Grow · field guide
What Soil to Use in a Raised Bed
The most common raised-bed mistake is filling it with bagged topsoil or native dirt. That stuff packs hard in a box, water sits on top, and roots struggle. The right mix does two jobs: it drains well so roots get air, and it feeds plants so they grow. Here is what to use and how much to buy.
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Why not just topsoil
Straight topsoil or bagged garden soil is built for the open ground, where worms and weather keep it loose. Drop it in a raised bed and it settles into a dense block. Water pools on the surface instead of draining, and roots end up sitting in soggy soil with no air. Plants in a packed box grow slow or drown.
A raised bed needs a lighter blend that holds some water but lets the rest drain away. You build that by mixing topsoil with compost and something gritty for air.
A simple mix that works
You do not need a fancy recipe. This three-part blend drains well, holds moisture, and feeds plants for a full season:
- About 50% screened topsoil for the base and body.
- About 30% compost for nutrients and to hold water.
- About 20% aeration like coarse sand or perlite to keep air in the soil and stop packing.
How much soil to buy
Figure out the volume before you shop so you do not run short or overbuy. The formula is length times width times depth, all in feet.
A 4 by 8 foot bed that is 12 inches deep needs 32 cubic feet of mix, which is about 1.2 cubic yards. Here is the math:
- Formula: length x width x depth (in feet) = cubic feet.
- 4 ft x 8 ft x 1 ft = 32 cubic feet.
- 32 cubic feet divided by 27 = about 1.2 cubic yards.
Test and balance
A cheap soil test shows your pH, which is how acid or alkaline the soil is. Most vegetables grow best when pH sits between 6.1 and 6.9. A test kit costs only a few dollars and takes about 10 minutes, so check it before your first planting.
If the number is off, lime raises pH and elemental sulfur lowers it. Retest after a few weeks to confirm the change took.
Top up every year
Compost breaks down and the level in your bed drops over time. You do not need to dig it all out and start over. Just add 1 to 2 inches of fresh compost each spring, mix it into the top few inches, and the bed is ready for another season.
Keep going
Questions, answered straight
Only as part of a mix. Straight topsoil packs hard in a box and drains poorly. Use about 50% screened topsoil, 30% compost, and 20% aeration like coarse sand or perlite.
A 4 by 8 foot bed that is 12 inches deep needs 32 cubic feet of mix, which is about 1.2 cubic yards. The formula is length times width times depth in feet.
A simple blend of about 50% screened topsoil, 30% compost, and 20% aeration like coarse sand or perlite. It drains well, holds some water, and feeds plants for a season.
No. The mix does not wear out. Just top up with 1 to 2 inches of compost each spring to replace what broke down, and mix it into the top few inches.
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