Effects of Soil Amendments and Soil Physicochemical Properties on Cadmium and Lead uptake in Tobacco Grown in Migori County, Kenya
Ngorwe, Evans Nyaenya
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Tobacco is a heavy metal accumulator and it is estimated that 1.22 billion people are involved in smoking tobacco while about 1.45 billion people are predicted to be smoking it by 2045. Smoking tobacco results in 5 million premature deaths yearly as a result of tobacco related illness; with 70% of the deaths occurring in developing countries, Kenya included. Tobacco has heavy metals and carcinogens that make it a leading cause of deaths in the world. Tobacco easily uptakes heavy metals depending on soil type. Cadmium and lead are heavy metals of greatest concern, because of their toxicity and cumulative nature. Tobacco bio-accumulates heavy metals incorporated in the soil as a result of fertilizers, soil mulches, pesticides, irrigated contaminated water and polluted run-off water that interacts with soil. This study investigated the use of low cost soil amendments to reduce the extent of uptake of cadmium and lead in tobacco. Field and pot experiments were conducted on contaminated soil to assess the uptake of cadmium (Cd) and lead (Pb) heavy metals by tobacco plant. The effects of soil amendments using hydroxyapatite (HA) and cow manure (CM) on tobacco plant growth on cadmium (Cd) and lead (Pb) accumulation was compared. In the field study, tobacco plant was planted in contaminated soil treated with cow manure (CM) and hydroxyapatite (HA) at the rates of 1% and 2% (w/w) and 0.75% and1.5% (w/w) respectively. In the pot experiment, contaminated soils was amended with cow manure (CM|) and hydroxyapatite (HA) at the rates of 1% and 2% (w/w) and 0.75% and 1% (w/w) respectively. Tobacco was planted in sandy-loamy soil with high concentration levels of Cd and Pb. The tobacco plants were harvested after 75, 90 and 105 days, dried, acid digested and analyzed for cadmium (Cd) and lead (Pb) by AAS. The data collected was analyzed by Analysis of Variance (ANOVA), correlation coefficient, and Student’s Newman Kuel’s test (SNK) to compare pot and field experiment. Experimental soil was found to have concentration levels of Cd of (32.20-95.73) μg/g, Pb of (45.87-107.67) μg/g, EC of (0.88-1.14) dSm-1, soil pH of (6.58-6.9), % OM of (2.67-18.69). Tobacco leaves harvested from 2% CM treatment had Cd (0.20-10.30) μg/g and Pb (6.0-44.40) μg/g, 1% CM treatment had Cd (4.30-27.60) μg/g and Pb (7.20-48.20) μg/g, 0.75% HA treatment had Cd (1.20-42.00) μg/g, while 1.5% HA treatment had Cd (0.60-24.00) μg/g and Pb (1.20-36.60) μg/g, non-amended soil had Cd (9.40-51.80) μg/g and Pb (9.80-47.80) μg/g while fertilizer added had Cd (15.10-53.60) μg/g and Pb (22.60-51.00) μg/g. All the tobacco leaves analyzed contained significantly different amount of cadmium and lead in the three harvests. First harvest had the highest levels of cadmium and lead ranging (0.2-27.2) μg/g and (8.4-42.42) μg/g respectively, while the third harvest had the lowest levels of cadmium and lead ranging from (3.2-16.42) μg/g and (1.0-39.2) μg/g respectively. The tobacco grown in 2% CM treatment had the highest height elongation of 63.5 cm while non-amended treatment had the shortest growth performance with 23.2 cm height elongation Although tobacco was grown in highly contaminated soils 2% cow manure and 1.5% HA treatments produced leaves with permissible levels of Cd and Pb for tobacco. Comparison between cadmium in field (12.98- 13.59) μg/g and pot (17.65- 18.95) μg/g experiment were significantly different, but lead had no significant difference. The amendments reduced the uptake of heavy metals in tobacco leaves, with 1.5% HA being more efficient by reducing lead by 39.69%, while 2% cow manure was most efficient in reducing cadmium by 67.20% as compared to the control. The tobacco grown in amended soils is within WHO/FAO limits. The results suggest that heavy metals uptake in tobacco is reduced by soil amendments indicating that tobacco can be grown in heavy metal polluted areas using soil amendments.