Why does the soil in my raised bed drop every year?

Two completely different things get called sinking. One is the annual settling that every organic-rich bed does as material is consumed by soil life, which is the system working. The other is the dramatic first-year collapse of a bed filled entirely with bagged compost, which is a filling error and will keep happening until the mineral content goes up.

What you'll need

• Screened topsoil
• Well-rotted compost
• A rake

Step by step

1. Measure how much it has actually dropped.Mark the inside of the bed at the fill level in spring and check it the following spring. Under 50mm a year is normal settlement. A quarter to a third of the total depth in year one is a filling problem.
2. Work out what it was filled with.Dig down 100mm and look. If it is uniformly dark, light and fibrous with no mineral grit, it was filled with compost. If you can feel sand and clay particles between your fingers, there is topsoil in there.
3. Understand normal settlement.Compost is largely carbon, and soil organisms consume it and release most of it as carbon dioxide. Half of what you add each year physically leaves the bed as gas. That is not a fault — it is the process that builds structure and feeds the plants — and it means an annual top-up is part of the system.
4. Understand the first-year collapse.Bagged and bulk compost is fluffed with air when it is delivered. Watering, rain and gravity compress it, and the organic fraction is simultaneously being eaten. A bed of pure compost loses a quarter to a third of its depth in one season and then dries out fast, because nothing mineral is holding water.
5. Top up an ordinary bed with compost each spring.Twenty-five to fifty millimetres of well-rotted compost spread on the surface. No digging, no mixing. That is the entire annual maintenance of a no-dig raised bed.
6. Top up a collapsing bed with topsoil, not compost.If the bed is compost-only, adding more compost repeats the failure. Add screened topsoil to bring the mineral content up towards 60 to 70 per cent, mixing it into the top 150mm, and top up with compost from then on.
7. Check whether it is sinking or the sides are spreading.A bed whose timber sides bow outward under load looks like it has sunk in the middle when the soil has actually moved sideways. Straighten and brace the sides before adding anything.
8. Check whether the ground under it has settled.A bed built over a filled-in hole, a stump, or fresh made-up ground sinks bodily. Look at whether the top edge of the frame is still level relative to the surroundings, which distinguishes this from soil settling inside a level frame.

Tips

• A bed that has settled to 50mm below the top of the frame is a good thing, not a problem — it gives you a lip to hold mulch and water without spilling over.
• Leaf mould breaks down more slowly than compost and settles less, so it is a good component in a bed you do not want to top up as often.
• The annual top-up is when to add slow-acting amendments if you use them: seaweed meal, rock dust, or a general fertiliser raked into the new layer.
• Woodchip or straw mulched on the surface breaks down and adds to the level over time, which offsets some of the annual loss on a bed you are not cropping hard.

If it doesn't work