Soil for Onions
Onions are a bit more picky about their soil than some other vegetables I’ve grown. I still think they’re an easy crop to grow, even for new gardeners, but you’ve got to get the soil right.
TL;DR
Onions grow best in loose, well-draining, sandy loam soil, but will grow in clay soils if amended with plenty of compost.
I’ve found onions grow best in soils with a pH between 6.3 and 6.8.
Most soils have ample amounts of phosphorus and potassium already present in the soil; however, I usually add an organic nitrogen additive before planting. Nitrogen is the most important nutrient for onions.
Fox Farms Happy Frog and SunGro Black Gold are my go-to commercial potting mixes for growing onions.
Best Soil For Onions
Onions require a few things from their soil: good drainage, good aeration, and lots of nutrients. I’ve grown onions in clay, but it has to be amended first otherwise you’ll end up with tiny or deformed bulbs.
This is what good soil looks like.
Onions grow best in loose, well-draining, sandy loam soil. If you can grab some moist soil and squeeze it in your hand it should form a ball and then crumble apart if you poke it with your finger. That’s perfect soil for onions in terms of aeration and drainage.
If the soil doesn’t fall apart, it’s too compacted and will need amendments as well as loosened up.
Onions also do best in really fertile soil. Unlike crops like beets which are willing to tolerate lower fertility soils, onions will not. They want soil that is rich in organic matter, especially nitrogen. I achieve this by either using fresh potting mixes or by adding compost. I’ll talk about both a bit later.
Soil pH for Onions
Most articles I’ve read say that onions grow in soils with a pH between 6 and 7. That may be true, but when I’ve tested pH levels and grown onions, I find that they do best when the pH is between 6.3 and 6.8.
Unlike some other vegetables, onions have never tolerated very acidic or alkaline soils when I’ve tried them.
With the exception of trusted potting mixes, which I’ll talk about later, onions are one of the few crops that I almost always test my soil before planting. You can send samples to your local extension office, but I usually use the Luster Leaf Rapitest because I’m impatient and I’ve found it to be fairly accurate.
Not only will the test tell you the pH level of your soil, but it will also show you any deficiencies in key nutrients such as nitrogen, phosphorus, and potassium.
How to Amend Clay Soils for Onions
If you live somewhere like I do, my soil is all clay and rock. It gets hard when dry and holds water like a sponge, not to mention the hard crust it forms on the top which prevents water from permeating the soil.
That won’t work for onions. Been there, and tried that. You don’t want to grow in that stuff. One year the pure clay soil caused my entire crop to rot. But don’t worry, here’s my workaround.
When I want to grow onions in clay, I usually start preparing my soil months in advance.
The first thing I do is remove about a third to half of the clay from the area I’m going to grow in. I then replace that with compost from my Hot Frog Composter and mix it in really well with the clay- especially in the top 6 to 8 inches of soil. I aim for 50% compost and 50% clay soil.
As I’m doing this, I also remove any rocks and large roots, and I break up thick clumps of soil. This will improve the aeration and drainage of the soil.
Sometimes I don’t have compost broken down yet. If I have a few months in advance, I will often bury my kitchen scraps about 6” under the clay and let the worms and microorganisms do their work. Then before I plant, I’ll mix it all up together and I usually have about the same end result.
The other option is I sometimes buy compost online, especially if I don’t have time to wait for my kitchen scraps to break down prior to planting. I like to buy Organic Compost from Blue Ribbon. The quality is good and I trust them. They’re also one of the few I’ve found that is certified organic.
One final word about clay soils. Even after amending them, in some weather conditions, the top layer can still form a hard crust. If my onion hasn’t emerged yet, I may come back through after planting and carefully loosen the top soil without disturbing the seeds.
Other Amendments for Clay Soil
Other people have told me that they use sand, manure, peat moss, and lime but I don’t mess around with that stuff. The way peat moss is harvested for commercial use destroys entire habitats and is terrible for the environment. Lime can create environments for some pests to thrive and it can change the soil pH. Unless you compost manure correctly, it can introduce bad bacteria into your soil. And sand, well, I’ve just never found a need for it.
That’s my laundry list of why I don’t use them. I’m sure they work. Give them a try if you want to, but I know my compost method works and it’s not harmful to anyone or anything. In fact, it’s pretty hard to screw up.
Nutrients Needed for Onions
This is what rich humus looks like.
The last thing I want to touch on is the fertility of the soil. Onions love soil that is rich in humus. Humus is that dark, crumbly, organic substance that forms when matter decomposes - in other words good compost.
Like all other vegetables, it requires three main nutrients: nitrogen, phosphorus, and potassium. Of these, nitrogen is probably the most important.
Onions require a steady supply of nitrogen. I almost always add blood meal into the top 3 to 4 inches of soil before planting. Blood meal is a slow-release, organic fertilizer that can provide a steady supply to the plant.
Phosphorus is essential for the early growth of seedlings. If you’re growing in clay soil, you probably don’t need to add any of this. Clay usually has a high retention of phosphorus already. Most commercial potting mixes also have ample amounts of this nutrient for onions.
If you’re ever unsure about any of the 3 key nutrients, use the Luster Leaf Rapitest that I recommended earlier. It’s easy to read and will tell you if you need to amend your soil for any of these. The test reminds me of a pregnancy test, but for soil.
The last nutrient, potassium, is important for cell wall formation and energy reactions within the onion plant. Low potassium levels can make onions more susceptible to cold injury as well. I rarely have to add any of this to my soil. Potting mixes and soils amended with compost usually have ample amounts.
Onions do need some micronutrients as well, especially zinc, magnesium, sulfur, and boron. I find that if my pH is between 6.3 and 6.8, there are usually plenty of these minerals present in the soil. If your soil is alkaline though, you may have some deficiencies such as boron and I would test it.
Best Potting Soils for Onions
Whether you’re growing onions in containers, or raised beds, or you just want to replace the soil in your yard, over the years I’ve landed on two potting mixes that I prefer to grow my onions in.
When using either of these, you can simply add the soil to your raised bed or container and not have to worry about anything else. It is the easiest way to get the right soil; however, it’s also a bit more expensive.
The two commercial soils that I buy are Fox Farms Happy Frog and SunGro Black Gold All Purpose Mix. I usually buy both and mix them together. Both have the right pH levels for onions and the right nutrient blend. The Black Gold mix has a controlled-release fertilizer in it that I find to be a perfect mix for my onions.
I’ve tried other soils, but I always come back to these for my raised beds and terracotta pots.
Best Soil For Green Onions
Green onions or bunching onions are an exception to most of this. While they too grow best in sandy-loam soils with good drainage and aeration, I’ve found that they are less picky about soil.
My guess is that it’s because we don’t grow them for big bulbs. I’ve grown green onions everywhere. I will often just scatter seeds in different places throughout my yard and grow them in between other plants.
Here’s a picture of some green onions growing in between some strawberries in my yard. I never fertilize them and I’ve never done anything to the soil. They just keep growing.
Soil for Onions FAQ
How do you prepare soil for onions?
It’s best to amend soils with 50% compost prior to planting seeds or sets. I almost always mix blood meal into the top 3 to 4 inches of soil as well prior to planting.
What type of soil is good for onions?
The best soil to grow big bulbs in is a sandy loam that has good drainage and good aeration. The soil also should be loose and rich in organic matter, especially nitrogen.
Is potting soil good for onions?
Yes, potting soil is great for growing onions in containers and raised beds and often takes the work out of amending the soil before planting. My go-to commercial mixes are Happy Frog and Black Gold All Purpose Mix.
Do onions like wet or dry soil?
Neither. Onions prefer consistently moist soil once established. They do not like to sit in soggy wet soil and can rot. They also don’t like dry soils. Onions are shallow-rooted and require consistent moisture to form bulbs.
