1.1 Background of the study
The disposal of spent engine oil (SEO) into gutters, water drains, open vacant plots and farms is a common practice in Nigeria especially by motor mechanics. This oil, also called spent lubricant or waste engine oil, is usually obtained after servicing and subsequently draining from automobile and generator engines (Anoliefo and Vwioko, 2001) and much of this oil is poured into the soil. There are relatively large amounts of hydrocarbons in the used oil, including the highly toxic polycyclic aromatic hydrocarbons (Wang et al., 2000). Also, most heavy metals such as V, Pb, Al, Ni and Fe, which were below detection in unused lubricating oil, have been reported by Whisman et al. (1974) to give high values (ppm) in used oil. These heavy metals may be retained in soils in the form of oxides, hydroxides, carbonates, exchangeable cations, and/or bound to organic matter in the soil (Yong, et al., 1992). Nevertheless, this is dependent on the local environmental conditions and on the kind of soil constituents present in the soil-water system. Ekundayo et al. (1989) have shown that a marked change in properties occurs in soils polluted with petroleum hydrocarbons, affecting the physical, chemical and microbiological properties of the soil. Oil pollution of soil leads to build up of essential (organic C, P, Ca, Mg) and non-essential (Mg, Pb, Zn, Fe, Co, Cu) elements in soil and the eventual translocation in plant tissues (Vwioko et al., 2006). Although some heavy metals at low concentrations are essential micronutrients for plants, but at high concentrations they may cause metabolic disorders and growth inhibition for most of the plant species (Fernandes and Henriques, 1991). However, plants respond differently to pollutants. Anoliefo and Vwioko (1995) reported that the contamination of soil with spent engine oil caused growth retardation in plants, with the effect more adverse for tomato (Lycopersicom esculentum) than pepper (Capsicum annum L.). Maize (Zea mays L.), a major cereal in Nigeria and many African countries, was chosen for this study because it has become increasingly popular and most farmers have adopted its cultivation. This study aims to evaluate the effects of spent engine oil on soil properties. In urban areas, various types of activities like agriculture, industry and transportation produce large amount of wastes which are classified as either agricultural, industrial, municipal or nuclear wastes (Onwuka et al., 2012). These wastes from various sources are deposited on the soil surfaces either deliberately applied as fertilizer, sprays or pesticides (Lauhanen et al., 2004) or inadvertently through small or large leaks (Adesodun, 2014) as solids, plastics, crude oil or spent engine oil. Some of these wastes can be recycled into some important products that can be used to meet with the challenges arising from increasing population of Nigeria. They can be recycled into manures and fertilizers for production of crops and animals among others (Onwuka et al., 2012).