Soil Preparation for Onion Plants

Preparing the soil before planting is a step many gardeners skip, but your onions' good growth absolutely depends upon fertile, well-drained soil. Always test your soil before planting to determine whether or not it’s deficient in any essential nutrients and/or minerals and to determine the pH of the soil. It’s easy to do a soil test yourself with one of our digital meters, or to have a test done by your local County Extension office.

The other goal of soil preparation is to loosen any compacted soil and make it friable (crumbly and aerated) so water can properly drain away from your plants.

NOTE: This is part 5 in a series of 9 articles. For a complete background on how to grow onion plants, we recommend starting from the beginning.

When To Prepare Your Soil

Soil preparation can be done any time the ground is not too wet (or frozen). Your onions can be planted even when temperatures are quite cool (but not freezing). Generally, as long as your soil is workable, it is fine to plant—but before you do, make sure your soil is the best it can be.

Soil Types

Know your soil! All regions of the country are different, even from township to township. Identify your soil type and amend it as needed. Onions grows best in a loamy, crumbly soil.

Clay and silt soils are made of very small particles. They feel slick and sticky when wet. Clay and silt hold moisture well, but resist water infiltration, especially when they are dry. Often, puddles form on clay or silt soils and they easily become compacted.

Loam is a mix of sand, silt or clay, and organic matter. Loam soils are loose and look dark and rich. When squeezed in your fist, moist loam will form a ball, which crumbles when poked with a finger. Loam soils normally absorb water and store moisture well. Loam soils can be sandy or clay-based, and will vary accordingly in moisture absorption and retention.

Sandy soils contain large particles that are visible to the unaided eye, and are usually light in color. Sand feels coarse when wet or dry, and will not form a ball when squeezed in your fist. Sandy soils stay loose and allow moisture to penetrate easily, but do not retain it for long-term use.
If your soil is so poor that it’s beyond amending, you can always create a raised bed. It can be any shape you like as long as it’s large enough to contain the number of onion sets you want to plant. The bed should be at least 6 inches deep.

You can even plant onions in a container (use at least a 1-gallon size or larger) and locate the pot where it will receive sun exposure 6-8 hours each day. Container-growing is better suited to growing scallions (very young onions, also called green onions or spring onions) vs. full-sized bulbs.

Soil pH

It’s also good to know whether your soil is acid (pH below 7.0) or alkaline (pH above 7.0). Onions prefer soil with a pH between 6.2 and 6.8. Your local county extension service can perform a soil test for you, or you can use one of our digital soil pH meters. If your soil is too acidic, mix in ground limestone; if it’s too alkaline, add peat moss.

How To Prepare Your Soil

Roots grow faster when they have ample room, and a larger root system means bigger onions!

After you have amended the soil pH and/or any nutrient/mineral deficiencies (if needed), prepare the area in which you want to grow your onions. They will grow best in raised beds or raised rows at least 4? high and 20? wide.

If you need to improve the soil in your planting bed, mix composted cow manure, garden compost or peat moss (up to 1/3 concentration) into the soil. Make sure the peat moss you get is either baled sphagnum or granular peat. You can also add our Coco-Fiber Potting Medium or 2 or more inches of organic material and evenly work it into your soil. Organic materials bind sandy soil particles so they retain moisture and nutrients better. They also break apart clay and silt particles, so that water can infiltrate and roots can spread.

Your lawn can also provide you with ideal organic materials such as grass clippings (not treated with herbicides) and shredded leaves. Not only will the grass and leaves break down to provide soil nutrients, but they will help loosen the soil as well. You can gather these in the fall with spring planting in mind.

Common soil amendments:

  • Compost
  • Sand
  • Composted manure
  • Lime
  • Peat moss (either baled sphagnum or granular peat)

Fertilizer First!

For the best growth and yield, onions need fertilizer right from the start. After you amend the soil and the bed is level, dig a trench measuring 4" deep and 4" wide and evenly distribute the displaced soil to each side of the trench to make “hills.” Into the trench, sprinkle 1/2 cup of fertilizer for every 10 linear feet of soil. Use a fertilizer with the middle number (phosphorus) that is higher than the other two, such as 10-20-10. Cover the fertilizer with about 3 inches of soil.

Once your soil is prepared to this point, you are ready to plant!

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