Phytoassessment of a waste engine oil-polluted soil exposed to two different intervals of monitored natural attenuation using African yam bean (Sphenostylis stenocarpa). 
The present study comparatively investigated the phytotoxic effects of waste engine oil (WEO)-polluted soil exposed to monitored natural attenuation up to 5 and 14 months respectively.  Soil was previously polluted with WEO at 0, 1, 2.5, 5 and 10% w/w oil in soil.  Although, there was significant reduction in heavy metal concentration of soil as well as total hydrocarbon contents, performance of Sphenostylis stenocarpa was greatly retarded when sown at 5 months after pollution (MAP), with death of all seedlings except in the control.  However, growth and yield performances were significantly (p > 0.05) enhanced at 14 MAP.  Computation of hazard quotient showed that ecological risk factor initially posed by the presence of heavy metals in the soil at 5 MAP was significantly (p > 0.05) reduced to safe levels at 14 MAP.