Nickel

Nickel’s clearest established function is as a cofactor for urease, the enzyme that breaks down urea generated within plant metabolism (and taken up from urea-based fertilizers) into ammonia and carbon dioxide. This links nickel functionally to nitrogen metabolism even though the plant’s nickel requirement is very small.

Nickel occurs naturally in most soils at levels normally sufficient for the small quantities plants require. As with most other cationic micronutrients, its availability decreases at high soil pH, and soil organic matter binds nickel in ways that can further reduce its solubility.

• Most agricultural soils contain enough nickel for plant needs
• High soil pH reduces nickel availability, similar to other divalent-cation micronutrients
• Documented deficiencies have been limited to soils very low in total nickel or very high in organic matter

Because urease activity depends on nickel, deficiency can allow urea to accumulate in leaf tissue, leading to leaf-tip necrosis — described as "mouse-ear" rosetting in some tree crops in documented cases. Confirmed field deficiencies remain limited relative to other micronutrients.

Nickel toxicity is uncommon in ordinary agricultural soils but can occur on naturally nickel-rich soils (such as those derived from serpentine rock) or on soils contaminated by industrial sources, restricting root growth and general vigour.

Nickel’s functional role is tied to nitrogen metabolism through the urease enzyme, connecting it to nitrogen and urea-based fertilization. It also shares soil chemical behaviour and uptake pathways with other divalent cation micronutrients, such as copper and zinc.