Here is a strange issue I've been tracking for the past few weeks. I have various cucumber seeds that I know are good. I know this because when I germinate in a plastic bag in the house on some paper towel I get 90%+ germination (seed pops open 24 hours later), and the seeds just keep on sending the roots out as long as they are in the bag.
However, from the very same bag of seeds, when I put them in the earth (outside) the majority appear to pop through the shell a tiny bit (say a mm or so) and then stop. I have dug through the soil on a lot of seeds and found this exact problem. Something is causing them to stop growing immediately after germinating.
This is happening even now in June, way beyond any frost concerns.
The earth I have appears fairly high in clay, but it's very loose when I put the seeds in and I've grown cucumbers, tomatos, etc. in it for the past few years.
I am saturating the soil heavily when I put the seeds in and sprinkling it each morning to ensure it stays moist OR I am saturating the soil when I put seeds in and covering with a plastic cup or similar. Depth is 1/4-3/8" I'd say.
My only idea right now is that the soil is so tough that when it's moist it is cutting out all the oxygen to the seed (?). Like I said, if I put seedlings in the ground they are good, and the seeds that do successfully germinate and poke out of the ground do fine.