A raised bed holds more soil than most people expect. A 4 by 8 foot bed filled 12 inches deep takes 32 cubic feet. That is 16 bags of soil.
Here is how to fill a raised garden bed so your plants grow well and the soil bill stays low. It works for wood, metal, and plastic beds.
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Not sure how much soil to buy? Enter your bed size in my raised bed soil calculator. It shows cubic feet, cubic yards, and the number of bags.
What you need to fill a raised bed
- A raised bed frame, set in place and level
- Plain brown cardboard with the tape removed
- Logs, branches, and leaves for deep beds (optional)
- Topsoil
- Compost
- Perlite or pumice
- A tarp or wheelbarrow for mixing
- A garden rake and a hose.
How to fill a raised garden bed
1. Measure the inside of the bed
Measure the length, width, and depth inside the walls. The walls take up space, so outside numbers give you too much soil.
Plug those numbers into the raised bed soil calculator. Leave the box checked to add 10% for settling. New soil sinks as air pockets close and compost breaks down.
2. Clear the ground and lay cardboard
Set the bed where it gets at least 6 hours of sun. Rake the ground inside the frame so it is level.
If the bed sits on grass or weeds, cover the ground with plain cardboard. Overlap the edges by a few inches so no light gets through. Wet it with the hose. The cardboard smothers the grass, then breaks down into the soil.
Do gophers or voles dig in your yard? Staple hardware cloth to the bottom of the frame before the cardboard goes in.
3. Fill the bottom of deep beds with free organic matter
Beds taller than 18 inches take a lot of soil. Most plants only need the top 12 inches. You can fill the space below with free material from your yard.
- Logs and thick branches on the bottom
- Smaller sticks and twigs next
- Fall leaves, straw, or grass clippings on top.
This layered method is called hugelkultur. The wood holds water like a sponge and feeds the soil as it rots. It does settle more than soil alone, so plan to top up the bed each year.
Skip gravel and rocks. A gravel layer does not help drainage. Water stays in the soil above the gravel until that soil is soaked.
4. Mix your raised bed soil
A good raised bed mix holds water, drains well, and feeds your plants. This is the blend my calculator uses:
| Ingredient | Share of the mix | For a 4 x 8 ft bed, 12 in deep |
|---|---|---|
| Topsoil | 60% | 21.1 cubic feet |
| Compost | 30% | 10.6 cubic feet |
| Perlite or pumice | 10% | 3.5 cubic feet |
| Total with 10% for settling | 100% | 35.2 cubic feet |
Dump each ingredient on a tarp and turn it with a shovel until the color looks even. You can also add a few inches of each to the bed and mix them in place.
For a small bed, or one on legs, you can use a lighter mix instead. Equal parts compost, peat moss or coconut coir, and perlite is a common recipe. My raised garden beds guide explains this mix. It costs more per cubic foot, so it suits small beds best.
5. Fill in layers and water as you go
Add 4 to 6 inches of soil, then water it well. Repeat until the soil reaches the top of the bed. Watering each layer settles the soil and finds dry pockets before you plant.
6. Let it settle, then top up and plant
Wait a few days to a week. The soil will sink as it settles. Add more mix to bring it back up near the top edge.
Now you can plant. My plant spacing calculator shows how many plants fit in your bed. To finish, spread a layer of mulch to hold in water and block weeds. The mulch calculator shows how much to buy.
How to fill a raised bed for less money
Soil is often the biggest cost of a new raised bed. Here are the ways to spend less:
- Fill the bottom for free. Logs, branches, and leaves take up space that soil would fill
- Buy in bulk for big beds. A landscape supplier sells soil by the cubic yard
- Compare the real price. Divide a bag price by its cubic feet. Divide a bulk price by 27, the cubic feet in one yard
- Split a load. Many suppliers have a minimum order, so share it with a neighbor
- Use your own compost. Homemade compost can replace part of the bagged compost
- Reuse dug soil. Soil you dig out of paths can go into the bed, if it is free of weeds.
One cubic yard equals 13.5 bags of 2 cubic feet. That math helps when you compare a bulk quote with bags at the store.
What not to put in a raised bed
- Pressure-treated wood scraps
- Black walnut wood or leaves, which harm many vegetables
- Weeds that have gone to seed
- Diseased plants
- Dog or cat waste
- Glossy or plastic-coated cardboard
- Fresh manure in a bed you will plant right away
- A layer of gravel or rocks.
How deep to fill a raised bed
Fill most beds with at least 12 inches of good soil. Some crops do fine with less, and root crops like more.
| Crops | Soil depth |
|---|---|
| Lettuce, radishes, and most herbs | 6 to 8 inches |
| Most vegetables, like peppers and beans | 12 inches |
| Tomatoes, carrots, and potatoes | 12 to 18 inches |
Not sure what to grow? Here are the best vegetables to grow in raised beds.
Top up your raised bed each year
The soil in a raised bed shrinks a little every season as plants and roots use it up. Each spring, spread an inch or two of compost over the top. In fall, my guide to winterizing raised garden beds shows how to put the bed to rest.
Where I buy raised beds and soil
Epic Gardening sells the metal raised beds I recommend. My code HOMEFORTHEHARVEST5 takes 5% off at checkout.
- Raised bed soil on Amazon: Bagged soil comes premixed, so you can skip the tarp
- Organic compost on Amazon: Mix it in now and top up with it each spring
- Perlite on Amazon: Keeps the mix light so roots get air
- Botanical Interests seeds: My favorite seed company for filling a new bed.
Questions about filling a raised bed
What do you put in the bottom of a raised garden bed?
Lay plain cardboard over grass or weeds first. In beds deeper than 18 inches, add logs, branches, and leaves next. Skip gravel and rocks. They do not help drainage.
How much soil do I need for a raised bed?
Multiply the length, width, and depth in feet. A 4 by 8 foot bed filled 12 inches deep needs 32 cubic feet of soil. That is 1.19 cubic yards, or 16 bags of 2 cubic feet. Buy 10% extra for settling.
Can I fill a raised bed with only potting mix?
Potting mix works in small or elevated beds. In a large bed on the ground, it costs more and dries out fast. A blend of topsoil, compost, and perlite holds water better.
Do I need to line the bottom of a raised bed?
Most beds work best with an open bottom. Roots can reach the ground below and extra water drains away. Add hardware cloth only if gophers or voles dig in your yard.
When is the best time to fill a raised bed?
Fall is a great time. The soil settles over winter, and leaves or wood in the bottom start to break down. A bed filled in spring works too. Give it a week to settle before you plant.
More raised bed guides
- Raised bed soil calculator shows how much soil to buy
- Raised garden beds compares wood, metal, and fabric beds
- Raised bed gardening covers planting and care
- Seed starting date calculator shows when to start seeds for your new bed.
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