The first time I built a raised garden bed, I had no idea how much soil I’d need. What looked like a pretty simple weekend project turned into a trip to the garden center, several heavy bags of soil, and a receipt that was much higher than I expected.
Deep raised beds can eat up a lot of soil, and filling the entire thing with expensive garden mix can get costly fast. That’s why many gardeners fill the lower part of a deep bed with organic materials they already have on hand, then save their best soil and compost for the area where most of the roots will grow.
Branches, leaves, untreated wood chips, cardboard, and other yard materials can take up space while breaking down over time. The exact materials you use will depend on the type of bed and what you plan to grow, but the basic idea is the same: don’t spend a fortune filling every inch with premium planting soil.
The top layer is where you want your best growing material. With a little planning, you can use less expensive soil while still giving your plants a healthy place to grow.
Wood Chips
Wood chips can fill some of the lower space in a raised bed while slowly breaking down and adding organic material. They also leave some room between the pieces, which can help keep the lower part of the bed from becoming tightly packed.
For a deep bed, use larger organic materials lower down and keep several inches of good-quality soil and finished compost near the top. Avoid using fresh wood chips directly around plant roots, since fresh material can temporarily affect nutrient availability as it breaks down.
Rocks, Stones, and Gravel
Rocks and gravel can take up physical space, but they aren’t usually the first thing I’d use to fill a deep raised bed. A thick layer of rock can take up room that could otherwise hold soil, and putting gravel beneath fine soil doesn’t automatically improve drainage.
If you have a small amount of stone on hand, it may be useful in certain areas of a raised bed or for specific drainage needs. For the bulk of a deep bed, though, organic material is usually a more practical way to reduce how much soil you need to buy.
DIY Soil Mix
Buying bags of premium raised bed mix can add up quickly, especially if you’re filling a large bed. Making your own mix can cost less, particularly if you can source some ingredients locally or make your own compost.
A basic blend can include quality topsoil, finished compost, and other organic material suited to your plants. Mixing your own also lets you make use of materials you already have instead of buying every ingredient by the bag.
Newspaper
Layering newspaper across the bottom of a newly built raised bed can help cover grass and weeds underneath. The paper breaks down over time, making it a useful temporary barrier while the bed establishes.
Lay down several sheets, overlap the edges, and wet the paper so it stays in place while you add the next layers. Use plain newspaper without glossy inserts or heavily coated pages.
Cardboard
Cardboard works much like newspaper, but it tends to hold together longer. Flattened boxes can help cover grass and weeds under a new bed while also letting you reuse household packaging.
Remove plastic tape and other non-paper materials before laying down the cardboard. Wet it thoroughly, overlap the pieces, and add soil or organic material on top so it stays in place while it breaks down.
Homemade Compost
If you already compost kitchen scraps and yard waste, you have a useful source of organic matter right at home. Finished compost adds nutrients and organic matter while improving soil texture.
Use fully finished compost in the main planting area, and leave partly broken-down material to finish composting before you rely on it to grow food. You want the material around your plants to be well decomposed and free of recognizable food scraps.
Garden Soil
Soil from another part of your yard can help reduce how much new soil you have to buy. It can work well in the lower part of a deep bed, while your best compost and planting mix can stay closer to the top.
Before moving soil into a raised bed, check it for persistent weeds, grass roots, and other problems. If the soil is full of weed roots, deal with them first so you don’t spend the growing season pulling the same weeds out of your new bed.
Straw or Straw Bales
Straw is lightweight, inexpensive, and easy to use in a deep bed. Loose straw can fill the lower space, while older straw bales can also work in certain raised bed setups where they’ll break down over time.
Use clean straw rather than hay if possible, since hay can contain seeds that turn into a weed problem later. Wetting the straw as you build the bed can also help keep it from shifting around while you add the upper layers.
Composted Animal Manure
Well-aged, fully composted manure can be a useful part of a raised bed mix. Gardeners use cow, horse, chicken, and rabbit manure, but fresh manure should not go straight around plants because it can be too strong and may contain pathogens or weed seeds.
Look for properly composted manure, and use it based on the needs of the plants you’re growing. Local farms and stables sometimes have composted manure available at a low cost, which can help stretch your gardening budget.
Grass Clippings
Grass clippings can add organic material to a raised bed, but they’re better used thoughtfully than dumped in as a thick layer. A large, wet pile of fresh clippings can mat together, heat up, and start to smell as it breaks down.
If you use clippings, spread them in thin layers, and use only grass that hasn’t recently been treated with herbicides or other lawn chemicals that could cause problems in your garden. Mixing them with other dry organic materials can also help keep them from compacting.
Biochar
Biochar is a charcoal-like material made by heating plant material with very little oxygen. Gardeners sometimes mix it into soil because it can improve soil structure and provide a home for microorganisms once it’s properly prepared and charged.
It’s a little different from the bulk fillers on this list because you’re not going to use huge amounts of it to fill a deep bed. A modest amount mixed into finished soil or compost is enough for most home gardening situations.
Vermiculite and Perlite
Vermiculite and perlite are lightweight materials often found in potting mixes. Perlite creates air space and improves drainage, while vermiculite is better known for holding moisture.
Neither one is a cheap way to fill a deep raised bed, so I wouldn’t use bags of either as bulk filler. They make more sense in smaller amounts when you’re mixing planting soil for crops that benefit from better aeration or moisture control.
Coconut Coir
Coconut coir is made from coconut husks and is often used as a replacement for peat-based materials. It holds moisture well and can help keep a soil mix from drying out too quickly.
Compressed coir expands once soaked in water, making it convenient to store and mix. Since coir is mostly useful for improving a planting mix rather than taking up large amounts of space, use it near the upper portion of the bed rather than treating it as a bulk filler.
Worm Castings
Worm castings are a concentrated organic fertilizer that can add nutrients and organic material to your garden soil. You don’t need much, which is why they work better as an ingredient in your planting mix than as filler.
Mix a modest amount into the upper soil layer where plant roots will reach it. If you make your own worm castings, you can have a steady supply without buying bags every time you build a new bed.
Leaf Mold
Leaf mold comes from allowing fallen leaves to break down over time. Once finished, it becomes a dark, crumbly material that improves soil structure and helps the soil hold moisture.
It’s especially useful if you have plenty of leaves available every fall. Add finished leaf mold to the upper layers of a raised bed or mix it with other organic materials to stretch your purchased soil.
Shredded Paper
Shredded paper can give you another way to reuse household materials in the garden. It breaks down over time and can add some bulk to the organic material in a raised bed.
Stick with plain, uncoated paper and mix it with other materials rather than adding a huge compacted pile by itself. Newspaper, cardboard, and shredded paper work best when they’re part of a larger mix of organic material.
Filling a deep raised bed doesn’t mean buying enough premium soil to fill it from the bottom to the top. Using cardboard, newspaper, yard materials, finished compost, and other suitable organic matter can help reduce how much soil you need to purchase.
