Heavy Metals in Agricultural Soils of Central Ethiopia: The Contribution of Land Use Types and Organic Sources to Their Variability

Abstract

In the present study, about 94% of the arable land had a phosphorus deficiency (available P < 10 mg &#183 kg &#45 1 ). To compensate for this deficiency, high amounts of phosphorous fertilization were applied to soils to get better crop yield. In the past two decades, soil organic matter has sharply declined from 34.8 g/kg in late 1980s’ to 12.0 g/kg in 2010. The soil has moderate to high cation exchanging capacity (CEC) in the range of 45 - 58 Meq/100 gm. The low soil organic carbon, high CEC and high phosphorus fertilization could lead to poor heavy metal availability to plants. Nevertheless, DTPA extractable concentrations were 40.5 mg/kg for copper, 35.5 mg/kg for iron, 134 mg/kg for manganese, 2.1 mg/kg for nickel, 2.6 mg/kg for lead and 53.5 mg/kg for zinc. All the available micronutrients seem to be adequate for plant growth. In terms of toxicity, the values of Cu, Fe, Mn, Zn, Ni and Pb found in the present investigation were lower than those of above critical value set by the international guidelines and other researchers. Variations in heavy metal content in the soil were attributed due to differences in land use types and disparity in organic sources amongst the plant and animal composites. For better crop productivity, more organic matter should be added to the soils. The traditional nutrient cycling practices, such as manure application, allow crop residue to decay on fields, fallowing and rotational cropping has to be followed to rehabilitate the soil and buildup both macro-and micro-nutrients to their natural level.