The top fertilizers for growing onions are:
- Ultimate Fertilizer 10-20-10 with Sulfur
- Hi-Yield 21-0-0 Ammonium Sulfate Fertilizer
- Jobe’s Organics Vegetable and Tomato 2-5-3
- Down to Earth Organics 4-4-4
This selection of fertilizer includes both synthetic and organic options. Synthetic options are fine-tuned and contain high levels of sulfur, for excellent onion flavor. However, organic fertilizers are an incredible choice that yields large, tasty onions.
Is Manure Good for Onions?
Untreated manure should not be used as a fertilizer for onions or any other plant. Pure manure harbors dangerous bacteria and pathogens that can make onions dangerous to eat. If you wish to use manure as a natural fertilizer, use only composted manure. The composting process destroys harmful bacteria, rendering manure safe for use as fertilizer.
- Raw manure should not be used to fertilize onions.
- Manure contains harmful bacteria such as E. coli and Listeria.
- Spreading manure on onions can cause anyone who eats the onions to become ill.
- Compost that contains manure can be used as an onion fertilizer.
- As compost decomposes, it generates heat that kills off harmful bacteria.
Although manure is often an ingredient in compost, it is not the only component. Proper compost combines manure with slow-decomposing materials such as wood chips and straw. This results in a nutrient-rich fertilizer that can be mixed into the soil to boost onion growth.
Is Granular or Liquid Fertilizer Better for Onions?
Granular fertilizer is the best choice for onions. Liquid fertilizers often wash out of the soil too quickly for onions to absorb them. Meanwhile, granular fertilizers break down over time. This gives slow-growing onions a chance to pull in all the essential nutrients. So, all the onion fertilizers we select are granular-type fertilizers.
- Onions grow best when fed with granular fertilizer.
- Using liquid fertilizer often results in underfed and undersized onions.
- Other plants may prefer liquid fertilizer.
Just because onions prefer granular fertilizer, doesn’t mean all plants do. Liquid fertilizers are an excellent choice for garden plants that benefit from foliar spray fertilizers. Some of the best fertilizers for new sod are liquid fertilizers.
The 4 Best Onion Fertilizers for Your Garden
Onions have specific nutrient needs at different stages of life. Below, we’ll walk through our choices for the best onion fertilizers. Additionally, we’ll explain when and how to use each one.
Best Starter Fertilizer: Ultimate Fertilizer 10-20-10
Onions require high levels of phosphorus when they are young. This nutrient is essential because it helps onions form the large root bulb that makes up the edible portion of the plant. Ultimate Fertilizer contains 20% phosphorus by weight, making it the perfect choice for onion seeds and seedlings. As an added benefit, Ultimate Fertilizer also contains 17% sulfur. Sulfur is responsible for rich, flavorful onions.
- Prepare onions for success with this 10-20-10 fertilizer.
- The high phosphorus and sulfur content in this fertilizer encourages healthy root growth.
- Spread the fertilizer along the bottom of the trenches, cover with 3 inches of soil, and plant your onion seeds.
To use your starter fertilizer to boost onion growth, dig trenches 3 inches deep where you plan to plant onions. Then, sprinkle Ultimate Fertilizer along the bottom of the trench. Use ½-cup of fertilizer for every 10 feet of trench. Cover the fertilizer with 3 inches of soil. Then, plant your onion seeds ¼-inch deep.
Top Monthly Onion Fertilizer: Hi-Yield Ammonium Sulfate 21-0-0
After your onion seeds have had one month to sprout and establish themselves, they require monthly applications of high-nitrogen fertilizer to fuel growth. Ammonium sulfate is the best choice because it contains 21% nitrogen, which helps your onions grow vigorously. Additionally, ammonium sulfate contains 24% sulfur, which will help you grow flavorful onions. Without enough sulfur, your onions may taste bland and watery.
- The high nitrogen and sulfur content in this Hi-Yield 21-0-0 fertilizer makes it perfect for developing onions.
- Begin fertilizing onions with 21-0-0 one month after planting.
- Side-dress your onions with ½-cup of 21-0-0 every 4 weeks to grow the best onions possible.
- Stop fertilizing 4 weeks before you plan to harvest your onions.
To use Hi-Yield ammonium sulfate, make your first application 4 weeks after you prepared the soil with starter fertilizer and planted your onion seeds. Begin by digging a thin trench between your rows of onions. The trench should be 3 inches deep. Each trench should be about 6 inches from the onion plants. Then, sprinkle ½-cup of 21-0-0 fertilizer into the trench. Cover the trench with soil and water your onions. Repeat this fertilizer application each month. Stop fertilizing 4 weeks before you harvest your onions.
Best Organic Starter: Jobe’s Organics Vegetable and Tomato 2-5-3
You don’t need to use the synthetic fertilizers we recommended above to grow incredible onions. There are fantastic natural options. Instead of 10-20-10 fertilizer, you can use an organic 2-5-3 fertilizer. Apply it at a 3-inch depth, just as you would with 10-20-10. However, use 2 cups of 2-4-3 fertilizer for every 10 feet of trench. Then, plant your onion seeds.
- You can replace 10-20-10 starter fertilizer with this organic 2-5-3 fertilizer.
- Repeat the steps for applying 10-20-10 fertilizer, just use 2 cups of 2-5-3 for every ½-cup of 10-20-10.
- Mix organic compost into your soil prior to planting for an added growth boost.
When using organic fertilizer, it’s also a great idea to mix compost into the soil in your onion bed. Spread 2 to 3 inches of compost on top of the soil. Then, use a shovel or hoe to mix the compost thoroughly into the top 6 inches of soil. This will ensure nutrients reach the onion root layer. Compost left on the soil surface will actually lose nutrients as it is exposed to UV rays.
Favorite All-Natural Fertilizer: Down to Earth Organics 4-4-4
You can use a balanced organic fertilizer to feed your onions each month. I prefer 4-4-4 organic fertilizer. Side dress your onions just as you would with 21-0-0 ammonium sulfate. However, quadruple the amount of fertilizer used. So, use 2 cups of 4-4-4 for every 10 feet of onions you are side dressing.
- This balanced, organic fertilizer is a great choice for monthly onion feeding.
- Side dress onions with 2 cups of 4-4-4 organic fertilizer for every 10 feet of your onion row.
- Once your onions are nearing harvest, stop fertilizing with 4-4-4.
Just as with synthetic fertilizer, stop fertilizing your onions 4 weeks before harvest. If you’re new to growing onions, you may not be certain how to judge harvest time. Read on to discover what signs tell you you should stop fertilizing your onions and prepare for harvest.
When Should You Stop Fertilizing Onions?
You should stop fertilizing onions in the middle of summer, typically in July. You will know onions don’t need any more fertilizer applications when the leaves begin to naturally turn yellow. At this time, feel the necks of your onions. If they are soft and pliable, it’s time to stop applying fertilizer.
- In mid-summer, typically late June or July.
- When outer onion leaves begin to turn yellow.
- As soon as the “neck” of the onion (the portion just above the bulb) begins to feel soft.
- You can harvest onions 4 weeks after your final fertilizer application.
After you stop monthly fertilizer applications for your onions, wait 4 weeks to harvest. This allows time for your onions to develop flavor. If you continue to fertilize your onions past this point, you won’t get the best onions possible
What Type of Fertilizer is Best for Onions?
Onions require two different types of fertilizer during their life cycle. At planting time, they require a high-phosphorus fertilizer that helps them form healthy young bulbs. A 10-20-10 synthetic fertilizer with high sulfur content is a great choice. However, you can substitute this with higher volumes of an organic 2-5-3 fertilizer.
Once your onions have established themselves, they require monthly feedings with plenty of nitrogen. Side-dressing onions with 21-0-0 ammonium sulfate fertilizer is the best choice. Alternatively, a balanced, organic fertilizer is also ideal. Try a 4-4-4 organic fertilizer for all-natural monthly feedings in your onion patch.