The deeper a raised bed, the easier it is on your back and the more flexibility you have about the crops you can grow, including those with deep roots. The snag is that filling up a raised bed 24 to 36 inches tall with topsoil can get really expensive really fast.

Luckily, there is no need to use only high-grade topsoil; there are several other alternative fillers that are not only economical but also improve the soil over time. Here are six organic materials you can use to fill up raised garden beds.

Logs

When filling raised garden beds, it’s important to be creative. Consider the organic material you have at your disposal, and see if it could possibly be a good fit for filling your beds.

“Filling the eight 3-foot-tall cedar beds in our homestead required creativity,” says Deborah DeSalvo, owner of Cold Brook Farm.

Taking advantage of materials right from her own property, she filled the bottom 24 inches of the raised beds with 2-3-foot sections of logs cut from downed trees.

“Any woods with sprouting potential, such as willow and sycamore, must be completely dead before using. And beds may absorb more water with newer logs,” DeSalvo points out.

As the logs decompose, they add nutrients to the soil, meaning bountiful harvests and lower maintenance over time. Just note that as this happens, the soil level sinks slowly and more topsoil needs to be added at the beginning of each growing season.

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Branches

Another excellent and no-cost organic filler material for the bottom of a raised bed is branches from suitable trees (the same exceptions for logs apply).

The advantage of branches is that they decompose faster than logs, but branches need to be cut into smaller pieces to become more manageable to put in place. Branches add nutrients to the soil as they decompose because they don’t stack tightly, making them good for aeration, too.

Wood Chips

If you can get your hands on arborist wood chips from a reputable local tree service, snap them up to fill your raised beds. Arborist wood chips are ground-up branches, twigs, and leaves; they are a type of mulch, but coarser and more irregularly sized than shredded mulch and wood chip mulch; as a result they break down slower.

Arborist wood chips add nutrients to the soil over time, retain moisture, provide aeration, and moderate soil temperature. When obtaining arborist wood chips, inquire about the type of trees in the batch to avoid unsuitable ones.

Straw

Straw bales work well as an inexpensive voluminous filler for the bottom of raised beds. The only caveat is that they decompose completely within a year, which is much faster than logs. Straw adds soil nutrients, improves aeration, and has good water-holding capacity; it also supports the activity of beneficial microorganisms.

Leaf Mold

Decaying leaves, also known as leaf mold, are another suitable filler for a raised bed before adding topsoil. Leaf mold is packed with microorganisms and worms that break down other organic materials. As they decompose, they add nutrients to the soil.

Simply collect leaves in the fall from any yard, provided no synthetic chemicals have been used, and let them sit over the winter so they begin to decompose. Add them to your raised bed in the spring.

Compost

Compost is not used by itself but rather worked into the topsoil of a raised bed after the lower portion has been filled with other organic materials such as the ones listed above. Compost adds nutrients and improves soil structure, aeration, and water retention, and supports beneficial soil organisms, including earthworms and microbes.

Ideally, the topsoil layer should be 12 inches deep. DeSalvo used 6 inches of topsoil that was saved from when building their home, mostly clay, and added 6 inches of organic compost.

Compost, combined with topsoil, needs to be added at the start of each growing season to make up for the sinking soil level due to the decomposing materials at the bottom of the raised bed. But four years after starting her raised beds, the amount needed to top up the beds has diminished significantly, according to DeSalvo.