a quick answer

On marigolds and nematodes:

I was taught that the roots release a compound that prevents the nematodes from being able to locate the tomato roots. The description I got was that it's like a bunch of guys running around wearing so much Axe that you can't find the one who's wearing Aqua Velva.

It's possible that's been disproven, but I know a few years ago Burpee's or somewhere was selling a special strain of marigolds that made more of the putative compound, and they were calling them "Nema-Gone" (and selling them for a higher amount than standard marigolds cost - and they claimed it killed the nematodes, which I wasn't aware happened)