Raised garden beds are an efficient way to make the most of your garden soil and space. Soil compaction is reduced because no one walks in the beds, which means you can install plants closer together and there is more room for their root systems. The height of the beds improves drainage, and fewer nutrients are lost to leaching and erosion. While there are kits available for do-it-yourself garden beds, it usually cheaper to build and fill your beds with salvaged, recycled or inexpensive materials.
The Cheapest Way
If you've already got a good layer of topsoil, the cheapest way to make a raised bed is to dig up an area of soil and till in organic amendments like aged manure, aged compost or well-rotted garden waste. This makes the most of your existing soil and gives you the advantage of an improved growing medium and increased drainage. To start a garden bed in a lawn, strip off the sod with a shovel and remove the topsoil to 4 to 6 inches deep. Lay the sod grass-side down, cover it with several layers of newspaper and replace the topsoil and any amendments you wish to add. The University of Missouri Extension Service points out that a raised bed of this type will last for a growing season, but irrigation and rain will eventually erode the soil back to the same level as it was before.
Recycling old lumber, concrete blocks, corrugated iron sheets or other building materials is an inexpensive way to frame a raised bed and prevent the soil from washing away. Ask around at building sites or check your local dump for free or inexpensive materials. The magazine "Urban Food Gardener" also recommends checking with wrecking companies who might sell lumber and framing timber at a reduced cost. If you're using salvaged lumber, avoid old railroad ties that have been treated with creosote as well as pressure-treated wood because these contain harmful chemicals like copper and arsenic that can leach into the soil and get taken up by plants. With concrete blocks or old pavers, you can simply stack them around the raised bed to hold the soil in. Salvaged lumber needs to be nailed together, or bolted at the corners with galvanized L-brackets, but the cost of this hardware is minimal.
Soil and Amendments
The cost of purchasing bags of soil to fill your raised beds with can add up, so it's more frugal to look for other sources of fill material, especially if your existing topsoil isn't very good. If you want to get your raised beds started right away, consider posting a classified ad asking for clean fill dirt left over from landscaping and building projects. Some municipal dumps have composting facilities which are good source of cheap organic material, and local farmers often sell or even give away cow, sheep or chicken manure. If you plan ahead, start composting yard and kitchen waste. Compost is usually ready to use within a year. You can also line the bottom of a raised bed with kitchen or yard waste in the fall and it will be rich growing material by spring. Other soil amendments are more expensive but might be worth splurging on, like blood, bone or seaweed meal, vermiculite or perlite, and lime.