EFFECT OF LIMING ON LEVELS OF POLLUTANT HEAVY METALS IN
Soil contaminated with heavy metals from agricultural and industrial sources leads to unhealthy food. The metals enter the food chain and are consumed by humans. Phosphate fertilizers too contain undetermined amounts of heavy metals and are widely applied on agricultural fields in Kenya together with lime to ameliorate effects of acidity with little regard to the chemical effects/ side effects. This research is a study involving on farm field experiments carried out in some selected fields in Uasin Gishu and Trans Nzoia counties to test the effect of liming and fertilization on heavy metal content in soils and maize grains. Samples were taken from different selected fields over four sampling periods between March 2009 to December 2009 covering both the dry and the wet seasons. The biogenic, reactive, readily available and cheap Mijingu Phosphate Rock (MPR 38.3% of CaO) was tested against agricultural grade lime (20.8% CaO), and diammonium phosphate (DAP) and monoammonium phosphate (MOP) as well as for blank (unfertilized) soils. Similar liming rates were compared for all the fields. The correlations between heavy metals concentration levels in soils with pH were determined. Seasonal variation of heavy metal concentration in soils was also determined. The soils were sampled at a depth of 15 cm using a soil auger, dried, ground, sieved and wet digested in a block digester. Freshly matured grains were also collected, dried, ground and sieved before digestion. Samples were then treated with a mixture of sulphuric acid/selenium powder and hydrogen peroxide to digest the organic material. To avoid loss of nitrogen in samples the samples were pre-treated with salicylic acid. Analysis for heavy metal contaminants was done for Zn, Cu, Pb, Cd, Mn and Cr by Atomic absorption spectroscopy. Results show insignificant correlations between the heavy metals and pH except copper metal (whose value of r = 0.770) which showed the strongest dependence on pH. Generally, liming of soils increased pollutant metal levels. However comparison of observed levels in the study area with threshold values set by the WHO and FAO, illustrates that the heavy metal content in soils and maize grains are within acceptable limits. Heavy metals concentrations dropped sharply from initially high soil values to low values in maize grains implying low uptake by the plants thus; Pb 380.3 ppm in soils and 137.9 ppm in grains, Cd 25.52 ppm in soils and 21.38 ppm in grains, Cu 99.98 ppm in soils and 7.71 ppm in grains, Cr 40.03 ppm in soils and 22.8 ppm in grains, Mn 1165.0 ppm in soils and 629.1 ppm in grains and Zn 243.8 ppm in soils and 150.0 ppm in grains.
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