By Rocio Silva, HOME GROWN. Updated October 2026.
The best soil mix for a vegetable garden depends on where the roots will live. A raised bed wants a blend of topsoil and compost. An in-ground garden wants its own soil improved, not hauled away and replaced. A pot wants soilless potting mix and nothing dug from the yard. Below are the ratios for each, where the numbers come from (four university extension services, cited at the end), and a table that turns your bed size into a shopping list, a step most soil guides leave out.
Best Soil Mix at a Glance
| Where you are growing | What to use (parts by volume) | Source |
|---|---|---|
| Raised bed on soil or lawn | 2 parts topsoil to 1 part compost, and never more compost than topsoil | University of Maryland Extension |
| Raised bed on pavement or hardpan | 1 part compost to 1 part soilless growing mix | University of Maryland Extension |
| Square foot garden box | Mel's Mix: equal thirds compost, peat moss or coir, and coarse vermiculite | Square Foot Gardening Foundation |
| In-ground garden | Your own soil, plus 2 to 3 inches of plant-based compost a year worked into the top 6 to 8 inches | Colorado State University Extension |
| Pots and containers | Soilless potting mix (peat or coir plus perlite or vermiculite), never garden soil | UNH Extension |
Whatever the mix, aim for a soil pH of 6.2 to 6.8, the range University of Maryland Extension gives for most vegetables and herbs.
Raised Bed Soil Mix: The Ratio That Works
For a bed that sits on soil or lawn, University of Maryland Extension (UMD) recommends filling it with compost and purchased topsoil at a ratio of 1:2 or 1:1. Put simply: two buckets of topsoil for every bucket of compost, and at most half and half. UMD's target for a raised bed that contains soil is 25 to 50 percent organic matter by volume, which is why every recipe leans so hard on compost.
We default to the 2:1 end. Compost keeps breaking down after it goes in, and you will add more every year anyway, so there is no reason to front-load it. If you are still deciding between a raised bed and an in-ground plot, our raised bed vs. in-ground comparison walks through the tradeoffs.
Beds on concrete, patios, or compacted ground
When roots cannot grow down into the ground below, the bed has to do all the work. For that case UMD switches to a 1:1 mix of compost and a soilless growing mix (the bagged blends of peat, bark fines, coir, perlite, and compost). Topsoil can make up as much as 20 percent of the volume, but only in beds at least 16 inches deep.
Peat and coir repel water when they are dry, so UMD's advice is to wet them thoroughly before they go into the bed.
What about Mel's Mix?
Mel's Mix is the square foot gardening recipe, and the Square Foot Gardening Foundation spells it out as equal thirds by volume, not weight: coarse vermiculite, sphagnum peat moss (or coconut coir), and blended compost, ideally from five different sources. It contains no mineral soil at all. A 4 x 4 box filled 6 inches deep takes 8 cubic feet, so about 2.7 cubic feet of each ingredient. We would use it for a small, shallow square foot box and stick with topsoil and compost for anything bigger.
How deep should a raised bed be?
UMD's depth minimums are written for beds on pavement or compacted ground, where the bed holds all the soil the roots will get: 8 inches for leafy greens, beans, and cucumbers, and 12 to 24 inches for peppers, tomatoes, and squash. We treat them as the floor for any new bed. An 8-inch bed grows Buttercrunch lettuce or a row of Contender bush beans without complaint. A big slicing tomato like Cherokee Purple wants 12 inches or more, so plan the depth around the deepest-rooted crop you want to grow.
How Much Soil Do You Need for a Raised Bed?
Multiply length by width by depth, all in feet, and you have cubic feet. Divide by 27 for cubic yards, which is how soil yards sell bulk. Measure the inside of the frame, not the outside, and use the real depth of the boards: a "2x6" is 5.5 inches wide, so two stacked make an 11-inch bed, not 12.
The table below does the math for four common beds. "Buy" includes 10 percent extra, our planning margin, because new beds settle and a spare bag is the easiest way to top one up.
| Bed (inside size) | Volume | Buy (with 10% extra) | 2:1 split: topsoil + compost | Premixed, 1.5 cu ft bags | Premixed, 2 cu ft bags |
|---|---|---|---|---|---|
| 4 x 4 ft, 12 in deep | 16 cu ft (0.6 cu yd) | 17.6 cu ft | 11.7 + 5.9 cu ft | 12 bags | 9 bags |
| 4 x 8 ft, 11 in deep (two stacked 2x6 boards) | 29.3 cu ft (1.1 cu yd) | 32.3 cu ft | 21.5 + 10.8 cu ft | 22 bags | 17 bags |
| 4 x 8 ft, 12 in deep | 32 cu ft (1.2 cu yd) | 35.2 cu ft | 23.5 + 11.7 cu ft | 24 bags | 18 bags |
| 3 x 6 ft, 18 in deep | 27 cu ft (exactly 1 cu yd) | 29.7 cu ft | 19.8 + 9.9 cu ft | 20 bags | 15 bags |
Check the cubic-foot figure printed on each bag, because sizes vary and some list liters or quarts instead (1 cubic foot is about 28.3 liters). Once you are past a cubic yard, get a bulk quote from a local soil yard as well and compare. Filling a new bed in fall gives it the winter to settle, and you can top it up before you plant in spring.
In-Ground Gardens: Improve the Soil You Have
An in-ground bed does not need a new soil mix. It needs organic matter added to the soil it already has, year after year. Colorado State University Extension (CSU) gives these yearly rates, with the compost spread on top and then worked into the top 6 to 8 inches of soil:
| Garden age | Plant-based compost | Manure-based compost, or salt content unknown |
|---|---|---|
| New garden, first three years | 2 to 3 inches a year | 1 inch a year |
| Established, after three years | 1 to 2 inches a year | ½ inch a year |
The manure column is lower because CSU warns that manure, manure composts, and biosolids can be high in salts that interfere with crop growth. CSU's long-term goal is soil that is 3 to 5 percent organic matter, built up over several years rather than in one load. For scale, a 10 x 10 foot plot needs about 17 cubic feet of compost for a 2-inch layer and 25 cubic feet for 3 inches, about 0.6 to 0.9 cubic yards.
Before you add lime, sulfur, or fertilizer, get a soil test so you are correcting what is actually off. Our guide to preparing garden soil covers clearing the ground, testing, and working compost in.
Containers: Potting Mix, Never Garden Soil
Garden soil fails in a pot. UNH Extension puts it plainly: garden soil "compacts easily, dries out quickly, yet drains poorly and can contain weed seeds and diseases." Use a soilless potting mix instead. Most are a water-holding organic material such as peat or coir, plus perlite or vermiculite to open up pore space.
It is the same advice our CEO, EJ Kaga, gave NBC News Select in 2021 about indoor pots: "Do not use soil that you dug from your outdoor garden in your indoor plants' containers. This can transfer harmful diseases and pests into your containers. Instead, buy or make your own potting soil." He was just as firm on drainage: "All indoor pots should have holes at the bottom, even if you're growing plants that love to be grown in wetter soils."
Size matters too. UNH Extension notes that 4 to 5 gallon containers are usually enough for a single plant like a tomato. A 5-gallon bucket holds about two-thirds of a cubic foot, so one 1.5 cubic foot bag of potting mix fills two of them with a little left over. More on picking pots and crops in our guide to growing vegetables in pots.
Soil Mix Mistakes That Cost a Season
Filling a raised bed with topsoil alone
Most topsoil falls well short of UMD's target of 25 to 50 percent organic matter by volume for raised beds, which is why compost makes up a third to half of every recipe. If you already filled a bed with straight topsoil, work a few inches of compost into the top of it rather than starting over.
Buying "organic soil" as if it were topsoil
Read the bag. UMD warns that bags sold as "organic soil" typically do not contain topsoil at all, meaning the mineral soil made of clay, silt, and sand. That makes them a poor stand-in for the topsoil in a 2:1 mix.
Pouring in peat or coir bone dry
Dry peat and dry coir repel water, so it beads up and runs off instead of soaking in. Wet them thoroughly before mixing, as UMD advises.
Putting gravel in the bottom for drainage
This one refuses to die. Washington State University's Linda Chalker-Scott calls it a myth: water does not move easily from a finer material into a coarser one, so it stays in the soil above the gravel until that layer is saturated. A gravel layer makes the bottom of a pot wetter, not drier. Use the same mix all the way down and make sure the drainage holes are open.
Containers without drainage holes
Even a good potting mix stays waterlogged in a pot with no way out. Chalker-Scott's bottom line is that planting containers must have drainage holes for root aeration, so drill holes in decorative pots and buckets before you fill them.
Piling on manure compost every year
Composts made with manure can be high in salts. CSU caps them at 1 inch a year in a new garden and half an inch once the garden is established. Plant-based compost, such as leaf or yard-waste compost, is the one CSU treats as low-salt, so make it your yearly top-up.
What We Would Put in a New 4 x 8 Bed
If you asked us how to fill one 4 x 8 bed, 12 inches deep, this weekend, this is the answer we would give.
- Skip the bags labeled "raised bed mix" or "organic soil" unless the label lists topsoil. Buy screened topsoil and a plant-based compost separately, or ask a soil yard for a topsoil and compost blend. UMD notes those blends can make an excellent growing medium for raised beds.
- Buy 24 cubic feet of topsoil and 12 of compost. That is the 2:1 split plus our 10 percent margin, rounded up.
- Mix on a tarp or in a wheelbarrow, watering as you go, so the bed goes in evenly blended and evenly moist.
- Fill to the top, water deeply, and top up where it sinks. Keep a bag or two back for this.
- Test the soil a year later. UMD suggests waiting a year so the components have time to fully mix and react. Do not panic at a reading near 7.0: UMD notes that productive raised beds high in organic matter often test 6.8 to 7.2.
- Add compost every year: an inch or two of plant-based compost, worked in, which is the rate CSU gives for established vegetable gardens.
Soil Mix Questions We Hear Every Season
What is the best soil to buy for a vegetable garden?
For a raised bed, screened topsoil plus a plant-based compost at 2 parts to 1, or a topsoil and compost blend from a soil yard. For pots, a soilless potting mix. For an in-ground garden, buy compost, not soil, and work it into what you have.
Can I fill a raised bed with only compost?
We would not. Every extension recipe we checked blends compost with topsoil or a soilless mix, at most half and half. Compost on its own also keeps breaking down, so the bed sinks year after year.
Is potting mix or garden soil better for vegetables?
It depends on where they grow. Potting mix is for containers, where garden soil compacts and drains poorly. In-ground gardens and raised beds on open ground want real soil with compost worked in. The exception is a raised bed on pavement, where UMD's 1:1 mix of compost and soilless growing mix is the better fit.
What pH do vegetables need?
Most vegetables do best between 6.2 and 6.8. Test before you adjust, because rich raised beds often read a little higher and still grow well.
Sources: University of Maryland Extension, Soil to Fill Raised Beds and Growing Vegetables in Raised Beds; Colorado State University Extension, Vegetable Gardens: Soil Management and Fertilization (CMG GardenNotes #711); UNH Extension, What is the best way to grow tomatoes in a container?; Linda Chalker-Scott, Washington State University, The Myth of Drainage Material in Container Plantings. Mel's Mix recipe from the Square Foot Gardening Foundation.
Bed filled and ready? Our 20-variety heirloom vegetable seed pack gives you enough variety to fill a 4 x 8 bed and find out what your new soil grows best, or browse all of our vegetable seeds.
