As a rule, the depth of planting depends on the size of seeds. But this is not invariable. Some quite large seeds, such as those of the bean, castor-oil bean, and some of the nuts, send out thick, clumsy seed-leaves, and if planted three inches deep, followed by slight soil-baking, they are unable to force their way upward. This quite numerous class, including the pumpkin and squash, should not be covered more than an inch deep, and in heavy soils still less. On the other hand, such plants as leave the seed-leaves or cotyledons in the soil, such as corn and the pea, will bear deep planting if the soil is not too compact to admit air freely. The close-observing horticultural student will soon notice that some plants, such as bean, pumpkin, and the orchard-fruit seeds, when germinating lift up the seed-leaves bodily into the air, while in others the seed-leaves remain in the soil. If deeply planted, the clumsy leaves may fail to lift the compact covering, while in such as corn and peas the seed-leaves do not emerge, but growth takes place upward from a growing point or plumule (Fig. 1). All fruit-tree seeds should be planted early, and planted no deeper than is necessary to secure the needed moisture. Even where small-fruit seeds are planted on the surface and pressed down firmly with a board early in the spring, they usually start the root downward in time to escape injury by drying. But it is safer to press down such seeds and cover with a board or mulch to lessen evaporation while the roots are starting. When the first upward growth is observed the covering is removed.
In planting nuts it is best to follow Nature as closely as possible. Plant shallow - not more than an inch deep - and press the soil down firmly. In early spring planting, when the surface soil is moist, we have planted black walnuts, butternuts, hickory-nuts, chestnuts, and oak acorns by merely crowding them down in mellow soil with the boot-heel, with a success never attained in planting over one inch deep.
In the pumpkin and bean, the seed-leaves (cotyledons) are lifted above the surface of the soil in germination. In the corn and pea, the cotyledons are not lifted above the surface of the soil in germination. (After Goff.)