Having The Best Fertilizer For Onion Must Be Paired With Something Else

As you are probably aware, when it comes to gardening, nothing is simple nor straightforward. There are always some new situations and difficulties which are just sitting around the corner to make things bad. If you have the best fertilizer for onion, this doesn’t mean that you just spread it around, and your job is done, no. This is merely the tip of the iceberg. There are few other things which must be done so that fertilizer is used correctly.

This article, along with my previous two about spring onions and ways to harvest and store onions will make an excellent trilogy, so you will have a beautiful set of guides which will cover most important parts of the art of onion growing.

Best fertilizer for onion


Does The Type Of Onions Influences The Fertilizer?

type of onions

Generally speaking, all of the varieties of onions have the same requirements when it comes to fertilizing. However, do not turn things other way around. Buying fertilizer and then adapting to buy the seed which fits it is counterproductive.

The differences between varieties are more of cosmetic ones. Some varieties will be more resistant to frost, while others will be reddish or purple. The bottom line is that your preference will make the decision here; just do as you see fit.

Improper Usage Can Do Irreparable Damage


Not all fertilizers are intended for the same usage. Some of them are milder and last a few days until absorbed, while others are stronger, but are releasing this amount of nutrients during a prolonged period. The first one can be manure tea, for example, and the other a slow-release fertilizer. Depending on the current situation, the particular type can be used, although the first one is more often used when it comes to onions.

Applying smaller quantities of fertilizer can make growth insufficient, which can be bad, especially if you are tight on space, and looking into good harvest. However, applying too much of it may even kill the plant, causing burns on the bulb or leaves. This is what I thought about when I said “irreparable” damage. Therefore, the best advice I can give you is to follow package labels and if you are not sure about how it will turn out, put a bit less of it.

Water Can Be Your Best Ally Or Greatest Menace

Suitable watering for onions is of vital importance. They might be small in size but are known as demanding when it comes to this liquid. But, it is up to you to decide whether the water will help or hinder you.

The requirements for water are highest at the beginning of growing cycle. Until bulbing phase, enough water and fertilizer must be provided. However, if you continue to water it as before, it is very likely that the taste will be diluted, and the bulbs will puff up unnaturally, which is not what for you are aiming.

My approach when it comes to onions was to set the area where the onions will be planted and to work in the fertilizer for at least three weeks before sowing. This way I was sure that the soil will retain nutrients, and that they will be quickly absorbed. After each round of tilling, I was watering the soil generously, so that water helped to infuse fertilizer.


Of course, since the onions are planted in early spring if the end of the winter is still troubled with frozen ground, this makes a whole process impossible. If this is the case with your town, I suggest you infuse the fertilizer or manure late in autumn, right after the onions are harvested.

Fertilizer Or Manure?


To be honest, I don’t think that these two exclude each other, especially when it comes to onions. They are heavy feeders as well, so extra food won’t harm them. Keeping numbers in line is of utmost importance, however, and this is where soil test kit and pH tester can show their full strength.

In fact, combining these two is essential for a successful harvest. The manure is implemented early, and because of beneficial bacteria and constant turning of the soil, it makes the ground well-aerated and puffy. This are ideal conditions for onions to grow, and I have always aimed for that.

Also, manure improves the drainage of the soil, while being able to tie up the moist, and this is another big plus for onions to grow. 

But, About Those Numbers…

numbers NPK

To clarify things a bit further, here is my approach when it comes to nutrients within the soil. As we know, fertilizer is marked with particular NPK index. This stands for Nitrogen (N), Phosphorus (P), and Potassium (K). These three elements are the building blocks and stepping stones which are making the plant stronger.

Each of these elements has its purpose, and according to onion’s demands, they need to be fulfilled.


is undoubtedly the most important to onions during the development phase. It makes leaves and the neck of the plant stronger and greener. Depending on the acidity of the soil, you may consider applying ammonium-sulfate (21:0:0) for slightly alkaline soil, or calcium-nitrate (15.5:0:0) for those a bit acidic. Fertilizing with these should be repeated for every 2 to 3 weeks until the bulbs begin to develop.


is the element responsible for proper development of the fruit of the plant. Since onions don’t bear fruits, the bulb will benefit from this ingredient. Also, this element is best used at the beginning of the cycle. In fact, working it in the soil can do wonders


the third component of every fertilizer is here to be sure that the plant is processing others in proper amounts. Think of it as a police officer; no need to have it too much, and is here to ensure adequate working for everything else

Checking The Acidity Regularly

checking the acidity

In case that you thought that after reviewing the pH value of the soil once is all that is needed, you would be wrong. This factor is in constant change. Rains, fertilizers, and even sunlight can affect and change it. Therefore, it is vital to keep it under control.

My custom is to check for pH value every week and see what happens next. I kept in mind two red lines which I couldn’t cross; 6.3 and 6.8. These are outlining the absolute values of pH for the onions to grow. Of course, you don’t have to be strict as I was because a few points over or under won't obliterate the whole harvest; I was merely interested into seeing whether it is true or not.

Leveling the pH value is not so difficult, all you need to do is to apply limestone to raise pH, or sulfur to lower it. These can be bought either online, or in every gardening center; diluted in water, and spread around by using the sprayer.


As I said at the beginning, having the best fertilizer for onion is an excellent thing, but to have fantastic results, you have to pay attention to much more. I hope that I managed to point out some important things when it comes to successful growing of onions and that this text will serve its purpose.

In any case, feel free to leave a comment, opinion or suggestion in the comment section below. I’m looking forward hearing from you.

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