It is thought that one or more biological organisms in the soil are suppressing nematode populations. Trying to document that a soil is suppressive is no easy task.
There are indications in the literature that crops grown under monoculture for many years in nematode infested fields will have high nematode populations for several years which will then fall and remain at low levels. It is thought that some biological organisms are functioning to maintain nematode populations at a low level.
It is not uncommon in chemical control field trials for chemically treated plots to have significantly higher nematode populations at harvest than untreated controls. At times, this has been interpreted as indicating the field is naturally suppressing nematode populations in the controls. In reality, the difference is likely due to the chemically treated plots having healthier root systems which are able to support larger populations than can be supported by the roots of the untreated controls.