"Flat" Partly Filled With Dirt
The "flat" is partly filled, on top of the layer of gravel, with specially prepared soil, which is then tamped down with a board, making a flat surface on which the seeds are sown. The soil is a mixture of fifty percent sand, forty percent loam, eight percent powdered moss, and two percent bone fertilizer.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 3 Chapter 4