nitrogen is a must for growing onions. Even though the plant itself has only a moderate requirement for nitrogen, their shallow root systems multiply the need. Most organic matter itself can be a good source of nitrogen, but adding nitrogen-based fertilizers to the soil will help your onions grow in a better manner.

Ever 100 square feet of growing land require an application of about 51/4 ounces of nitrogen. This quantity can be supplied by either 2.6 pounds of blood meal, 4.6 pounds of cottonseed meal, or even 3.3 pounds of 10-10-10 fertilizer.

Phosphorous and Potassium

It is generally believed in the harvesting community that onions require high volumes of organic matter and nitrogen. While that is true, onions require higher levels of Potassium and Phosphorous.

A deficiency in either of these nutrients can lead to several diseases in the onions because the plants do not develop the required immunity. Another reason why these nutrients are s important is that they promote a robust root-system formation.

You must surely use fertilizers that have high phosphate and potash content to ensure good growth. Generally, onions require about 4.8 ounces of potash and 2.2 ounces of phosphate per 100 square feet of growing area.

Wood ash and bone meal are rich sources of these two nutrients. Therefore, you can add 6 pounds of wood ash to the soil along with 1.1 pounds of bone meal to supply the correct percentage of these nutrients. You can also use about 3 pounds of any commercial-grade 10-10-10 fertilizer to the soil for great growth.

Minor Nutrients

Even though onions require minor nutrients in a minor amount, they are necessary for growth. Nutrients such as Sulphur, zinc, manganese, and copper help onions grow perfectly. You can add two pounds of kelp meal or azomite per 100 square feet of soil to provide these nutrients.

Fertilizing an Onion Foliage

Onions require a lot of fertilization because they are heavy feeders and require help in growing strong roots. With the right volume of fertilizers, you can promote a rich and robust root system to develop. Fertilizers also help the onions to sprout at an early age.

Kind of Fertilizers Required

The adequate amount of potassium and phosphorous content in fertilizers is important to supply strength to the plant. This strength is utilized in creating a healthy root system that helps in establishing the crop. If you want your onions to taste mild and sweet, you will require Sulphur.

Since onions are low-forming and have shallow roots, they require proper Nitrogen supply. The shallow root system makes the onions’ absorption rate slow down immensely. With the help of nitrogen, plants grow rapidly and absorb more nutrients from the soil.

You must make sure that you only use fertilizers that are rich in Ammoniacal Nitrogen and not Nitric Nitrogen. The latter form is known for being unsuitable for onion crops as they deliver great losses to the foliage.

Generally, the supply of nutrients should include 1/3rd of nitrogen, ½ of potassium and Sulphur, and ½ of phosphorous. Therefore, using a 10-10-10 fertilizer is a good option. However, you can also use fertilizers that have a lower volume of nitrogen. Other suitable NPK ratios are 12-12-17, 12-10-20, and 11-15-15.

You can also use any nitrate-based fertilizer to make your soil’s nutrients available for the onion. Therefore, compromising on sulfate for nitrate is not too bad an idea. This will allow you to chop the onions tear-free.

If you are not a fan of commercial and chemical-based fertilizers and want to grow your crop organically, you will have to make some extra efforts. To achieve this kind of result, you will have to add a thick layer of rotten manure or compost to the soil. This will supply most of the important nutrients to the soil.

Generally, fertilizers derived from blood meal, feather meal, fish, and bone meal are great for onions. All these ingredients are a rich source of any of the three most important nutrients required by the plant.

When and How Often to Fertilize Onions

All Allium species have two stages of growth. The first stage is the vegetative phase, and the second is the bulbing phase. It is important to make the plants ready for both these phases.

The vegetative phase is when the onions focus on green growth and maximizing foliage. The plants use this foliage for synthesizing sugars that are required for the bulbing stage. The bulbing stage is when the onions grow bulbs and produce the vegetable that is required of them.

The best way to utilize the nutrients you will provide the plants is by fertilizing the soil twice. As mentioned earlier, the first fertilization should be done before sowing the onions. You can conduct this fertilization through preliminary irrigation. The first irrigation is not to be confused with the soil preparation, including tilling and adding organic matter into the soil.

The second fertilization should be conducted about a month after the first one. Another indicator of when the time of second fertilization has approached is to be on the lookout for sprouting. You must irrigate the soil immediately after the onions start sprouting.

The second fertilization is done at such a time because you’ll stop adding fertilizers after the plants start bulbing. Onions bulb about a month after sprouting. Once the onions start to the bulb, it is best to leave the onions alone. They can grow in drier and less fertile conditions once the bulbs start to appear.

How to Fertilize Onions

As mentioned before, you will need to fertilize the soil twice throughout the 90 days of growing time to onions. Your fist fertilization is done before the onions are set in. Since this fertilization does not include adding chemicals to the crop directly, you can simply add fertilizers to the soil during pre-plantation irrigation.

For the second type of fertilization, you will require to follow a different method. It is best to dig up pits around your onion’s patch. Once the spot is ready, you can fill the ditches with your choice’s fertilizers before filling the soil again.

Make sure that you make the soil loose with each fertilization since that will allow the bulbs to grow bigger and push through the soil at ease.

The 4 Best Fertilizers for Onions

By now, you know all about fertilizing onions; you can proceed to buy the fertilizer that will make harvest easy for you. We have consolidated a list of the four best fertilizers for onions that you can use on your onion patch. These fertilizers will help in growing huge onions that are disease-free and have robust root systems. So, without further ado, let’s proceed to the reviews.

1. Homesteader Hobbles Organic Onion Fertilizer

NPK ratio – 3-4-3

Homesteader Hobbles have come up with an organic fertilizer that is perfect for onions. This organic fertilizer is designed specifically for the plant in question and is therefore made to be optimum for onion growth.

The Homesteader Hobbles Organic Onion Fertilizer is non-GMO and completely organic to allow better plant growth. There are many other properties of the fertilizer that will allow you to yield great-looking and healthy onions.

Since the fertilizer has a high phosphate level, it will allow your onions to set deeply and create bigger bulbs. The adequate nitrogen level allows the vegetable to grow faster and allows you to produce many bulbs on a small land.

The source of all the nutrients provided is completely organic. They come from blood meal, feather meal, fishbone meal, Sulphate of potash, gypsum, and alfalfa meal.

The best way to use this fertilizer is by sprinkling two full teaspoons of the fertilizer around each onion plant. You must mix the fertilizer gently into the soil and irrigate after application.


  • Great NPK ratio
  • Complete organic and GMO-free
  • Designed especially for onions


  • Very mild fertilizer only suitable for extremely fertile soil

2. Hoss Tools Ammonium Sulfate Fertilizer for Onions

NPK ratio – 21-0-0

As you already know, Sulphur is an important ingredient that fertilizers for onions should have. However, it is common to see many fertilizers negate the importance of this nutrient. The Hoss Tools Ammonium Sulfate Fertilizer is great for adding Sulphur and nitrogen to the oil. Not only will this help in forming better bulbs, but it will also increase foliage production.

This fertilizer is perfect for all allium species such as onions, shallots, garlic, and leeks. It helps in increasing the foliage during the vegetative phase. With increased foliage, the crops will grow bigger bulbs during the bulbing phase.

The Hoss Tools Ammonium Sulfate Fertilizer is an incredibly fast-acting source of nitrogen and sulfur to the plants. It can either be applied through side-dressing. However, since this fertilizer is water-soluble, it can also be applied through injections.

The recommended amount of application is 2 cups per 40 feet of growing land while side dressing. It is important to add water to the crops immediately after application to allow the soil to soak the nutrients.


  • Increases foliage
  • Helps in growing bigger bulbs
  • Provides Sulphate to the onions
  • It can be applied in multiple ways.


  • Does not have any phosphorous or potassium

3. Greenway Biotech Cottonseed Meal Fertilizer

NPK ratio – 5-2-1

Using synthetic fertilizers can be detrimental to the plant’s growth instead of helping them grow. As the GMO level of the fertilizer increases, it becomes dangerous to the plant. Therefore, the world is moving towards GMO-free fertilizers. The Greenway Biotech Cottonseed Meal Fertilizer is one such product. It allows you to grow plants in a healthy manner.

The 5-2-1 Cottonseed Meal fertilizer is efficient in helping in growing healthy onions. With the help of the extra kick of nitrogen, you will surely see an increase in the foliage along with an increase in the size of the bulbs. Other than nitrogen, phosphorous, and potassium, this fertilizer also includes lead and mercury.

This is a great fertilizer for your onions if your soil is too alkaline. This fertilizer adds acidity to the soil. The release potash, nitrogen, phosphorous, and other nutrients slowly. Therefore, the nutrients are available for the plants to imbibe for a long time.

With the help of this fertilizer, you can help get robust roots in your onion plantation. It can help aerate even dense soil. However, since you will be using loose soil for your onion plantation, you will be able to get very oxygenated onions.

This fertilizer is also great at holding organic matter. Therefore, you can add as much organic matter as you want and not worry about the nutrition not reaching your plants.


  • Non-GMO
  • Aerates even dense soil
  • Holds on to the organic matter
  • Promote vigorous growth
  • Contains other metallic nutrients required by onions


  • Low potassium content

4. Burpee Organic Bone Meal Fertilizer

NPK ratio – 6-8-0

Who does not want big onions? The best way to go about growing big onion is by providing organic fertilizers to the soil. The Burpee Organic Bone Meal Fertilizer is a great choice for you if you want a big onion growth.

This fertilizer aids the onions’ vegetative growth. It also helps in forming stronger roots and bigger bulbs. The growth volume is increased due to the nitrogen content in the fertilizer. Whereas the phosphorous allows the roots to be strong and well-set.

This fertilizer is great at providing calcium to your onions since it is made of bone meal. Calcium is one nutrient that is often negated in onion plantation but is essential for healthy onion bone growth.

Since the nitrogen in this fertilizer is water-insoluble, it is released very slowly into the soil. Therefore, your onions will have access to the nutrient for a longer period. The same is with all the other nutrients in the fertilizer. Since the bone meal is a natural fertilizer, it does not have any inorganic additions. This fertilizer also has an OMRI certification, which proves its efficiency.

The best way to apply this fertilizer to the soil is by sprinkling one teaspoon of the fertilizer per plant. You must mix the fertilizer into the soil properly and do not forget to irrigate later.


  • Helps in vigorous foliage growth
  • Increases the root strength
  • Completely organic
  • It contains calcium and promotes healthy onion bone growth.
  • OMRI certificated


  • Does not have any potassium

Frequently Asked Question

Given below are some of the questions that are frequently asked regarding fertilizing and growing onions. Read through them if you face similar issues and queries regarding the topic.

1. How many onions can I expect to grow out of single produce?

Onions are an extremely high-yield crop. With only a single plantation, you can expect a large number of onions that you can harvest. With proper care and fertilization, you can expect about 50 onions to grow in one square foot of growing land.

Since you can grow onions in any odd place alongside any kind of crop, you can expect to have plenty by the end of the season.

2. What is causing my onions to be too small?

There are two reasons why your onions could be too small. First is that you might have planted the seeds too close to each other. The second reason is that the onions might have a nitrogen deficiency. When the onions are planted too close to each other, they keep colliding. Therefore, they do not get enough space to grow to their full size.

3. In which month should I plant onions?

Cool-weather is perfect for growing onions. Therefore, if you live in mild-winter regions, you will want to plant your onions in the fall season. The best month to plant onions in such regions is October. If you live in moderate temperature areas, growing onions in the spring season are perfect. Therefore, the month of August is great for planting onions.

4. Is it okay if my onions flower?

Onions do flower naturally. However, bolted onions do not store well. Therefore, if you see an onion flowering, it is best to snip it and consume it first. Bolting onions will rot quickly. This is a problem that the best of onion-growers face from time to time.

5. How much sun do onions need?

Onions require a lot of sun to grow perfectly. On average, onions require at least thirteen hours of sunlight on a daily basis. The bulbing phase especially requires the maximum amount of sunlight that the plants can get. This is why it is best to plant onions when it is mildly cool. Too much heat from the sun will burn the crops.

Final Words

Onions require a lot of fertilization, feeding, and watering to grow to their full extent. Since they are high-feeders, it is important to make sure that your plants get the right nutrients. With the help of the best fertilizers, you can get a high-yield of onions that can be great for commercial or personal use.