Start with a drainage layer, then soil, then amendments

A raised bed works best when you build it in layers from bottom to top: drainage material first, then your main soil, then compost or other organic matter mixed in. The drainage layer keeps water from pooling at the root zone. The soil holds nutrients and structure. The amendments feed your plants through the growing season.

Most gardeners use a straightforward three-layer approach. Put 2 to 3 inches of gravel, wood chips, or broken pottery shards at the bottom. Add 6 to 8 inches of garden soil or topsoil in the middle. Mix in 2 to 3 inches of compost, aged manure, or leaf mold on top, or blend it into the soil layer before planting. This setup works for vegetables, herbs, and ornamentals without special adjustment.

Key Takeaways

  • Drainage material at the bottom prevents water from sitting around roots; gravel, wood chips, or broken clay work equally well.
  • Garden soil or topsoil makes up the bulk of your bed and should be purchased by the cubic yard or bag, not dug from your yard if it is heavy clay.
  • Compost or aged manure mixed into the top layer feeds plants and improves soil structure without burning roots.
  • A raised bed 12 inches deep needs roughly 1 cubic yard of soil and amendments combined; measure your bed length, width, and depth to calculate how much to buy.
  • Avoid fresh manure, peat moss, and wood from treated lumber, which can harm plants or leach chemicals into soil.

Drainage layer: what goes on the bottom and why

The drainage layer sits directly on the ground and lets water move through instead of pooling. Use gravel (pea gravel or larger), wood chips, or broken clay pots. Aim for 2 to 3 inches. This layer does not need to be perfect — the goal is airflow and water movement, not a sealed barrier.

If your raised bed sits on compacted soil or clay, the drainage layer matters more. If it sits on sandy or already-loose ground, it matters less but still helps. Do not use landscape fabric under the drainage layer; it traps water instead of letting it escape. Do not use sand as your drainage layer — sand compacts and blocks water just like clay does.

Wood chips break down over time and add organic matter to the soil below, which is fine. Gravel stays put and never breaks down. Either works. Avoid treated wood chips or anything that smells like creosote, which can leach into soil.

Soil: the main body of your bed

Garden soil or topsoil makes up 60 to 70 percent of what goes into your bed. Buy it by the bag or cubic yard from a garden center, landscape supply company, or online. Do not dig it from your yard unless you know it is already loose and dark — most yard soil is too heavy or too poor to use straight.

Garden soil is topsoil mixed with compost or other organic matter. Topsoil is the upper layer of existing soil, usually darker and richer than subsoil. Both work in a raised bed. The difference is that garden soil is already partially amended and costs a bit more. Topsoil is cheaper but needs more compost mixed in to feed plants.

A raised bed 4 feet long, 2 feet wide, and 12 inches deep holds about 1 cubic yard of soil. Measure your bed and divide the volume by 27 (the number of cubic feet in a yard) to find how many cubic yards you need. Most bags hold 2 cubic feet, so a cubic yard is roughly 13 to 14 bags. Buying by the yard is cheaper if you have a way to move it.

Amendments: compost, manure, and organic matter

Compost is the most common amendment. It is decomposed plant material — leaves, grass clippings, kitchen scraps — and it feeds soil microbes and plants. Mix 2 to 3 inches into the top of your soil layer, or blend it throughout. You can buy bagged compost or bulk compost by the cubic yard. Homemade compost works just as well if it is fully broken down (dark, crumbly, no recognizable scraps).

Aged manure — usually from horses, cows, or chickens — adds nitrogen and organic matter. "Aged" means it has sat for at least six months and no longer smells sharp. Fresh manure burns plant roots and can carry pathogens. Buy aged manure bagged or in bulk. Mix it in at 2 to 3 inches, or less if you are planting root vegetables, which can split in very rich soil.

Leaf mold is decomposed leaves and works like compost. Peat moss holds water and adds organic matter but does not feed soil the way compost does, and it is expensive. Skip it unless you are growing acid-loving plants like blueberries. Coconut coir is a peat alternative that holds water similarly but is less common and costs more.

Mix amendments into the top 4 to 6 inches of soil before planting, or layer them on top and let them settle over a season. Either way works. A bed filled in spring and planted right away will settle slightly over the first month — that is normal.

How to calculate and buy materials

Measure your raised bed length, width, and depth in feet. Multiply them together to get cubic feet. Divide by 27 to get cubic yards. Round up.

Example: A bed 4 feet long, 2 feet wide, and 12 inches (1 foot) deep is 4 × 2 × 1 = 8 cubic feet. Divided by 27 = 0.3 cubic yards, or roughly one-third of a yard. You would buy one cubic yard and have material left over, or buy 7 to 8 bags of soil and 3 to 4 bags of compost.

For a typical vegetable bed (4 × 2 × 12 inches), buy 1 cubic yard of garden soil or topsoil and 0.25 to 0.5 cubic yards of compost. For a shallower ornamental bed (4 × 2 × 8 inches), buy 0.6 cubic yards of soil and 0.2 cubic yards of compost. Landscape suppliers can help you convert bags to yards if you ask.

What to avoid putting in your raised bed

Do not use fresh manure — it is too hot and will burn roots. Do not use treated wood chips or sawdust from pressure-treated lumber, which contains copper or arsenic. Do not use peat moss unless you have a specific reason (it is expensive and does not feed soil). Do not use sand as a drainage layer or soil amendment — it compacts and holds water.

Do not fill your bed with only compost or only manure. Both are too rich and can cause nutrient imbalances or salt buildup. Do not use yard waste that includes diseased plants, weeds with seeds, or grass clippings treated with herbicide. Do not use soil from a bed where you grew plants with serious disease the year before — start fresh or sterilize it.

Do not use subsoil (the pale, dense layer below topsoil). It has few nutrients and compacts easily. Do not use clay dug from your yard without amending it heavily — it will compact in a raised bed just as it does in the ground.

Filling and settling your bed

Layer your materials in order: drainage at the bottom, soil in the middle, amendments on top. Water each layer lightly as you go to help it settle. Fill to within 2 inches of the rim — the bed will settle over the first few weeks, and you want room for mulch.

If you are filling in spring and planting right away, the bed will be slightly loose. That is fine. If you are filling in fall and planting in spring, the bed will compress and settle more, and you may need to add a bit more soil in spring. Either way, the bed becomes more stable and compact over the first season.

After planting, add 1 to 2 inches of mulch (wood chips, straw, or shredded leaves) on top to retain moisture and suppress weeds. Do not pile mulch against plant stems — leave a small gap around each plant to prevent rot.

Frequently Asked Questions

Can I use soil from my yard in a raised bed?

Only if it is already loose and dark. Most yard soil is too heavy (clay) or too poor (compacted, few nutrients). If you dig a hole and it is hard to break up, do not use it. Buy garden soil or topsoil instead. If your yard soil is already good, you can use it as part of your mix — blend it with compost to improve it.

How deep does a raised bed need to be?

Eight inches is the minimum for shallow-rooted plants like lettuce and herbs. Twelve inches works for most vegetables, including tomatoes and peppers. Eighteen inches or deeper is best for root vegetables like carrots and parsnips. Shallower beds dry out faster and need more frequent watering.

Do I need to replace the soil every year?

No. Add 1 to 2 inches of compost or aged manure on top each spring to replenish nutrients. After three to five years, the original soil will have broken down and compacted enough that a full refresh makes sense. You can remove the top layer, add new soil and compost, and reuse the old soil in other parts of your garden.

What is the best soil mix for vegetables?

A mix of 60 percent garden soil or topsoil, 30 percent compost, and 10 percent aged manure works well for most vegetables. If you do not have manure, use 60 percent soil and 40 percent compost. Adjust based on what you are growing — root vegetables prefer slightly less compost to avoid forking, while heavy feeders like tomatoes benefit from extra compost.

Can I use mulch as a drainage layer?

No. Mulch (wood chips, straw, shredded leaves) breaks down and compacts over time, blocking drainage. Use gravel, larger wood chips, or broken pottery for the drainage layer. Save mulch for the top of the bed, where it suppresses weeds and retains moisture.