Can you be more specific? Are your seeds germinating and then underperforming or are you having a germination issue? Also, what, beside potting soil have you added to the container?

Germination is my main problem. Up to this point I have started them in a smaller container (like the long shallow flower boxes) and then transplanted them into the earth boxes at a great loss to the sprouts. They will germinate in the shallow flower boxes but not in the earth box. The reason I transplant, is because they get bigger in the earthbox.

The content of the soil, is some (bagged) topsoil, manure, compost, and a small amount of miracle grow. It is the same mix that I am using in the flower boxes. The location is the same for both also.

Would you suggest keeping them covered with an opaque cover until they sprout? (sort of like when plants grow under a board outside they seem to sprout well...?) Or are they needing direct sunlight on the soil? Since they're bottom watered should I be watering from the top?

I've even tried sprouting them in a jar (like I was growing sprouts to eat), and then planting the sprout in the soil, but nothing came up.

Thanks for your input.

Are you sure this is the right time to grow lettuce? Lettuce is a cool weather crop and heat will affect both germination,
growth, and taste. I am in MO, so I may be wrong, but I thought that in CA, lettuce growing season was more in the fall and winter. Also, I once read that greens are grown in different areas of CA at different times of the year for that reason.

Perhaps you might want to contact your University Extension office and get a plant guide that will give you the dates to plant certain crops. You might be doing absolutely nothing wrong except planting at the wrong time.
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I guess that it is getting too warm for the lettuce, I thought that being in the partial shade might help, but I guess that I will try again as the weather turns cool.

Thank you for your suggestion, I will also look into the more heat tolerant varieties of lettuce.

How deep are you planting the seeds? Lettuce needs some light to germinate (not direct sunlight).

If germination is problematic then the soil temps are likely above 75F. One way around this is to bring the container inside, particularly a cool basement.

Once it germinates it can go back outside.

No guarantees that it won't be bitter or bolt quickly, but it is what I will be trying as soon as my container lettuce/greens doesn't germinate outside. I am trying to see if I can grow lettuce right through July/August in my zone.

This past week, I have completely emptied the earthboxes, and replaced the potting mix with new. I then replanted lettuce seed in them (just barely covering them). I don't think that the soil temps are above 75, it is still cool to the touch during the day(partial shade). I have noticed that the other lettuce that I transplanted has turned bitter. Starting to rethink the lettuce situation. Any suggestions on salad greens for the summer?

Not ready to give up yet.

Pick up some 'Jericho' lettuce. It's a Romaine bred to grow in the Sinai Desert, and is probably the best warm weather Lettuce out there.

Look through the seed catalogs for heat tolerant varieties.

Mustards are heat tolerant. Mizuna is a popular one with a taste that isn't mustard like. Black Seeded Simpson is an heirloom with decent heat tolerance and Simpson Elite is an 'improved' version with more heat tolerance. Buttercrunch has some heat tolerance as does 'Green Ice'. Also look for mixes of greens labelled for summer growing. Mesclun mixes as well will offer a variety of greens and you may find some labeled as heat tolerant.

Also ignore what I said above about poor germination above 75F. I am finding that to not be true. I have a container in the shade where lettuce germinated in a couple days with a soil temp of 80F (by day). Last year I tried it in the sun and got no germination unless I brought it inside the basement for germination. I didn't measure the temp of the soil then, but since it was in direct sun and shallow I am guessing much higher than 80.