Effects of Soil Amendments on Growth and Metal Uptake by Ocimum gratissimum Grown in Cd/Zn-Contaminated Soil

Hydroponic and pot experiments were conducted to assess the uptake of heavy metals (Cd and Zn) by a common crop plant, African basil, Ocimum gratissimum. In addition, the effects of soil amendments, hydroxyapatite (HA) and cow manure on plant growth and metal accumulations were compared. In the hydroponic study, plants were exposed to various concentrations of Cd (2.5 and 5 mg L-1) and Zn (10 and 20 mg L-1) for 15 days. O. gratissimum was shown to be a Cd accumulator more than a Zn accumulator. Cadmium concentration in its shoots exceeded 100 mg kg(-1). In the pot experiments, soils from a heavily Cd-contaminated site (Cd 67.9 mg kg(-1) and Zn 2,886.8 mg kg(-1)) were treated with cow manure and HA at the rates of 10% and 20% (w/w), and 0.75 and 1.5% (w/w), respectively. Plants were grown in the greenhouse for 3 months. The addition of cow manure resulted in the highest biomass production and the lowest accumulations of Cd in plant parts, while HA was more efficient than cow manure in reducing Zn uptake. Leaves of African basil showed a decreased Cd concentration from 1.5 to 0.3 mg kg(-1) (cow manure) and decreased Zn concentration from 69.3 to 34 mg kg(-1) (HA). This clearly demonstrates the efficiency of HA and cow manure in reducing metal content in leaves of plants grown on high metal-contaminated soil to acceptable or close to acceptable values (0.2 mg kg(-1) for Cd, 99.4 mg kg(-1) for Zn).