Ultramafic or serpentine outcrops represent less than 1% of continental surfaces and display a very unusual geochemistry. Ultramafic soils lack essential elements, which either play a fundamental role as plant nutrients: K and P, or as essential cations involved in pedological processes: Al and Ca. Whereas serpentinite bedrocks give birth to Cambisols with high levels of Ni availability, peridotites induce over-expressed weathering of soils that tend to form Ferralsols (i.e. laterites) in tropical conditions. Nickel-hyperaccumulator plants are reported in many ultramafic areas of the world. Their abundance is usually high on ultramafic Cambisols with high available Ni. In temperate regions, all ultramafic soils can host these plants. In tropical regions, they are mostly found on serpentinites because they host Cambisols. Hyperaccumulation of Ni involves (1) efficient root absorption from diffusion-limited pools, (2) active xylem uploading, (3) accumulation in storage organs, (4) complexation with carboxylates, and (5) phloem-driven redistribution of Ni in emerging leaves or reproductive organs. Hyperaccumulators ensure the biogeochemical recycling of Ni in surface horizons of ultramafic soils under available forms through decaying of Ni-rich leaves. This phenomenon could be involved in the “Allelopathy Hypothesis”. Other elements recycled by hyperaccumulators are Ca and K. Hyperaccumulators thus enhance the replacement of excess Mg in soil cation exchange capacity by Ca, K and Ni; this explains the role that these plants may play as pioneer species in ultramafic ecosystems.