Choosing the Best Fertilizer for Onions

Growing onions successfully can be a daunting task, but if you are planting in the right conditions, in healthy, living soil, and using the proper fertilizers, then you will have a much better time making sure you grow a tasty onion crop. Let’s explore some ways to ensure you are choosing the right fertilizer for your onions. 

Organic or Chemical Fertilizers for Onions?

Onions love to eat! So make sure they are getting plenty of nutrients in their diet. But when choosing between chemical-filled groceries at the store versus natural, organic ones, wouldn’t you rather be eating the all-natural option? Well, so would your onions! 

Nutrient Release: Slow and Steady Wins the Race

Organic fertilizers provide nutrients in a slow and steady capacity, as opposed to a direct hit. This allows for your onions to steadily absorb their nutrients over time. On top of just a slow release, chemical fertilizers only supply nitrogen, phosphorus, and potassium, while organic fertilizers can supply those plus micronutrients such as sulfur, magnesium, and zinc, which are crucial for onions.

You also run the risk of over-fertilizing with chemicals which can lead to excessive top growth at the expense of bulb development (especially if you give your onion too much nitrogen). 

Soil Health

Don’t just think about this onion crop, think about future onions or other plants that will live in this same soil! Going the all-natural route is making sure your soil stays healthy, not just for this onion crop, but future vegetation. Don’t elect to kill your hearty soil by dumping chemicals on it. 

Organic fertilizers improve the soil structure and increase its organic matter content, which enhances water retention and aeration, crucial for onion crops that need well-drained soil. 

Organic fertilizers also stimulate the growth of beneficial microorganisms in the soil, promoting healthier soil ecosystems that improve nutrient cycling and availability to plants.

What is the Best Organic Fertilization Option for Onions?

Now that we have established chemicals are not in the best interest of your onions, what should we use on them? Let’s take a look! 

Nutrients of Importance

For the best quality onions, you want to avoid nitrate nitrogen such as a 10-10-10 unless sulfur and molybdenum are properly addressed. Onions like a fair amount of nitrogen in its natural form (amino acids), such as feather meal, bone meal, soybean meal, soy protein, and blood meal. Nitrogen will help give the bulb more depth. Nitrate nitrogen and manures are not desired and will cause them to rot prematurely and greatly reduce shelf life.

Aside from nitrogen, calcium is important early in the growth process, and for longer shelf life in storage. Copper improves overall disease resistance and reduces cracking of the outer layer. Lastly, Potassium gives onion bulbs more width and is especially important in the last two months before harvest. 

How to Provide These Important Nutrients for Onions

The type of fertilizer you use on your onions will depend on what nutrients they are lacking. So conducting regular soil tests is crucial to choosing the right fertilizer. Some of the best options to ensure your onions are growing strong and healthy are: 

7-7-7 NatureSafe

The dry-soluble 7-7-7 NatureSafe will provide a balance of nitrogen, potassium, and phosphorus, and is one of the most popular options to apply for onion plants. 

Explorer 16-0-0

This dry-soluble 16-0-0 will provide your onions with the nitrogen they need from amino acids. Use this for a boost in nitrogen and energy for nutrient uptake. 

Potassium Sulfate (0-0-50)

This soluble 0-0-50 is 50% potash and 18% sulfate and is recommended to use in combination with a source of carbon such as Rejuvenate, Key Factor, or molasses.

BioQuest Foundation Microbial Inoculant

Bioquest Foundation is a microbial inoculant designed to provide the soil with live beneficial microbes to enrich soil health and provide proper nutrients to the crops growing in it. We have tested this inoculant with onions specifically and found it to work effectively and efficiently. Below is a testimonial and case study from one of our farmers:

“I am very impressed with the BioQuest Foundation and EnSoil Algae products. My onions were in very bad condition, seeming with a bad case of damping off. I called in to Keystone Bio Ag and Kevin came out to look at them and recommended to use BioQuest Foundation and EnSoil Algae, which I did along with a carbohydrate. It really turned them around. Instead of being brown and looking like they were going to die, they greened up tremendously after several applications in one weeks’ time. I am convinced it saved my crop.” – R. Stoltzfus

Application and Harvesting

First of all, planting at the right time is of utmost importance before any type of fertilization is even being considered. To get the maximum size out of your onions, plant them at the end of March (if you are in the Northeast). 

As you approach harvest, walk your fields every day, from one end to the other. Observe the plants; are diseases setting in? Especially watch for center rot. Every several days, pull a 5 foot section in several areas of the field. Check for “Slippery Skin”, which is when the onions become soft and mushy, generally starting with the outside skin of the onion splitting, as a result of a structural calcium or copper deficiency.

The goal is to leave the onions out as long as possible without getting too much disease, of course keeping your workforce in mind if you would suddenly need to harvest. As long as the onions are staying sound, you will typically see around 5-6 bins per acre increase from leaving them out an extra week. Of course, this can go the other way fast if the onions aren’t sound and we get lots of rain. Rainy, high humidity days can trigger some really fast disease development.

Cobalt applications during the last month before harvest helps to keep the plants going longer by slowing die-down. Die-down will eventually get the onions to a point where you have to harvest them even if there is minimal disease pressure there. The tops of the leaves will dry down as the onions go into senescence, which will mean that the plants actually start getting shorter. That’s okay, as long as you don’t get a lot of rain.

After harvest, storage is also critical. Store in a cooler for the longest storage life: 35 degrees and 65-70% humidity is ideal. 

Where to Buy Organic Onion Fertilizers

Keystone Bio-Ag is a leader in regenerative ag products, including dry, granular fertilizers as well as liquid fertilizers for foliar applications. Contact us for a soil test or for a regenerative farming consultation. You can also find our fertilization products at one of our many dealers across the country.