This study aimed at investigating the impact of spent engine oil on soil physicochemical properties and the microorganisms of the soil. Two nursery beds of 5m distance from each other was created at agricultural research farm land, University of Maiduguri, Nigeria and 250ml volume of spent engine oil was added to the first nursery bed for the period of 4weeks (application was three times per week) and 500ml of the spent engine oil was added to the second nursery bed for the same duration. A control soil experiment was equally set up and treated with normal water. The physicochemical properties and bacteria of the respective soils were determined by standard methods. The soil pH was slightly acidic, there was no difference with the control, and the electrical conductivity was low in both the control as well as the contaminated soil samples. There was an increase in organic matter, organic carbon, sodium and potassium content in the soil compared to control. There was phosphorus decreased in nitrogen as well as magnesium, phosphorus decreases at the top but increased at depth in the contaminated soil samples. The bacteria identified in the control at the top soil (Proteus species, Corynebacterium species, and Bacillus subtilis) and depth (Bacillus subtilis, Staphylococcus epidermis, Corynebacterium species, and Proteus species), reduced with the application of the spent engine oil at 250ml volume (to Corynebacterium species and Bacillus subtilis) both at top and depth and at 500ml volume of spent engine oil, Corynebacterium species and Bacillus subtilis occurred at top and Escherichia coli, Corynebacterium species, and Bacillus subtilis, at depth. This may be due to the increased application of spent engine oil which was not present in the control and soil contaminated with 250ml volume of spent engine oil. This means that spent engine oil may have a unique property that triggers the occurrence of E. coli. It can therefore be concluded that the spent engine oil altered the physicochemical properties and bacteria of the soil.
