Probably no agricultural practice varies more than the depth to which seeds are sown. Some farmers pay very little attention to depth, but simply allow the seed to be placed where the drill happens to penetrate. The percentage germination of the seed as well as the vigor of the young plants is without doubt influenced by the depth of sowing, and certainly these factors are very important in determining the yield of crops.
Harris, F. S. and Maughan, H. J., "Bulletin No. 164 - Factors Affecting the Depth of Sowing Various Crops" (1918). UAES Bulletins. Paper 130.