What is the best soil mix for a raised bed?

For a square foot garden, the standard is Mel's Mix: one-third blended compost, one-third peat moss (or coconut coir), and one-third coarse vermiculite, measured by volume. It is light, drains well, holds moisture, and comes pre-loaded with nutrients so you do not need to add fertilizer the first season.

How much soil do I need for a 4x4 raised bed?

A 4 feet by 4 feet bed that is 6 inches deep needs about 8 cubic feet of mix (4 x 4 x 0.5 = 8). Split into thirds, that is roughly 2.7 cubic feet each of compost, peat or coir, and coarse vermiculite - about four 5-gallon buckets of each ingredient.

Can I use garden soil or topsoil in a raised bed?

It is not recommended for a square foot bed. Native garden soil and bagged topsoil compact, drain poorly in a shallow frame, and can bring in weed seeds and disease. The whole point of Mel's Mix is to skip those problems with a clean, light, blended medium.

Is peat moss or coconut coir better?

They perform almost the same in the bed. Coconut coir is the more sustainable choice because peat is harvested from slow-to-regenerate bogs, and coir rehydrates more easily once dry. Peat is slightly acidic, which some crops like. Either works; pick coir if sustainability matters to you.

The Best Raised Bed Soil Mix: Mel's Mix Recipe

The exact recipe for square-foot-garden soil, how much a 4x4 bed needs, and what to swap when an ingredient is expensive or hard to find.

The soil is the one thing beginners cannot afford to get wrong, and it is also the easiest to get right in a raised bed. In square foot gardening you do not improve the dirt already in your yard - you fill the frame with a purpose-built blend called Mel's Mix, named after the method's creator, Mel Bartholomew. It is light enough to run through your fingers, drains freely, holds moisture between waterings, and already contains the nutrients your plants need.

The Mel's Mix recipe

The recipe is three equal parts, measured by volume, not weight, per theSquare Foot Gardening Foundation:

Ingredient	Share	What it does
Blended compost	1/3	Feeds the plants - holds the nutrients, so no added fertilizer is needed
Peat moss or coconut coir	1/3	Makes the mix light and fluffy and helps it hold water
Coarse vermiculite	1/3	Keeps the mix loose and retains water so roots spread easily

The single most important detail is the compost. The Foundation stresses that it should beblended - made from at least three, ideally five, different sources (for example composted cow manure, mushroom compost, leaf mold, worm castings, and municipal compost). A single compost source gives a narrow band of nutrients; blending several covers the whole range, which is why Mel's Mix needs no chemical fertilizer in its first season. Use coarsevermiculite, not the fine grade sold for seed starting - the coarse particles are what keep the mix airy.

How much a 4x4x6 bed needs: about 8 cubic feet

The math is simple. Volume is length x width x depth, so a 4 feet by 4 feet bed filled 6 inches (0.5 feet) deep is:

4 ft x 4 ft x 0.5 ft = 8 cubic feet of mix.

Split into thirds, you need about 2.7 cubic feet of each ingredient. In buckets, that is roughly four 5-gallon buckets each of fluffed peat or coir, coarse vermiculite, and blended compost - a figure the Foundation uses too. Here is the shopping list for common bed sizes at 6 inches deep:

Bed size (6 in deep)	Total mix	Each ingredient (1/3)
4 ft x 4 ft	8 cu ft	~2.7 cu ft
4 ft x 8 ft	16 cu ft	~5.3 cu ft
4 ft x 2 ft	4 cu ft	~1.3 cu ft

Buy a little extra. Peat and vermiculite are sold compressed and fluff up when you open them, and the whole mix settles an inch or two after the first few waterings. A 10 to 15 percent buffer saves a second trip to the store.

Mixing it without wrecking your back

Spread a large tarp on the ground or driveway.
Pour the peat or coir, vermiculite, and compost onto it in the right proportions.
Wet the pile lightly as you go - dry peat and vermiculite throw fine dust, so dampen them and wear a mask.
Grab two corners of the tarp and roll the pile back and forth. It mixes far faster than a shovel and spares your back.
Shovel the finished mix into the bed and level it off just below the frame's top edge.

Cheaper and more sustainable swaps

Coconut coir instead of peat moss. Peat is harvested from bogs that take centuries to reform, so coir is the greener pick and rehydrates more easily. The Foundation lists coir as an accepted substitute that performs the same in the bed.
Make your own compost. The costliest third is compost. Building a home pile from kitchen scraps and yard waste cuts the bill over time and gives you the "blended" variety naturally.
Do not skip the vermiculite to save money. It is the pricey ingredient, but leaving it out gives you a dense mix that compacts and drains poorly - the exact problem the method exists to avoid.

Topping up in later years

Mel's Mix does not wear out - it just loses a little compost as plants use up nutrients. Each season, before you replant a square, work in a trowel or two of fresh blended compost. That is the entire maintenance routine; you never rebuild the bed from scratch. Depth guidance and the standard bed dimensions here followWashington State University Extension, and the Foundation keeps a set ofMel's Mix resources with a soil calculator if you want to double-check your quantities.

With the bed built and filled, the fun part is deciding what goes in it. Seewhat to plant in a 4x4 garden for ready-made layouts, check the plant spacing chart for how many of each crop fit per square, or open the bed planner to design your own.

The Best Raised Bed Soil Mix (Mel's Mix)