Introduction: Ultramafic (serpentine) outcrops occupy about 1% of the planet and are constituted by igneous or metamorphic rocks with low content in silicon (< 45 % SiO2) more than 70 % of ferromagnesium minerals and usually with high contents of several toxic trace elements (TEs) (Brooks, 1987). The soils developed over these rocks are characterized by deficiency in organic matter and essential nutrients, unfa...