We have nothing but sand in our yard, so I have dug a hole 4 feet by 12 feet and 18 inches deep, filled with potting soil. I mixed steer manure just under the seeds, but they are not doing well (cucumbers, beets, lettuce, beans and zucchini). A few cukes and zucchini have come up, but I see no signs of the lettuce or beets. Should I have mixed sand with the potting soil, or what can I do at this point?
These articles should be helpful: https://www.gardeningknowhow.com/garden-how-to/seeds/using-potting-soil-in-the-ground-to-start-seeds.htm
https://www.gardeningknowhow.com/special/organic/the-importance-of-soil-for-an-organic-garden.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/what-is-soil-made-of-creating-a-good-garden-planting-soil-type.htm
How long does it take for commander lettuce to reach maturity, and how do I really know it's ready? My lettuce has been in the ground for about 41 days. It has started forming heads but I'm not quite sure how big or firm they get or how to know they are ready for harvesting.
Commander lettuce takes 50-55 days to reach maturity, so it sounds like you have some more time. When ready to harvest, the head will feel firm. Right now, the heads likely feel rather loose.
I have tons of tiny black beady things all over the lettuce in my garden. They are the size of tiny dirt granules. . . looks like someone sprinkled compost over my plants, but no one did. Are these pest eggs of some sort?
It is likely the frass (insect debris) from a pest of some kind. Since you have not seen the pest, we don't know what kind it is, but we have found neem oil to be very effective against almost all harmful pests in the garden. This article has more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
It could possibly be black aphids. A simple store bought pest control, at a local garden store will get rid of them.
How do I know when to cut leaf lettuce growing in an oblong planter? Where do you cut it, and will it grow back to cut again?
This article will help with that: https://www.gardeningknowhow.com/edible/vegetables/lettuce/growing-lettuce.htm
I always grow Iceburg head lettuce and I never get a head. It kind of grows straight up like a tree. Why is this happening?
It may have bolted. This article will explain: https://www.gardeningknowhow.com/edible/vegetables/vgen/what-is-bolting-what-it-means-when-a-plant-bolts.htm
Is it best for me to sow my seeds indoors to plant in my fall crop here in Iowa?
Indoors you will have a little more control over the temperatures that the plants will be exposed to, so that is an advatage. The disadvantage is that you may not be able to provide enough light. If you think that you would be able to provide enough light, then I would start them indoors.
Does head lettuce look leafy at first? Then does the head start later in the middle?