Some weeks back ABC tv's gardening program described a discovery
by the CSIRO that mustard is an effective organic control for soil
nematodes.

They said to plant mustard seed as a green manure, and when it is
4 - 5 cm in height, dig it into the moist soil. The key mechanism
is a substance in the mustard that decomposes into isothiocyanates
and these effectively fumigate the soil and destroy your nematodes.

(I believe I have in the past posted a different organic technique,
one that involves watering the soil with a solution of molasses.)
--
John Savage (my news address is not valid for email)