Abstract
This study was conducted on the urban agricultural soils at the banks of three metropolitan and suburban rivers namely Challawa, Jakara and Watari in Kano, Kano State, Nigeria to determine the total, exchangeable and soluble concentrations of the heavy metals Cu, Cr, Ni, Zn, Pb and Cd . The soils are all slightly alkaline (mean pH 7.42 – 7.77), with moderate CEC (12.63 – 23.11cmol/kg), medium to high organic carbon (7.16 – 11.27g/kg). The mean ranges of total Cu, Cr, Ni, Zn, Pb and Cd were 4.95 – 5.99, 5.85 – 165.66, 54.03 – 57.77, 55.07 – 255.52, 42.84 – 68.12 and 0.59 – 11.81mg/kg respectively. All the values were lower than the maximum allowable concentrations in soils, except for Cd at Jakara and Challawa. The mean ranges of exchangeable metals were 0.57 – 1.18, 1.69 – 29.26, 13.89 – 15.59, 14.16 – 49.01 10.48 – 18.31 and 0.39 – 1.20mg/kg for Cu, Cr, Ni, Zn, Pb and Cd respectively. The mean soluble concentrations were 0.27 – 0.48, 0.61 – 15.91, 5.60 – 7.15, 4.90 – 17.42, 2.78 – 9.36 and 0.00 – 0.60mg/kg for Cu, Cr, Ni, Zn, Pb and Cd respectively. Although appreciable amounts of all the metals were detected in the various forms, variability exists between the sites with concentrations found to be significantly higher (p≤ 0.05) in areas irrigated with waters receiving industrial and domestic sewages namely, Challawa and Jakara than Watari which was the control and significant relationship (p≤ 0.05) exists between exchangeable and total metal forms on one hand and such soil properties as organic carbon, available P, total N and basic cations on the other.

Key words: Heavy metals, bio-avaliable, Kano, Urban, Agriculture