Seeds can be as picky as Goldilocks. They don't thrive if planted too deep or too shallow; they want a depth that's just right. And the magic ideal planting depth can vary depending on time of year and weather conditions. Understanding exactly how the extremes can affect germination, root development and overall plant health will help you pinpoint the optimal planting depth to start your garden off right, any time of the year.
Shallow planting can result in a shorter wait for visible sprouts. However, this is not necessarily a good thing. A quicker emergence means earlier exposure to adverse conditions; this in turn may mean slower development overall.
Planting too deep will delay or even prevent emergence. Seeds contain a small reserve of food to tide plants over until they have developed sufficiently to make their own via photosynthesis. A plant too deeply sown may use up all its food store and starve before reaching the surface.
Another danger of delayed emergence, this one specific to corn and other grasses, is for the plant to outgrow its coleoptile before it emerges. This is the sheath that protects the shoot tip from direct exposure to rough soil. If a corn plant outgrows this while still underground, its vulnerable young shoot and leaves could get shredded.
Because the soil nearest the surface tends to dry out easily, seeds planted too shallowly may not get enough moisture. Lack of water can subsequently lead to delayed or uneven germination and poor root development. If you're planting early, when cooler weather means less evaporation, you may be able to get away with planting more shallowly.
Planting too deep, on the other hand, leads to excessive moisture. This can result in "damping off," a condition wherein young plant growth simply rots away. Damping off is caused by fungi that thrive on the unrelenting moisture and attack new shoots and roots.
Another property of shallow soil is greater fluctuations of air temperature. Seeds planted too shallowly will be less protected from extremes of heat and cold, and the resulting stress is another obstacle to strong root development.
Soil at greater depth tends to be cooler, as it is less susceptible to air temperature and direct sunlight. Planting too deeply can therefore result in temperatures too cool for timely germination, or any germination at all.
Normal application of herbicides may injure your plants if you plant them too shallowly. Plants will also be more susceptible to damage from urea at such a shallow depth.
Settling and Erosion
You may have planted your seeds at the perfect depth, only to have conditions change right out from under you. A fine soil may settle, reducing the thickness of earth above the seed. Wind erosion may blow away some of the soil up top. These effects may leave your seeds planted too shallowly after all. It may be a good idea to plant about a quarter inch deeper than seed packet instructions recommend in order to allow for erosion and settling, or step on the soil to compress it before measuring planting depth.