Abstract
Heavy metals status of agricultural soils should be monitored in order to prevent soil-plant pollution. This study evaluates the effect of season, agronomic practice and soil mineral composition on the levels of some heavy metals (Pb, Mn, Ni, Fe, and Zn) in the arable and oil palm soils of three Farm Settlements in Ogun-State Southwest, Nigeria. Soil samples were collected in two consecutive seasons between 2010 and 2012 and digestion was carried out using Standard Wet Acid Digestion method. Total heavy metals in the digest were determined using Flame Atomic Absorption Spectrophotometer (FAAS). The total levels of heavy metals (in mg/kg) found in the sampled soils were as follows: in the rainy season Mn (28.4 - 34.2), Fe (1599.7 - 2013.2), Pb (11.0 - 16.9), Zn (100.5 - 112.9) and Ni (11.3 - 13.8) and in the dry season Mn (32.1 - 40.1), Fe (1701.4 - 2455.5), Pb (13.0 - 18.7), Zn (105.7 - 110.4) and Ni (15.5 - 16.3). Levels of the heavy metals found in the sampled soils were significantly lower (p < 0.05) than their permissible levels in agricultural soils. Although the levels of heavy metals determined in dry season were higher than those of the rainy season, the only seasonal difference was that of Fe in Sawonjo soil significant at p = 0.05. The level of heavy metals in oil palm soils was significantly higher than the level in arable soils (p < 0.05). At present, pollution level of heavy metals in the sampled soils is low and poses no environmental risk, yet, they have to be regularly monitored before they bio-accumulate into toxic. Government should therefore set up soil monitoring agency and provide irrigation facilities to encourage dry season farming.
Highlights
Heavy metals pollution in agricultural soils has become one of the global challenges facing food production and the sustainability of life
Variation in Soil Properties Variation in general soil properties between the arable and permanent agro-ecosystems: There are variations in some of the soil properties determined in the soils of the two agro-ecosystems but only variation among organic carbon, organic matter, nitrogen and C/N ratio are significant at p = 0.05 (Table 1)
Little variation observed in some of the soil properties of sampled soils from the two agro-ecosystems may be as a result of differences in the plant soil requirements and root type
Summary
Heavy metals pollution in agricultural soils has become one of the global challenges facing food production and the sustainability of life. Heavy metals (though, some are useful to plants and animals when present in only traces amounts as micronutrients) become dangerous when they persist and accumulate to the level of toxicity [2] [3]. Heavy metals such as Cu, Fe, Zn, Mn, Co and Se, essentially aid metabolism but at a higher concentration can adversely affect human health and her growth, and sometimes, they can result in death. If the agricultural soils are not well monitored, in terms of heavy metals level in the soil, there is no way the country would be able to accomplish such a laudable goal by 2020. Heavy metals in the soil need to be effectively monitored before an irreparable damage is done to farmlands
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