Answer: Measure the height, width and length of your bed to determine total cubic feet. This will tell you how much soil by volume. Keep in mind it will settle and the organic matter will need to be renewed regularly over time. Using organic mulches and growing cover crops during fallow periods will help to maintain the soil volume on an ongoing basis.
Ideally you would use top soil taken from your property along with organic matter. If this is not possible you may have to purchase some top soil. Top soil is not a regulated material so quality can vary widely. Ask for soil tests on the soil before you purchase it for this reason. Also, loosen the soil beneath the bed and mix that into the new material to avoid having a distinct layer line between existing soil and new.
About a third of your volume should be organic matter such as good quality compost, rotted down autumn leaves, or very well aged, composted stable manure/bedding.
You may also need to add a small amount of coarse sand or fine grit to assure good drainage, depending on the top soil you use.
I would strongly suggest you work with your local county extension for suggestions on the best amendments in view of what you have available locally, as well as for assistance with the soil tests and interpreting the results. Best of luck with your project!
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