Can't think of any vegetable seeds that require light to germinate - flowers, yes. There is an old rule of thumb to plant any seed 2x the thickness of the seed deep. So generally, yes, you are right - the smaller the seed the more shallow it is planted.

Of course there are always exceptions so the planting depth listed on the seed packet is usually the best guideline.

If planted too deep they will usually still germinate in the same amount of time as it is soil temp and moisture that causes germination. But once they germinate they may die before they can push through the soil on top of them to get to the light they then need. Tiny seeds have tiny weak stems.

Dave

DD

Lettuce, the light-colored seeds, requires light to germinate. I always sow them uncovered but must keep them moist until they germinate. Oregano also germinates exposed to light. Maybe for the reasons you cite, plants with tiny seeds have evolved to germinate mostly on top of the soil -- as you say, they hardly have the strength to push through a layer of soil. If you are direct-seeding your lettuce into the garden, plant seeds 1/4 to 1/2 inch deep, in rows or beds. The dark seeds of lettuce should be planted at this rate. The light seeds need light to germinate, so if covering those, cover them just barely. When the seedlings emerge, thin accordingly.

http://www.texasgardener.com/pastissues/novdec04/salads.html

KG, for seeds that require a soil covering to germinate, you should cover the seed with soil twice as deep as the seed is deep when lying flat, or twice as deep as the seed's smallest dimension. That isn't a lot of soil in some cases.

Here is a link that might be useful: How deep do I sow my seeds.