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What to Fill Grow Bags With

Home/Resources/Raised Beds & Planters/Filling Grow Bags
Elena Marsh
Written & researched by Elena Marsh Gardening & Landscaping Editor
Updated October 2026
Wide environmental view of a person's hands pressing seedlings into dark soil in a timber raised bed, in warm late-afternoon light

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Fill grow bags with a soilless container mix, never garden soil or topsoil. The blend that performs most reliably is roughly 60 percent quality potting mix, 30 percent finished compost and 10 percent perlite or pine bark fines by volume, with a slow-release granular fertilizer stirred through at the label rate. That combination stays loose under repeated watering, drains at the rate fabric expects, and holds enough moisture that a 10-gallon bag does not need watering twice a day in July. Garden soil in a bag compacts into a brick within weeks.

What are the actual proportions?

ComponentShare by volumeWhat it does
Peat or coco coir based potting mix60%Water holding and structure
Finished compost30%Nutrition and biology
Perlite, pumice or pine bark fines10%Air space, prevents compaction
Slow-release granular fertilizerPer labelSustained feed through the season
Garden soil or topsoil0%Compacts, carries weed seed and disease

Push the aerating fraction to 20 percent for tomatoes, peppers and anything grown in a hot southern summer where you will be watering daily. Drop the compost to 20 percent for herbs and Mediterranean plants such as rosemary and lavender, which resent a rich, moisture-retentive mix.

Low-angle view of a person's hands pressing seedlings into dark soil in a timber raised bed, in bright midday sun

Why does garden soil fail in a fabric bag?

Structure. Field soil holds its air space because of aggregates built by roots, fungi and soil fauna over years, and because it sits in a continuous profile where water can keep moving downward. Lift the same soil into a container and you destroy the aggregates in the shoveling, remove the continuity, and then compact what is left with every watering. Within a few weeks the mineral particles settle into a dense mass with almost no air-filled pore space, and roots in a waterlogged, oxygen-poor mix cannot take up nutrients even when the nutrients are present.

Extension horticulture programs make this point consistently in their container gardening guidance: potting mixes are engineered to hold air at container depth, and soil is not. There is a second, more practical reason too. Garden soil brings weed seed, and it can bring soilborne disease, which in a bag has nowhere to go.

Overhead view of a person's hands pressing seedlings into dark soil in a timber raised bed, in warm late-afternoon light

How much do you need to buy?

Work in cubic feet, because that is how mix is sold. A rough conversion: 7.5 gallons is 1 cubic foot. So a 5-gallon bag takes about two thirds of a cubic foot, a 10-gallon bag about 1.3, and a 15-gallon bag about 2. A row of six 10-gallon tomato bags therefore needs about 8 cubic feet, which is four standard 2-cubic-foot bales. Mix settles noticeably after the first two or three waterings, so fill to the rim and plan to top up with an inch of compost once the bag has settled.

Three-quarter view of a person's hands pressing seedlings into dark soil in a timber raised bed, in soft overcast light

How do you keep the cost down?

Buying bulk potting mix by the bale rather than in small retail bags cuts the price per cubic foot substantially, and making your own compost fraction removes the most expensive third of the recipe. What does not work is bulking the mix out with cheap filler at the base: in a bag you are paying for root volume, and giving up four inches of it to wood chips costs more in yield than it saves in mix. The same argument, applied to much larger volumes where it does start to make sense, is in the cheapest way to fill a raised bed.

For crop-specific volume, see what size grow bag for tomatoes, and for the bags themselves our best grow bags roundup ranks the fabrics owners report holding their shape when filled with a heavy, well-watered mix.

We never buy, test or physically handle the products we rank. Recommendations come from aggregated owner reviews, star ratings, review volume and expert consensus — see how we rank.

More from this hub: Raised Beds & Planters · Best grow bags · Best soil mix ratio · Are grow bags reusable · How we rank

Straight Answers

Frequently Asked

Can you use raised bed soil in a grow bag?

It works less well than a container mix, though it will not kill anything. Bagged raised bed blends are formulated for a large volume in contact with the ground, where a heavier texture is an advantage and drainage happens through the open bottom. In the confined volume of a bag, that same texture holds too much water in the lower third and compacts into a dense layer by midsummer. If it is what you have, cut it roughly half and half with a peat or coir-based potting mix and a scoop of perlite.

How much mix does a grow bag hold?

Close to its stated gallon rating, though fabric bags flex and settle so plan on a little more. Roughly speaking, a 5-gallon bag takes about 0.7 cubic feet, a 10-gallon takes about 1.3 cubic feet, and a 15-gallon takes about 2 cubic feet. A standard 2-cubic-foot bale of potting mix therefore fills one 15-gallon bag, or a 10-gallon bag with a third of the bale left over. Mixes settle 10 to 15 percent after the first few waterings, so top up.

Do you need to add fertilizer to a grow bag mix?

Yes, for anything grown longer than about six weeks. Fabric bags drain freely and are watered often, which flushes soluble nutrients out through the sides and base faster than a plastic pot does. Blend a slow-release granular fertilizer into the mix at planting per the label rate, then supplement with a diluted liquid feed every two to three weeks once fruit sets. Leafy crops harvested young generally get through on the starter charge alone.

Is coco coir better than peat moss in grow bags?

For most container vegetables they perform similarly, with coir holding a small edge in rewetting behavior — dried peat becomes hydrophobic and sheds water, while coir takes water back more readily, which matters in a fast-drying fabric bag. Coir is also pH-neutral where peat is acidic, so a coir mix needs less lime. Buy coir that is labeled washed or low-sodium, because unwashed coir can carry salt levels high enough to stunt seedlings.

Can you fill the bottom of a grow bag with filler material?

Not usefully in a bag under 15 gallons, where you need every inch of root volume. In larger bags, some gardeners put a few inches of coarse wood chips at the base to reduce cost and weight. Understand the trade-off: you are giving up root zone, and a coarse layer under a fine mix creates a perched water table that keeps the layer immediately above it wetter. Peanuts, gravel and packing foam are all worse than simply using a smaller bag.