EFFECT OF SPENT LUBRICATING OIL ON THE GROWTH AND YIELD RESPONSE OF COWPEA (Vigna unguiculata) (L.) Walp.

Abstract

Spent lubricating oil is usually obtained after servicing and subsequent draining from automobile and generator engines which contains heavy metals and polycyclic aromatic hydrocarbon found to be dangerous to biota. The effect of spent lubricating oil on the growth and productivity of cowpea Vigna unguiculata (L) Walp was investigated in a screen house, using potted plants in soil contaminated with different concentrations of spent lubricating oil. The seeds were sown at the screen house for six weeks and then transplanted to the soil polluted with spent lubricating oil. 3kg of soil was mixed homogeneously by hand with 50ml, 75ml and 100ml of spent lubricating oil and and the control (0ml). Spent lubricating oil in soil generally resulted in delayed seed emergence, poor plant growth and mortality at higher concentrations (100ml) of sokoto local Vigna unguiculata. The growth parameters revealed that, application of spent lubricating oil had a significant reduction (p<0.05) on the plant height, number of leaves, leaf area and the stem girth of the two cowpea seedlings studied. At week 2, there was a decrease in the parameters measured from the control (0ml) to the treated plants, in that the oloyin Vigna unguiculata at week 2 had 10.2cm as the height while the sokoto local Vigna unguiculata had 19.8cm. In both cowpea seedlings, sokoto local seedling was more tolerant to spent lubricating oil than oloyin seedling. There was no significant difference in the pH of soil following the spent lubricating oil application. Also, there was no significant difference in the concentration of heavy metal between plant parts apart from iron that increased in the root of oloyin Vigna unguiculata. The amount of heavy metals (Copper, Cadmium, Iron, Lead and Nickel) present in soil polluted with spent lubricating oil polluted soil could be responsible for the retarded growth, chlorosis and yellowing of the leaves observed at three weeks after transplanting. It was observed also that the heavily polluted soil appeared darker in colour than the less polluted soils. The darker soil absorbed a lot of heat which could be responsible for the inhibition of crop seed germination. It was also found that soils contaminated with spent lubricating oil at 75ml and 100ml of pollution become waxy and usually does not allow water to penetrate or sink from above and when wet, the soil become water-logged. Therefore, sokoto local Vigna unguiculata seedling can tolerate low levels of spent lubricating oil pollution or contamination than oloyin Vigna unguiculata seedling.