[Redbaron]

I have found that tilling marigolds is like moving 3 steps backwards to go 2 steps forward. Counterproductive. Yes marigolds do help kill RKN. Tilling ruins the soil structure that allows predators nematodes to do their part.

Carolyn is right. Organic matter is important! But it is important to also consider the organic matter "highways" the predatory nematodes use are largely structured by soil glues (like glomalin). Tilling in especially sandy soil destroys that soil structure, even if you are tilling in organic matter. Much better to let the soil biology incorporate it. Most those organisms also have soil glues too, like the slime on earthworms.
Remember RKN are much smaller than predatory nematodes. They can flow right through the sand with the water. The predators being larger need channels.