**3.5 Nickel**

Nickel is naturally present in the soil from the weathering of rocks, which may contain about 0.009%, and from volcanic eruptions [25]. The maximum permissible nickel limit in agricultural soils is 75 mg.kg−1 [19]. Nickel uptake by plant roots is highly dependent on the pH, CEC, soil texture, water content, redox potential, organic matter content, as well as on the concentrations of competing ions such as Ca2+, Cu2+, Mg2+ and Zn2+ [25, 26].

### **3.6 Plomb**

Natural lead levels in the soil range from 5 to 20 mg.kg−1. The maximum lead content in agricultural soils set by the European Union is 300 mg.kg−1. Once in the soil, the behaviour of lead depends on its speciation and also on the characteristics of the soil [27–29]. As lead is generally very bound to soil colloids, it is one of the least mobile metallic micropollutants in the soil. However, it can accumulate in the roots with little diffusion to other plant organs [30]. Lead can also be delivered directly to plants through the discharge of lead-containing fuel from automobiles. This type of discharge usually occurs when plants (lettuce, cabbage) are grown on the roadside in many African countries.

#### **3.7 Zinc**

Zinc is a ubiquitous metal in the earth's crust. According to Taylor et al. 1964 in Ondo, 2011 [31], the average zinc content of the West African land crust is between 40 and 100 mg.kg−1. In soil, the important factors controlling zinc mobility are clay and organic matter contents and pH. liming, the addition of high clay content soil, iron or phosphorus reduces the transfer of zinc to plants. Thus, clays and organic matter retain zinc, while an acid pH favours its release into the soil solution [24, 32].
