Answer: The soil needs to be organic and humusy yet well drained, and for azaleas it must also be acidic in pH. It is usually best to use some of the native soil, then add organic matter such as good quality compost and/or milled spagnum peat moss and/or leaf mold (rotted down autumn leaves) and/or very well aged stable manure and bedding and/or pine fines (small bits of pine bark) or similar materials, plus some coarse sand or fine grit to assure it drains well.
While it may seem like a good idea to buy topsoil, this is an unregulated material and may or may not be suitable. If you want to use topsoil please have it tested before you fill the bed with it. If you use topsoil, also add the organic matter and depending on the type of topsoil you may also need to add the sand to assure it drains well.
Once you have made your soil mix, it is a good idea to have it tested to check fertility and pH to make sure it is suitable for the plants you want to grow. Then amend as needed based on the test results before you plant.
After planting, use an organic mulch year round to help continue providing organic matter over time as it breaks down.
Your local county extension should be able to help you with the soil tests and interpreting the results and may have suggestions of sources for organic matter to add to the bed as you will need a surprisingly large quantity of material to fill it.
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