THE EFFECTS OF SOIL AMENDMENTS ON THE GROWTH AND YIELD PERFORMANCE OF COWPEA (VIGNA UNGUICULATA CV. IFE BROWN) SOWN IN A PETROLEUM HYDROCARBON-POLLUTED SOIL

Abstract

The present study was on the phytoassessment of a waste engine oil (WEO)-polluted soil after soil amendment, using Vigna unguiculata cv. Ife Brown. Top soil (0 - 10 cm) of constant was measured into buckets. The WEO was poured into the measured soil and thoroughly mixed to obtain 2.5% w/w oil-in-soil concentration. Polluted soils were mixed with manure (MN) and/or sawdust (SD) at 10% w/w respectively. However, when more than a soil amendment was applied, they were both added together in equal proportions to make 10% w/w substrate in soil. Urea (UR) was also applied to soil at 4g/kg soil. The treatments available during the study included, polluted soil (PS), PS+UR+SD+MN, PS+SD+MN, PS+UR+MN, PS+UR+SD, PS+MN, PS+SD and PS+UR an the unpolluted soil (UPS). The entire set up or control was left in an open shade for 3 months without mechanically disturbing the soil. Thereafter the status of the soil was phytoassessment using cowpea. Results showed significant reductions in heavy metal contents of soil. After three months of planting, copper in the polluted soil ranged from 4.8 - 6.4mg/kg while lead (Pb) was from 0.89 - 1.64mg/kg in the amended soils compared to 2.08mg/kg in the unamended but polluted soil at three months after sowing. Comparatively, cow dung amended soils had relatively less heavy metal and hydrocarbons compared to other soil amendments or combinations. Results of the phytoassessment showed that there was depressed vegetative growth and yield performance of cowpea in the oil-polluted soil. Soil amendment however enhanced plant growth and survival. Estimated grain yield was lowest in polluted but not amended soils (222.3kg/ha), compared to 956.04kg/ha in unpolluted soil. However, grain yield was highest in PS+MN (147878.35kg/ha).