Additional nitrogen may be necessary for adequate crop production because decay organisms in the soil will utilize available nitrogen as they break down the green manure crop. Sometimes people resort to using home remedies to control nematodes such as planting marigolds or mixing sugar or lye into the soil. Of these three, only French marigolds are effective in controlling nematode populations, and their effectiveness has limitations and is often questionable. Some people think that marigolds secrete a toxic substance into the soil that kills nematodes and that planting a few marigolds around annual plants in infested soil will prevent infection. This is not true.
Marigolds primarily act as a trap crop.
Nematodes are able to enter their roots but are unable to complete their life cycle. The trapped nematodes die without reproducing. The type of marigold is also important. French marigolds, Tagetes patula , are more effective in controlling root knot nematodes than the African marigold, Tagetes erecta , which is also referred to as the American, Big, or Aztec marigold. To be effective, marigolds must be planted as a solid crop and grown for 90 to 120 days to effectively reduce the nematode population sufficiently to grow annual plants without treatment.