**4. Materials and methods**

Nigeria, the most populous country in Africa, is situated on the Gulf of Guinea in West Africa with a land area of 351,649 sq mi (910,771 sq km); and total area of 356,667 sq mi (923,768 sq km). Nigerian population is estimated to be152,217,341 with a growth rate: of 1.9%; Its neighbors are Benin, Niger, Cameroon, and Chad. The lower course of the Niger River flows south through the western part of the country into the Gulf of Guinea. Swamps and mangrove forests border the southern coast; while inland areas are hardwood forests.The vegetation of Nigeria northwards include derived savannah,guinea savannah,sudan savannah and sahel savannah. Seven major soil groups in Nigeria include Alfisols, Ultisols, Inceptisols, Entisols, Vertisols, Oxisols and Histosols. Arable farming is a major socioeconomic activity while crude oil prospecting is a chief source of national revenue. Oil and oil-related activities plus urban wastes constitute significant mechanism of pollution.

Soil samples were collected by random sampling in polluted soils studied. Soil samples were collected from epipedons (0-15 cm depth) only. The soil samples were sieved using 2-mm sieve before they were subjected to various laboratory analyses.

Cation exchange capacity was measured by ammonium acetate method at a pH 7 (Soil Survey Staff, 2003). Soil pH was determined using a 1:1 soil :water ratio (Soil Survey Staff, 2003). Total carbon content of soils was estimated by loss on ignition using LECO equipment (Leco Corp, St. Joseph, M. I.). Exchangeable Ca and Mg in ammonium acetate extracted solution (leachate) were estimated with atomic absorption spectrophotometer (Perkin Elmer Model) while exchangeable K and Na were measured by flame photometry. Total nitrogen was estimated by microkjeldahl (Bremner, 1996). Available phosphorus was obtained using Bray P No.2 method (Olsen and Sommers, 1982).

Soil heavy metals were extracted with 1 M ammonium acetate (NH4OAc) [1:4 ratio of soil / 1 M NH4OAc extraction solution : 60 minutes reaction time (Reed and Martens, 1996]. After extraction, the suspension was centrifuged at 7500 x g relative centrifuge force for 30 minutes and the supernatant was passed through a Whatman No.42 paper. The filter paper was allowed to drip dry. The metal concentrations in the supernatant after centrifugation were determined using Inductively Coupled Plasma Optical Emission Spectroscopy, IRIS N701776 (Thermo Jarrell Ash Corporation). The total petroleum hydrocarbon (TPH) was determined using Fourier Transform Infra-red spectrometry (FTIR) (QAL/AM/S 16) at wavelengths ranging from 2800-3200 cm. The digestion and analytical procedures were checked by analysis of DOLT-3 Matrix Certified Reference Material with known concentration for heavy metals (Cantillo and Calder, 1990).
