Nematodes and cowpea cover crop (resistance varies with cultivar
and nematodes eventually adapt):
http://edis.ifas.ufl.edu/in516
Other nematode resistant cover crops:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2620786/
Environmental requirements of sunnhemp (grown in summer):
http://ucanr.org/sites/asi/db/covercrops.cfm?crop_id=40
One would use most of these like marigolds, but grown in between
actual crops rather than interplanted with them. I imagine that
turning the top growth of sunnhemp under would require some
equipment. If your equipment is a shovel, probably better to mulch
with the harvested top growth and turn it under after it breaks down.