This study was conducted to assess the growth performance of Amaranthus cruentus on spent-engine-oil- (SEO) contaminated soil bioremediated with bacterial inoculants and processed biostimulants: compost, sole cocoa pod husk (CPH), and sole cow dung (CD). SEO with 5%, 10%, and 15% densities was artificially contaminated with sterilized soil. Subsequently, biostimulants and bacterial inoculants, Staphylococcus aureus and Bacillus cereus, were applied. The treatments were replicated three times, adopting a completely randomized design. After the biodegradation assay that lasted for 70 days, Amaranthus cruentus seeds were sown, while its growth parameters, plant height, number of leaves, and stem girth were monitored for 6 weeks after sowing (WAS). The results generated indicate that the applied biostimulants and bacterial inoculants accomplished significant (P < 0.05) reclamation of the SEO-contaminated soil as the test plant recorded substantial growth compared with the controls that recorded no growth. At 2 WAS, compost influenced the highest plant heights (6.5, 7.4, and 7.5 cm) compared with other biostimulants at 5%, 10%, and 15% SEO contamination levels respectively. At 4 WAS, all biostimulants produced the same number of leaves (5.0) at 5% SEO contamination level, the compost recorded the highest number of leaves (6.0) at 10% SEO contamination level, while all biostimulants produced the same number of leaves (4.0) at 15% SEO contamination level. The study showed that the combination of biostimulants and bacterial inoculants significantly reclaimed SEO contaminated soil. Therefore, reclamation of petroleum-impacted environments with the combination of biostimulation and bioaugmentation is recommended.
