Nickel is a micronutrient that plays a role in plant development. Although once considered toxic, it was recognized as an essential nutrient in 1987 when research showed some species need it to complete their life cycle. Plants require nickel in very small quantities and absorb it from the soil as a divalent cation (Ni²⁺).
Nickel’s Roles in Plant Health
Nickel’s primary function in plants is its role in nitrogen metabolism as a component of the urease enzyme. Urease is responsible for the hydrolysis of urea into ammonia and carbon dioxide. This conversion allows the plant to use nitrogen from urea, whether from fertilizers or internal metabolic processes. Without sufficient nickel, urease activity is impaired, preventing the plant from processing urea.
Beyond its involvement with urease, nickel contributes to other processes. It is a component of enzymes in bacteria, such as hydrogenases, which are important for nitrogen fixation in legumes. This means leguminous crops may require nickel to support nodulation and efficient nitrogen fixation. Nickel also has a function in plant defense mechanisms and stress tolerance, as it can influence the production of certain secondary metabolites used to resist pests. Additionally, it is a component of superoxide dismutase, an enzyme that helps protect the plant from oxidative damage.
Identifying Nickel Deficiency Symptoms
The most direct symptom of nickel deficiency is the toxic accumulation of urea, which causes necrotic lesions on leaf tips. In certain species, particularly pecans, a severe deficiency causes a malformation known as “mouse-ear,” where leaflets are small, rounded, and blunt instead of pointed. This symptom appears on new spring growth.
Other symptoms can include chlorosis (yellowing) of younger leaves, reduced leaf size, and poor plant vigor. In woody plants, a deficiency can manifest as stunted growth, delayed bud break, loss of apical dominance, and the death of terminal buds. The appearance of leaf tip burn, especially when using urea-based fertilizers, is a strong indicator of insufficient nickel.
Soil and Environmental Factors Causing Deficiency
Nickel availability is influenced by soil chemistry, especially pH. It becomes less available for plant uptake as soil pH rises above 6.7, making alkaline soils a primary cause of deficiency. Excessive liming can also raise soil pH and induce a nickel shortage.
Soil composition also dictates nickel availability. Sandy soils with low organic matter are often naturally low in nickel. Competition with other metallic nutrients can limit uptake, as high concentrations of other divalent cations like zinc (Zn²⁺), copper (Cu²⁺), manganese (Mn²⁺), and iron (Fe²⁺) can inhibit absorption. Cool and dry soil conditions can also reduce uptake.
Managing and Correcting Nickel Levels
A nickel deficiency can be confirmed through plant tissue and soil analysis. While soil standards are not established, normal leaf tissue concentrations are between 0.05 and 5 parts per million (ppm). Symptoms often appear when levels drop below 1 ppm in sensitive species like pecan.
The most effective correction method is a foliar spray of a soluble nickel compound, like nickel sulfate (NiSO₄). Foliar applications are fast-acting because the nutrient is absorbed directly through the leaves. Recommended spray concentrations are very low, around 0.03 to 0.06 ppm, since plants require only trace amounts. For pecans with “mouse-ear,” sprays are applied in spring after bud break.
Soil applications are an option but are less common because most soils have adequate nickel. Preventative management includes maintaining a soil pH below 6.7 and ensuring balanced fertilization to avoid excessive competing cations. Precise application is necessary, as nickel can become toxic above 10 ppm in sensitive plants.