What to fill a raised garden bed with

Why ordinary garden soil is not enough

Native soil dug from the yard tends to compact when it is piled into a frame, and bagged topsoil on its own can set almost like concrete after a few waterings. Roots need a mix that holds moisture and nutrients while still letting air reach them. That balance comes from combining materials that each do one job well.

A simple, reliable mix

A common and dependable starting point is roughly equal parts of three ingredients:

• Screened topsoil gives the bed body and mineral content.
• Finished compost feeds soil life and holds moisture.
• A light, airy material such as aged bark fines, coco coir or perlite keeps the mix from packing down.

Blend the three thoroughly before filling rather than layering them, so roots do not hit an abrupt change partway down. If your compost is homemade and still a little coarse, that is fine for most vegetables.

Filling a deep bed without overspending

Beds taller than about eighteen inches take a lot of mix to fill. A common approach is to put coarse organic material in the bottom third: untreated logs, branches, leaves and plain cardboard. These break down slowly and feed the soil from below.

Two things to plan for. The bottom layer settles as it decomposes, so expect to top the bed up for the first year or two. And fresh wood ties up some nitrogen while it breaks down, so keep it well below the root zone and add a little extra compost or a nitrogen-rich amendment to the top layer. Save your best mix for the top twelve inches, where most roots grow.

Working out how much you need

Multiply length by width by depth, all in feet, to get cubic feet. A four by eight foot bed that is one foot deep needs about thirty-two cubic feet of mix, which is a little over one cubic yard. Bagged products are usually sold in one to two cubic foot bags, so for several beds, ordering a blended mix by the cubic yard is often cheaper. Buy about ten percent extra to allow for settling.

Keeping it fertile year after year

Raised bed soil gets lighter and lower as organic matter breaks down and crops use nutrients. Each spring, add an inch or two of compost to the surface and mix it lightly into the top few inches before planting. Mulching with straw or shredded leaves during the season slows evaporation and adds more organic matter as it breaks down.

Compost alone can run short on some minerals over time. Worm castings, kelp meal and rock dust are popular ways to add them back, and Thrive Garden's soil amendments include several of these. A basic soil test every couple of years tells you whether anything is actually lacking before you add more.

Common mistakes

• Filling with only bagged topsoil or only compost.
• Using fresh manure, which can burn young plants; let it age first.
• Walking on the bed and compacting the mix.
• Letting the bed sit bare over winter instead of mulching or planting a cover crop.

Checking that your mix is right

A good raised bed mix feels loose and slightly springy when you squeeze a handful. It should hold together briefly when damp and then crumble apart, not stay in a tight ball like clay or run through your fingers like sand. After a good watering, it should drain within a few minutes, with no water sitting on the surface. If it stays soggy, add more of the light, airy component. If it dries out within a day, add more compost.