passages.id: split_1_test_1882_passage
passages.type: progene_text
passages.text: Leaf analysis of the maize plants grown in soils contaminated with crude oil a week before planting ( preplant treatment ) revealed mean levels of heavy metals ( 6.18 ppm Zn2+ , 0.62 ppm Cu2+ , 26.24 ppm Fe2+ , 10.84 ppm Mn2+ , 2.96 ppm Pb2 + and 3.88 ppm Co2+ ) which are higher than the maximum permissible levels ( MPL ) for maize in tropical soils .