Chard has weird seeds. They are usually multigerm (so multiple seeds in one), and the seed is chunky and I've found it often doesnt want to let the cotyledons out. I start my chard indoors, and if it sprouts where the seed essentially raises up out of the soil, I'll just make sure to keep them moist by spraying with water, and sometimes I'll use tweezers to remove the shell of the seed off of the seedling.

I've heard daikon radishes are a great plant to break up the soil since they get so big, and you can just let them rot in place. I've never grown any since I don't own any land, and everything I grow is in potting mix or soil that I've purchased.

I did do peas and rye as a cover crop on some bigger planters this past spring, with the intention that I'd kill it off in sections as I planted stuff. That was a big mistake cause it's pretty hard to kill rye like that. If i do it again, it'll have to be peas with something else to provide some structure.