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How to fill a raised garden bed cheaply

Deep beds eat soil. What extension services say about filling the bottom with branches and logs, how much soil it saves, and the settling and nitrogen costs.

The short answer

Fill only the depth your plants need with soil mix, and fill the rest of a very deep bed with wood. A 4 × 8 ft bed 24 inches deep holds 64 cubic feet; putting branches in the bottom 8 inches saves about 21.3 cubic feet of bought soil. The catch is that wood breaks down: the bed settles and needs topping up, and fresh wood takes nitrogen from the soil.

Branches and logs in the bottom

A University of Georgia county extension agent suggests putting branches in the bottom of very deep beds to fill space; they break down into growing material over time. Building a bed on buried wood is called hügelkultur.

The University of Minnesota Extension notes that as these materials break down you will need to keep adding soil. Washington State University Extension goes further for hügelkultur mounds: they collapse as the wood decays and have to be rebuilt.

Nitrogen

South Dakota State University Extension warns that fresh wood can deplete the nitrogen in the soil; it advises against freshly cut wood and suggests packing nitrogen-rich organic material around the wood. Utah State University Extension gives a rate for uncomposted woody material mixed into soil: ¼ pound of actual nitrogen per 100 square feet for every inch of woody material.

Keep enough real soil on top

Leave enough soil mix above the wood for the roots: at least 10 inches for most crops, according to UGA, and more for deep-rooted crops. The depth guide lists root depths by crop. Avoid a hard line between layers: Colorado State University Extension says a sharp change from one material to another slows water and roots.

Other ways to spend less

Build the bed shallower over loosened ground; UGA says to till the soil below a bed that is less than 10 inches tall. Buy the mix in bulk once you need more than a few cubic yards; the bagged vs bulk soil calculator compares your quotes. And do not put gravel in the bottom to save soil: it holds water in the soil above it (see what goes in the bottom).

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