Covering Seeds with Sand
After seeds are planted in the flats or elsewhere, they are covered with a sprinkling of sand of just the right depth. A very common mistake of the amateur gardener is to cover the seeds too deeply. This gives the cotyledons needless work in crowding up to the light. Sometimes it prevents the germination of seeds that otherwise might have grown satisfactorily.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 12 Chapter 1