The Best What To Use As Filler In A Raised Garden Bed References. Web topsoil belongs as part of the soil mixture used in raised beds, but you cannot only use topsoil to fill your new garden bed. Never use plastic as this will retain the water at the root level and you’ll end up with root rot problems.

It bulks up the amount of soil you have in your garden bed. This will help you in the amount of cubic yards of soil you will need to use in addition to the other additions. However, plants need plenty of nutrients, and topsoil doesn’t contain enough organic matter for your plants to grow as needed.