Raised growing beds are used to solve many different gardening problems, most often soil problems. For example, the raised planting beds in the photo are built on top of pavement to line a used car sales lot. You might want to use a raised bed to overcome soil problems such as poor drainage, or to provide better accessibility for the disabled gardener, or to make maintenance easier. Raised beds can be constructed of many different materials and in many different styles. But all raised beds have one thing in common: they need dirt or soil in them. (Gardeners call it soil, rather than dirt.) Follow this guide to what kind of soil to use and how much soil and compost are needed.
How Much Soil to Fill a Raised Bed?
A rule of thumb is to fill the raised planting bed with one half organic matter such as compost and one half soil, plus some sand or fine grit mixed throughout for drainage. Filling a raised bed requires a surprisingly large volume of material. Figure the cubic measurement of the bed to determine how much you need: width x length x depth. Soil and compost are sold in bags, by the tractor scoop, or by the cubic yard. A cubic yard is three by three by three feet or 27 cubic feet. Accessibility may determine whether you use bagged soil or purchase the soil in bulk.
What Kind of Soil is Best for Raised Bed Gardening?
You will need a mixture of materials to provide a soil mix with good fertility, good drainage, and good moisture holding abilities along with a suitable pH.
If you have access to decent garden soil, you can certainly use that (along with extra organic matter and probably coarse sand or grit to assure good drainage.)
But most gardeners find they need to bring in additional soil when building a raised planting bed or planter because of the large amount needed to fill a bed. Importing top soil to fill the raised bed can have mixed results. Top soil is not a regulated material, so all kinds of soil can be sold as top soil. A reputable supplier should be able to give you soil test results and provide samples to reassure you about the quality of their product. A good local nursery or your county extension should be able to recommend a source.
Add Organic Matter to Soil or Top Soil in Raised Planting Bed
Generally it is best to loosen the native soil at the bottom of the bed and mix that with your added soil to avoid creating two distinct layers. Also add ample organic matter, especially good quality compost, in addition to the soil. Mix them together thoroughly. Once you have thoroughly mixed the materials, run another soil test to verify the pH and fertility levels in the bed. This will help you develop an ongoing maintenance plan for your plants. Ideas on Less Expensive Raised Bed Soil
Raised Bed Needs Ongoing Soil Care
The soil will settle over time, so plan on regular additions of organic matter to maintain soil depth in the bed. Using organic mulch year round will also help feed and replenish the soil on an ongoing basis. Another option is to cover and protect the soil surface by growing a cover crop every winter, then turn that under each spring as a source of organic matter.
More Information on Raised Beds or Planters
Raised beds can help solve many different gardening problems and offer many opportunities for growing specialty plants or gardening where it might not otherwise be possible. For more details on the benefits of raised beds and how to plan and build raised garden beds, see How to Plan and Construct a Raised Bed. I hope this helps you get started on installing or building – and filling -- the raised beds for your garden. Remember, great plants need great soil!