Customers building their first raised bed almost always ask if they can just use topsoil or whatever bagged soil is cheapest that week. You can, but it usually disappoints — raised beds need a different mix than ground-level planting, and getting it wrong shows up as poor drainage or compacted soil by midsummer.
Straight topsoil compacts hard in a raised bed because there's no surrounding earth to help it drain and no worms or soil life established yet to keep it loose. It's fine as a component, but not as the whole fill.
What we mix for customers building new beds is roughly one-third quality compost, one-third topsoil, and one-third an aerating material — coarse sand or perlite for smaller beds, or a bagged raised-bed mix for larger ones. That ratio drains well, holds enough moisture between waterings, and gives roots room to spread without fighting compaction.
Boise's native clay soil is a big part of why this matters more here than in some climates. Clay holds water and compacts badly, so if you're filling a bed with a mix that has too much clay-heavy topsoil in it, you're recreating the exact drainage problem raised beds are supposed to solve.
Existing beds benefit from an annual top-dress of an inch or two of fresh compost each spring rather than a full soil replacement — it's cheaper, and it keeps feeding the soil biology that's already established instead of resetting it every year.
If you're not sure how much soil a bed of your dimensions needs, bring us the length, width, and depth and we'll calculate it and load it for you — this is one of the most common reasons customers use our delivery service, since a full raised bed's worth of soil is heavier than it looks.