A seed is an embryo plant and contains within itself virtually
all the materials and energy to start off a new plant. To get the most
from one's seeds it is needful to understand a little about their needs,
so that just the right conditions can be given for successful growth.
Why does it matter how deep, so long as the seed is covered up?
It does matter, and the reason depends on the seed involved. Plants that
need light for good germination tend to have small seeds, and seed size
provides a clue to why light is needed. A tiny seed, germinating too far
down, will run out of the food stored in it before reaching the surface.
Only after the tiny leaves reach daylight can the seedling make its own
food through the process of photosynthesis. If the seed detects light, it "knows" it
is shallow enough to germinate successfully. Other seeds need to be in the
dark to germinate well. These are frequently the seeds of species that need
a lot of moisture in order to germinate, and being down away from the soil
surface makes sense for them. The soil surface will dry out more quickly
than soil that is even an inch or two lower. Light tells these seeds they
are too close to the top, where they are likely to dry out and die as the
seedling tries to develop. There are, of course, seeds that do not respond
to light one way or the other, and need only be deep enough that they remain
moist in order to germinate.
The definition of germination is emergence of the root tip (radicle)
from the seed coat (testa). Depth does not appear to have much or any effect
on seedling germination, but not too surprisingly depth does play a role
in seedling emergence. A seed contains an embryonic plant in a resting condition,
and germination is its resumption of growth. Seeds will begin to germinate
when the soil temperature is in the appropriate range and when water and
oxygen are available. Optimum soil germination temperatures will vary greatly
from one species to another. With soil temperatures extremely low or excessively
high, growth of the seed is both slow and erratic or germination is entirely
prevented. Not all of your seeds will sprout at the same time due to constant
temperature fluctuations typically observed in nature.