Every raised bed loses soil volume over time, and it surprises nearly every first-year gardener. This is not a sign you did something wrong — it is simply how organic soil behaves, and there is a predictable amount to plan for.
Why it happens
- Organic matter — compost, aged manure, peat — physically decomposes and shrinks in volume as microbes consume it.
- Settling and compaction from watering and foot traffic near the edges compresses air pockets that were present when the bed was first filled.
- Plant roots and harvesting remove some soil mass along with the crop itself, especially with root vegetables.
How much to expect
A newly filled bed with a high percentage of compost or uncomposted organic matter can drop by as much as 20–25% of its volume in the first year alone as material breaks down and settles. A more mature, mineral-heavy mix will drop closer to 5–10% annually.
What to add back, and when
Top off with finished compost each spring before planting rather than raw organic matter, which will simply repeat the same sinking cycle. Aim to bring the level back to within an inch of the bed's rim — a bed that sits several inches below its frame loses growing depth and dries out faster along the exposed sides.
Organic Matter Decomposition Is the Main Culprit
Compost and other organic amendments continue breaking down after a bed is filled, reducing volume as microorganisms digest the material — this is normal, expected, and actually a sign of healthy soil biology at work, not a defect in how the bed was originally filled.
How Much Top-Off Is Actually Normal Each Year
Expect to add roughly 2-4 inches of fresh compost or soil annually to a raised bed with a heavily organic fill mix — beds filled primarily with mineral soil rather than compost-heavy mixes settle less dramatically but also offer less of the nutrient benefit that a compost-rich mix provides in the first place.
