For the bottom layer of deep raised beds, consider using a mix of organic materials like logs, branches, and leaves, or even cardboard to smother weeds. These materials break down over time, enriching the soil and improving drainage. A layer of well-rotted compost can also be added.
Logs, branches, leaves, and straw are excellent options. They provide space for aeration and decompose slowly, releasing nutrients into the soil. Use cardboard to suppress weeds, and it will naturally break down and enrich the soil. A layer of well-rotted compost provides nutrients and improves soil structure. Avoid using inorganic materials like gravel at the bottom, as they can interfere with root growth and may not provide the same benefits as organic matter. Hugelkultur: This method involves layering logs, branches, and other organic matter at the bottom of the raised bed, followed by a layer of soil.
The material you choose should promote good drainage, especially in deep beds. The organic matter you use will decompose over time, so it's a good idea to replenish the soil with more compost or other organic matter regularly. While you might not need to dig deep into the bottom layer, consider the plants you'll be growing and whether they need to reach that depth.