Seed size, type of emergence, and the type of soil in which the seeds are planted determine the optimum depth of planting. The seed must be able to imbibe enough water to germinate before the soils surrounding it dries out. Thus, large seeds require a considerable amount of water to germinate. Planting deeper than 1 inch insures that large seeds are completly covered with soil and take in enough water for germination to proceed. Firming of the soil after seed placement provides good seed to soil contact, which results in water movement from the soil into the seed.
Plant species exhibit different types of emergence .
In some crops, the cotyledons remain in the soil as either the mesocotyl or epicotyl elongates and pushes to the surface. In others, the hypocotyl elongates pulling the cotyledons to the soil surface. When comparing crops with similar seed size, those that must pull their cotyledons through the soil for emergence should be planted shallower than those in which the cotyledons remain in the soil. Depth of planting is also influenced by soil type. Deeper plantings are possible in lighter soils than in heavy soils .