Filling Your Raised Bed

Healthy soil will help you produce healthy crops. Ideal vegetable garden soil should be loose, deep, and crumbly. It both holds water for root uptake and allows excess rainfall to quickly percolate downward. So how do you fill a raised bed? Should you add topsoil, compost, or potting soil? The answer depends on the situation. Read the scenarios below to learn about options and develop an approach that works best for you!

Scenario 1:

Reduce costs by foregoing an enclosure. Photo credit: Jon Traunfeld

Reduce costs by foregoing an enclosure.
Photo credit: Jon Traunfeld

Raised beds without a wood enclosure- formed by “pulling up” loose, fertile topsoil into a 2-6 inch raised bed with gently sloped sides. Spread 1-2 inches of compost on the area before forming the beds.

Scenario 2:

This raised bed is almost 6 inches above grade and was filled with existing soil and compost. Photo credit: Jon Traunfeld

This raised bed is almost 6 inches above grade and was filled with existing soil and compost. Photo credit: Jon Traunfeld

Raised bed with an enclosure located in an existing garden- If the soil is in good shape (topsoil intact, not compacted, drains well) increase the soil depth by adding add 4-6 inches of compost (homemade or purchased) and mix it with the top 4-inches of soil. You could also add a 2-3 inch layer of topsoil from the walking space around the raised bed and replace it with woodchips, bricks, or weed barrier fabric.

Prior to adding the compost, loosen soils with high clay content by pushing in your garden fork and rocking it back and forth. Move the fork 6-8 inches and repeat across the entire bed. Photo credit: Jon Traunfeld

Prior to adding the compost, loosen soils with high clay content by pushing
in your garden fork and rocking it back and forth. Move the fork 6-8 inches and repeat across the entire bed. Photo credit: Jon Traunfeld

Scenario 3:

Raised bed placed on lawn– kill the grass and weeds by covering the area with plain cardboard multiple layers of newspaper, or weed barrier fabric (can take 6-8 weeks). Then fill the bed with a mixture of compost and purchased topsoil in a 1:2 or 1:1 ratio. There are vendors who sell topsoil mixed with compost.

You could also fill the bed with compost and a soilless growing mix in a 1:1 ratio. The latter contain ingredients like peat moss, bark fines, vermiculite, perlite, coconut coir, and compost. As in Scenario 2, you can also remove add the topsoil adjacent to the raised bed.

Turf and weeds were smothered and raised beds built on top of the ground without disturbing the soil. It was filled with adjacent soil, compost, and soilless growing media. Plant roots will grow through the raised bed soil into the ground below. Photo credit: Jon Traunfeld

Turf and weeds were smothered and raised beds built on top of the ground without disturbing the soil. It was filled with adjacent soil, compost, and soilless growing media. Plant roots will grow through the raised bed soil into the ground below. Photo credit: Jon Traunfeld

If the raised bed is at least 6 inches deep and it’s time to plant it’s ok to cut the grass and weeds at ground level and cover the area with the selected growing medium. The vegetation will die and decompose in place.

An example of a soilless medium. It’s more economical to purchase a compressed bale of media. Products containing peat or coconut coir are hydrophobic (repel water) and should be thoroughly moistened before adding the raised bed. Photo credit: Jon Traunfeld

An example of a soilless medium. It’s more economical to purchase a compressed bale of media. Products containing peat or coconut coir are hydrophobic (repel water) and should be thoroughly moistened before adding the raised bed. Photo credit: Jon Traunfeld

Scenario 4:

Raised bed on hardscape or heavily compacted soil- should be at least 8 inches deep for leafy greens, beans, and cucumber, and 12-24 inches deep for pepper, tomato, and squash. Fill the bed with compost and a soilless growing mix in a 1:1 ratio. Topsoil can be added (up to 20% by volume) for beds that are at least 16 inches deep.

This bed sits on a concrete library balcony. It’s about 16 inches deep and is growing a nice crop of tomatoes. Notice the perlite (white specs) in the soilless growing medium. Photo credit: UME Master Gardeners, Frederick Co.

This bed sits on a concrete library balcony. It’s about 16 inches deep and is growing a nice crop of tomatoes. Notice the perlite (white specs) in the soilless growing medium. Photo credit: UME Master Gardeners, Frederick Co.

Some tips and caveats:

  • Submit a soil sample to a soil testing laboratory if you will be using the existing soil (more accurate and complete and usually less costly than DIY soil testers). Pay for a basic soil test and have the lab test the soil for leadif you plan to grow food in your raised bed. The lab will send back results (soil pH, nutrient levels, etc.) and fertilizer and soil amendment recommendations.
  • Topsoil sales are not regulated in Maryland. Go to a reputable nursery or topsoil dealer and ask questions about where the soil comes from, how it’s been treated, and soil test results. Examine the soil before purchase or delivery. Topsoil should be dark and crumbly with an earthy smell. Do not purchase if the soil is foul smelling, has grayish mottling or a chalky texture. Some sellers have a mix of topsoil and compost which can make an excellent growing medium for raised beds.
  • Minimize the amount of digging and tilling needed to prepare the soil for planting. Disturbing the soil brings weed seeds to the surface where they can more readily germinate.
  • Over time the quality of the native soil below the raised bed will be improved through the addition of organic matter and root growth of crop plants.
  • Organic matter improves the structure of soils that are high in clay so that roots can better grow and take advantage of available water, air, and nutrients. It can be grown (cover crops and living plant roots), added as fresh organic materials (manure, grass clippings, leaves), or added to soil as compost. You need 3 cu. yds. of compost to add a 1 in. layer to 1,000 sq. ft. of ground or 8.33 cu. ft. (12, 5-gallon buckets) to cover 100 sq. ft.

 

By Jon Traunfeld, Extension Specialist

5 Comments on “Filling Your Raised Bed

  1. Loved the thoughts on how to address different growing circumstances. Unlike the kids in Lake Wobegon, not all of our garden situations are “above average.”

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  2. It still seems odd to me that so many of us ‘buy’ soil. Even where the soil is of inferior quality, I just add other material (without adding soil) accordingly. I mostly use just compost. In fact, that is all I have used for the past many years. There have been times where a cover crop got tilled in, and other time when surface debris got tilled into sandy soil.

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  3. Yes, when soil is in decent shape it’s good to keep improving it with living plants/roots and organic matter additions. We have many situations where people are growing on vacant lots or hardscapes and need to bring in the initial growing media.

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  4. Thank you for this! Outside of rainfall, can you tell us your thoughts on the watering of this soil as well? We’re researching this dynamic currently and are looking for some expertise on the matter. Some of the tap water can have high levels of chlorine and chloramines, and my thought is that this would affect the soil PH negatively if watered with tap often. Thanks for your time. – the berkey water team

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  5. Raised bed “soil” that is entirely compost and/or soilless grow mix will dry out more rapidly than raised beds that contain a mineral soil (garden soil).

    All types of potable water are fine to use for watering raised bed flower and food gardens. I’m not aware of any research suggesting soil p or plant growth problems associated with chlorinated water.

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