Nickel (Ni) & Cobalt (Co)
These two micronutrients are treated in one chapter here as they are only required in trace amounts by plants and do not have to be added to fertilizers. The quantities of nickel and cobalt in fertilizing salts present as “impurities” are sufficient for providing the plants with sufficient amounts of those nutritive elements.
These metals are absorbed by plants in the form of Ni2+ and Co2+ ions. They are part of different enzymes, e.g. urease, which contains nickel and catalyses the degradation of urea to ammonia and carbon dioxide, or of dipeptidases, containing cobalt and splitting dipeptides into two amino acids.
Higher nickel concentrations in the soil are toxic for terrestrial plants, as they cause an iron and zinc deficiciency. In regions with Serpentine rock high in nickel there is often a special flora of nickel-resistant plant species.