July 27, 2026

Compost for Raised Beds: Getting the Mix Right (and Keeping It Moist)

Every "best compost for raised beds" list ends the same way: a ratio and a row of products. What none of them tell you is the thing that actually decides your summer — a raised bed drains fast and dries unevenly, so the mix you choose is really a choice about how often you'll be standing there with a hose. Get the ratio right and you water less. Learn which bed dries first and you stop watering the ones that don't need it.

Start with volume, then the ratio

Before you buy a single bag, work out how much fill the bed actually swallows. Multiply length by width by the depth you plan to fill — most crops are happy with 25–30 cm of good soil. A 1.2 × 2.4 m bed filled 30 cm deep is about 0.86 cubic metres, which is a lot more compost than people expect on their first trip to the garden centre.

For the mix itself, a forgiving starting point is roughly one-third compost to two-thirds topsoil (or a topsoil-and-aeration blend). Compost is the sponge — it holds water and feeds the plants — while the mineral soil gives the bed structure so it drains predictably and doesn't collapse. Buy a little extra of everything: fresh fill settles several centimetres in the first season, and topping up is normal.

Why "all compost" backfires

Filling a bed with pure compost feels generous, but it tends to disappoint. As the organic matter finishes breaking down, the level sinks fast — you can lose a third of your fill height in a year. It also swings between extremes: soggy and airless right after watering, then baked into a water-repellent crust once it dries. And it's often far richer in nutrients than most vegetables want, which pushes leafy growth at the expense of fruit.

Blending compost with topsoil fixes all three problems at once. The mineral fraction anchors the bed, keeps drainage steady, and dilutes the nutrient load so you're feeding, not force-feeding. Think of compost as the amendment that makes good soil great, not as the soil itself.

Raised beds dry unevenly — and that's the part guides skip

Here's the honest complication. A raised bed has more surface exposed to sun and wind, and gravity pulls water straight out the open bottom, so it dries faster than the same soil in the ground. That's a spring gift — beds warm early and drain after rain — and a midsummer tax, when a hot week can pull a bed dry in days.

It gets more uneven than that. Two beds side by side rarely dry at the same rate. A bed with more compost holds moisture longer; one in full afternoon sun, or planted with thirsty squash, empties first; a bed near a fence or wall can shelter from rain entirely. "Water the raised beds" is the wrong instruction — usually one bed is gasping while another is still damp from three days ago.

The finger test still works: push a knuckle into the root zone and if it's dry down there, water. But doing that across every bed, every day, in the heat is exactly the chore people quietly abandon in July. For more on reading the root zone versus the surface, see our guide on soil moisture and when to water.

Let a probe tell you which bed is dry

This is where a soil sensor earns its place. A buried capacitive probe — an Ecowitt WH51 is the best-value option and pairs to any Ecowitt gateway — mapped to a specific bed turns "I think that one's dry" into a number you can act on. Siltwell sends a "this bed is low" alert when the bed's two most recent readings are both below its threshold. Two readings, not one, so a single odd blip doesn't trigger a false alarm.

The default threshold is 25%, and that's a starting point, not gospel — you tune it per bed once you've watched how each one actually behaves over a couple of weeks. A compost-heavy bed and a free-draining one will settle at different numbers, which is exactly the point: the probe confirms the uneven drying you'd otherwise be guessing at, so you water the bed that's dry and skip the ones that aren't.

One boundary worth stating plainly: Siltwell alerts you that a bed is dry — it doesn't open a valve or run your irrigation. There's no automation on the watering itself. It replaces the guessing, not the watering can.

Rain and microclimate: the garden-level picture

Watering isn't only about individual beds. Siltwell also gives the whole garden a single hold / covered / water call — hold when about 5 mm or more of rain is forecast in the next day or two, covered when that much already fell in the last three days, water when neither is true. That's deliberately garden-level, and with a connected station it reads the rain that actually landed on your yard, not a regional guess. Use it to avoid the classic mistake of soaking beds the evening before a downpour.

Your beds also run their own temperature. If you're curious before buying hardware, the public microclimate checker compares the forecast low to what nearby home stations recorded over recent nights and tells you how much cooler or warmer your area tends to run — an area indicator, not a guaranteed reading for your exact plot.