Secondary succession is more common and usually occurs faster than primary succession because the substrate is already present.
Secondary succession occurs on a surface where an ecosystem has previously existed. This happens after some sort of disturbance. Vegetation has already been present in the area, but it no longer exists.
Primary succession only occurs on a surface where no ecosystem existed before. There is no soil and it needs to form. This process takes time, as pioneer species must colonize the area, they must die, and as this happens over and over again, soil forms.
Further Readings –