Raised bed soil sank after a month: why, and how much to top up

Illinois Extension says it plainly: "Settling soil is simply a fact of raised bed gardening." A freshly filled bed has very little soil structure. Every time you dig, plant or pull a weed, you knock it about a bit more, and the mix packs down.

Source: Illinois Extension, refreshing raised bed soil, checked October 6, 2026.

The second reason is the compost. Most bed mixes are heavy on it, and Oregon State University Extension warns that newly filled beds sink as the compost in the mix keeps decomposing. Its raised bed fact sheet adds that organic matter decomposes and disappears, so you need to add more of it often.

Source: OSU Extension EM 9308, using compost, checked October 6, 2026; OSU Extension FS 270, raised bed gardening, checked October 10, 2026.

Water does the rest. Bagged mix comes out of the bag loose and full of air. The first few soakings close those air pockets and wash fine bits into the gaps, so the level can drop quickly at first.

• A few inchesof compost or good topsoil each year keeps up with settlingIllinois Extension, refreshing raised bed soil, October 6, 2026
• 1/4 to 1 inof compost a year on an existing bed surfaceOSU Extension EM 9308, using compost, October 6, 2026
• About 8 inof soil-and-organic mix roots most vegetables wellOSU Extension FS 270, raised bed gardening, October 10, 2026
• 5.33 cu ftraises a 4 x 8 ft bed by 2 inchesArithmetic

1. 1

   Let it settle first

   Water the bed well a few times and wait a week or two before you measure. Top up too early and you'll be doing it again next month.

2. 2

   Measure the gap in four spots

   Stand a ruler on the soil and read the distance up to the top of the frame. Do it near each end and twice in the middle, then average. Readings of 1.5, 2, 2.5 and 2 inches average out to 2 inches.

3. 3

   Work out the cubic feet

   Inside length × inside width in feet × the gap in inches ÷ 12. For a 4 x 8 ft bed and a 2 inch gap: 8 × 4 × 2 ÷ 12 = 5.33 cubic feet.

4. 4

   Turn it into bags

   Divide by the cubic feet printed on the bag and round up. Miracle-Gro's raised bed soil comes in 1.5 cu ft bags and Kellogg's mix in 2 cu ft bags, so 5.33 cubic feet is 4 of the first or 3 of the second.

   Source: Miracle-Gro raised bed soil product page, checked October 6, 2026; Kellogg Garden raised bed mix product page, checked October 6, 2026.

5. 5

   Pick what goes on top

   Illinois Extension suggests compost or high-quality topsoil. Oregon State suggests a quarter inch to 1 inch of compost a year on an existing bed. If your gap is deeper than that, fill the rest with the same mix you used to fill the bed.

   Source: Illinois Extension, refreshing raised bed soil, checked October 6, 2026; OSU Extension EM 9308, using compost, checked October 6, 2026.

6. 6

   Write it down

   Note the date, the gap and the bags on the raised bed soil record. Next year you'll know roughly what to buy before you measure.

Length × width × gap ÷ 12. Divide by the cubic feet on your bag and round up.

Plan on topping up again next year, because the organic matter keeps breaking down. The two extension figures differ: Illinois says a few inches, Oregon State a quarter inch to 1 inch of compost. Your own record is the better guide, so measure each spring and write the gap down.

Source: Illinois Extension, refreshing raised bed soil, checked October 6, 2026; OSU Extension EM 9308, using compost, checked October 6, 2026.

A bed that has dropped a little still works. Oregon State's fact sheet says a soil-and-organic mix about 8 inches deep is enough for most vegetable roots, so a 12 inch bed that now sits at 10 inches of soil has room to spare. The top-up keeps the level where you want it.

Source: OSU Extension FS 270, raised bed gardening, checked October 10, 2026.