The Importance of Nickel in Plant Nutrition

Nickel (Ni) is a micronutrient found in plants in minute quantities. Plants absorb Ni as the divalent cation Ni²⁺. It is required in very small amounts, with the critical level appearing to be about 1 part per million. Although its role in plant growth and development has often been overlooked, as it was previously classified as a toxic metal element, a positive impact on several processes within the plant has been confirmed.

Nickel was the last nutrient added to the list of essential plant nutrients. This addition occurred in the late 20^th century. The recognition of nickel as an essential nutrient was based on research conducted in the late 1980s by Brown and colleagues, who determined that nickel is necessary for plants. The American Association of Plant Food Control Officials officially recognized nickel as an essential nutrient in 2004.

Role of Nickel in Plant Physiology

Nickel is an essential component of several plant enzymes, most notably urease, which is responsible for hydrolyzing urea into ammonia and carbon dioxide. This process is crucial for nitrogen metabolism in plants. Without nickel, toxic levels of urea can accumulate in plant tissues, leading to necrotic lesions on leaf tips.

Nickel also plays a role in nitrogen fixation, a process where certain bacteria convert atmospheric nitrogen into a form that plants can utilize. This is particularly important for leguminous crops which rely heavily on nitrogen fixation for their growth.

Additionally, nickel seems to be involved in enzymatic processes related to amino acid synthesis, protein metabolism, and also the production of secondary metabolites.

Nickel Deficiency and Excess in Plants

Nickel deficiency is very rare as the element is usually present in sufficient amounts as a contaminant in the soil and water. No Nickel deficiencies have been observed under crop-growing conditions, but in crop research settings, researchers have reproduced deficiency symptoms. Minor deficiencies may not show visible symptoms and may be misdiagnosed though they can reduce growth and yield. Nickel is rather mobile in plants. Severe deficiencies therefore first manifest as chlorosis in the old leaves, necrotic leaf tips, and inhibited growth. In legumes, deficiency symptoms include whole-leaf chlorosis and necrotic leaf tips due to the accumulation of toxic urea levels.

Nickel deficiency has been observed in some nursery plants and tree crops. In woody plants, nickel deficiency can cause shortened internodes, weak shoot growth, death of terminal buds, and eventually, the death of shoots and branches.

Nickel concentrations reach phytotoxic values at 10-100 mg per kg dry weight. Its high content limits plant growth and inhibits photosynthesis and transpiration. A reduction in the intensity of nitrogen fixation has been found in legumes. The toxicity of nickel is rather explained by the Ni:Fe ratio in plants than its absolute concentration.

Importance of Nickel in Agriculture

Nickel’s role in agriculture extends beyond plant health. As it participates in the nitrogen metabolism within the plant, it directly enhances nitrogen use efficiency and thereby the right nickel levels may reduce the need for excessive fertilizer application. This is particularly important in modern agriculture, where the goal is to minimize environmental impact while maximizing crop yield.

Some studies also suggest a positive effect of nickel on plants’ tolerance to abiotic stresses through support of antioxidant enzyme activity. This makes nickel control especially important in regions prone to extreme weather conditions.

Managing Nickel Levels

Ensuring adequate nickel levels in soil is essential for optimal plant growth. Nickel is typically found in sufficient quantities as a contaminant in soil, water, and fertilizers. However, its availability can be affected by soil pH and the presence of other metals like zinc, copper, iron, cobalt, cadmium, and magnesium. High levels of these metals can induce nickel deficiency, making it necessary to monitor and manage soil composition.

Foliar applications of nickel may be an effective way to address deficiencies. For example, studies have shown that foliar nickel applications improved mineral nutrition status, urease activity, and the physiological quality of soybean. This method ensures that plants receive the necessary nickel levels directly, bypassing potential soil-related issues. Nickel salts (such as sulfates and nitrate) and organic nickel ligands (like lignosulfonates and heptogluconates) are effective as foliar fertilizers. The Ni-lignosulfonate form is preferred for field applications due to potential safety concerns for field workers associated with other foliar-applied sources.

Conclusion

Nickel, though required in small quantities, impacts plant health and agricultural practices. It is important for plant nitrogen metabolism, nitrogen fixation, and various enzymatic processes. While nickel deficiency is rare, it can adversely affect plant health and growth. Conversely, excessive nickel levels are phytotoxic and can inhibit plant growth. Therefore, understanding and properly managing nickel levels in soil, water and fertilizers is essential for farmers to ensure healthy crops, optimal yields, and contribute to a more sustainable and productive agricultural system.