BEFORE YOU BUY
A soil quantity is not a soil recipe
Separate the space you need to fill from the growing medium you choose. A ground-connected bed and an elevated container have different conditions.
Describe the bed before ordering
Does the bed sit on garden soil with an open bottom, or does it have a contained base above the ground? Is it placed on a hard surface? These details matter when selecting a growing medium. One generic mix recommendation cannot account for all of them.
The University of Maryland’s raised-bed guidance separates conditions such as existing soil and hard surfaces. Use guidance that matches your situation instead of taking a recipe for one kind of bed and applying it to every planter.
Match the questions to the base underneath
For an open-bottom bed on existing garden soil, the condition of that soil matters because roots can extend below the frame. University of Maryland Extension recommends testing the existing soil when planning amendments. A bed on concrete or pavement needs a plan for its contained root zone and drainage; it cannot be treated as the same situation.
Use the distinction below when asking a supplier or extension service for advice. It is a way to describe the site, not a universal shopping recipe.
| Where the bed sits | What to establish before buying |
|---|---|
| Open bottom on garden soil | Existing soil condition, test results and depth still needed |
| Concrete or another hard surface | Drainage, available root-zone depth and suitable fill |
| Elevated planter with a contained base | Drainage openings and a medium intended for containers |
Avoid taking away useful container space
Maryland’s container guidance explains that a gravel layer at the bottom does not improve drainage and takes away growing space. Check that drainage openings can work and use a suitable container medium instead of assuming a stone layer will solve the problem.
The same guidance recommends moistening dry potting mix before filling and leaving room at the top for watering. Account for the fill level you actually intend to use when measuring the missing depth. Do not fill to the rim in the calculation if your final soil surface will be lower.
Use the calculator for volume
Measure the plantable interior and the depth you need to add. Plot & Soil converts that space into volume and a rounded purchase order. It does not test your soil, prescribe a compost percentage or estimate the finished weight.
For example, an interior 120 cm long and 60 cm wide needs 108 litres for a 15 cm layer. If only a 5 cm top-up is missing, it needs 36 litres. With 40-litre bags and no purchase allowance, those are three bags and one bag respectively. These depths illustrate the calculation; they are not root-depth recommendations.
If you are comparing suppliers, write down exactly which product each quote covers. Equal volumes of different materials can behave differently in the bed. The cheaper total does not establish that a product is suitable.
120 × 60 × 15 ÷ 1,000 = 108 L; a 5 cm top-up = 36 L
Leave growing decisions with the right local information
Choose growing-medium advice for your site, crops and drainage conditions. Check with a local extension service or a qualified local nursery when the bed’s base, soil history or growing conditions are uncertain. Take the interior measurements, a description of the base and the product label with you. Those details make advice more useful than a volume figure alone.
Photographs on this site illustrate the idea of a garden. They are not measured planting examples or evidence of a particular growing method. Use the labelled diagrams and your own measurements for planning.