Nematodes
I have read that nematodes can be eliminated for several years by planting
a crop of marigolds for an entire season. Also that companion planting of
marigolds can be helpful if the infestation is not too severe and the crop
not too susceptible. The marigold of choice seems to be the "french" type,
which is actually closest to those originating in Mexico. Another point
raised is that a biologically active garden soil which is rich in a variety
of microrganisms is less likely to be troubled as there are kinds of fungi
and bacteria occuring that are inimical to nematodes. I am very much in
favor of maintaining a true "living" topsoil and I believe I have been able
to do so. A soil environment that is out of natural balance can lead to
problems like nematodes. I know my soil is rich with bacteria as i have
witnessed just how fast the little buggers can take out my fence or support
stakes! I gave up on a wood fence in favor of a living fence which I like
better anyway. Life and gardening are full of little compromises and
trade-offs.