When building a raised garden bed, you'll want to make sure that the soil has good drainage while also retaining moisture. There are a few different materials that can be used to create a bottom layer for the bed:
This is a permeable fabric that allows water to pass through while keeping weeds and other unwanted growth from coming up through the soil.
Laying down cardboard on the bottom of your raised bed is a great way to suppress weed growth while also retaining moisture in the soil.
Using a layer of gravel at the bottom of the bed can improve drainage and prevent the soil from becoming waterlogged.
If you have a lot of compost available, you can use it to create a nutrient-rich base layer for your garden bed.
Keep in mind that the choice of materials for the bottom layer of your raised garden bed will depend on your specific needs and the type of plants you'll be growing.