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Growing medium for a bed, bench or container run
The volume most people underestimate
Filling a raised bed costs more than building it, and the gap between the two surprises nearly everyone. A 4 by 8 foot bed at 12 inches deep holds 32 cubic feet of medium. Bagged potting mix commonly comes in 1.5 or 2 cubic foot bags, so that bed is sixteen to twenty-one bags.
The unit confusion makes it worse. Bagged growing medium is frequently sold in dry quarts rather than cubic feet, and one cubic foot is 25.71 dry quarts. A 50 quart bag is therefore under 2 cubic feet, and a bed needing 32 cubic feet needs about sixteen and a half of them. Buying by quarts without converting is how people arrive with a third of what they need.
Settling adds to it. Fresh medium settles by roughly 10 to 20 percent over the first season as organic matter breaks down and the structure compacts, so a bed filled level in spring will be noticeably down by fall. Ordering 10 to 15 percent over the calculated figure and topping up annually is the realistic approach rather than an indulgence.
Depth by crop
Bed depth determines what you can grow, and it also determines how forgiving the bed is about watering, which matters more under cover than outdoors.
6 to 8 inches supports salad, most herbs, radish and other shallow-rooted crops. It is the minimum for a bench-top bed and it dries fast, so it needs consistent irrigation.
10 to 12 inches is the general-purpose figure and supports most vegetables including brassicas, beans, chard and bush crops. This is the depth to build to unless there is a reason not to.
16 to 18 inches supports deep-rooting crops properly: tomatoes, long carrots, parsnips and anything grown to full size over a long season. It also gives a substantially larger water buffer, so the bed tolerates a missed watering rather than punishing it.
That water buffer is the underrated argument for depth in a greenhouse. Under cover there is no rainfall, so every drop is applied deliberately, and a deep bed with a large moisture reserve is far more forgiving of an irregular schedule than a shallow one or a container. Container-grown tomatoes in a 5 gallon bag need daily watering at peak; the same plant in an 18 inch bed does not.
What to fill a greenhouse bed with
Garden soil alone is the wrong answer, and it is the most common mistake in filling a raised bed.
Garden soil in a contained bed compacts, drains poorly and loses the structure that soil biology and root action maintain in open ground. It also brings weed seed and any soil-borne disease present in the garden, and in a greenhouse those problems concentrate rather than dispersing because the bed is never rained through or frozen deeply.
A workable greenhouse bed mix is roughly one third good topsoil or loam, one third compost for nutrition and biology, and one third material providing structure and drainage such as coarse bark, perlite or coir. The exact ratio matters far less than the principle: nutrition, structure and water-holding are three separate jobs and one material does not do all three.
For containers rather than beds, a proper container medium such as a rich potting mix or organic potting mix is the right choice, because containers need far better drainage than beds and topsoil in a pot compacts into a brick. For seed starting, neither is appropriate and a fine, low-nutrient starting mix is correct.
Beds, containers and benches
The three ways of holding medium in a greenhouse behave differently and suit different situations.
In-ground or raised beds hold the largest volume, buffer moisture and temperature best, and allow deep rooting. They fix the layout permanently and consume floor area that a bench would leave free underneath. A galvanized bed raised on legs brings the working surface to a comfortable height, though the supported weight is substantial: a 4 by 2 foot bed 12 inches deep holds about 8 cubic feet, which wet is on the order of 400 to 600 pounds.
Containers are flexible and movable, which is their whole advantage. Fabric grow bags have a specific benefit over rigid pots: roots reaching the fabric wall meet air and stop rather than circling, which produces a better root system. They also fold flat out of season, which matters in a building with no spare space. The cost is a small water reserve and therefore frequent watering.
Bench-top beds, essentially shallow containers on staging, suit salad and herbs and put the crop at working height. The 6 to 8 inch depth limits what they grow and makes irrigation consistency important. Capillary matting beneath evens out moisture across a bench considerably better than watering pot by pot.
Buying and mixing
Three practical points that affect the cost significantly.
Bulk beats bags above roughly 20 cubic feet. A cubic yard is 27 cubic feet, and bulk-delivered soil and compost is dramatically cheaper per cubic foot than bagged product. For a single 4 by 8 bed at 12 inches, which is 32 cubic feet, bulk is worth investigating. The constraint is access for delivery and somewhere to put a pile.
Compost is the component to buy well. Structure material and topsoil are commodities; compost quality varies enormously and it is what supplies both nutrition and the biology that keeps a bed functioning. Home-made compost, if you have it, is generally better than anything bagged, and a compost thermometer is how you tell whether a heap is working properly and reaching temperatures that kill weed seed.
Mix before filling, not after. Layering components into a bed and hoping they blend does not work; the layers persist and roots meet abrupt transitions in texture that they struggle to cross. Mix on a tarp or in a barrow and fill with the blended material. A riddle is useful for removing stones and breaking up lumps as you go.
Top up annually rather than replacing. A functioning bed improves over years as biology establishes, and the standard maintenance is adding an inch or two of compost to the surface each year and letting worms and water carry it down.
