Nickel is a chemical element that plants require in extremely small quantities, earning it the designation of a micronutrient. It was the most recently added element to the list of plant nutrients considered essential, with its role formally established in the late 1980s. Despite the minute concentration required—often less than 0.5 milligrams per kilogram of dry plant weight—nickel is fundamental for a plant to complete its life cycle. A deficiency can severely compromise plant health and development.
Essential Functions of Nickel in Plant Life
The primary role of nickel in plant physiology is its function as a metallic component of the urease enzyme. Urease catalyzes the breakdown of urea, a nitrogen-containing compound, into ammonium and carbon dioxide. This process is a foundational step in nitrogen metabolism, as ammonium is a usable form of nitrogen necessary for growth and protein synthesis.
Without sufficient nickel to activate urease, the plant cannot efficiently process urea, leading to a dangerous buildup of the compound within its tissues. This accumulation creates a toxicity that disrupts normal cellular functions and causes visible damage. Nickel also plays a role in the mobilization of stored nitrogenous compounds during seed germination; seeds from severely deficient plants can be completely inviable.
The requirement for nickel is particularly pronounced in leguminous plants, such as soybeans and cowpeas, which engage in biological nitrogen fixation. Nickel is a component of enzymes, including urease and hydrogenase, used by the nitrogen-fixing bacteria living in the root nodules. An adequate nickel supply supports the bacteria’s activity, thereby enhancing the plant’s overall nitrogen uptake and improving early growth.
Recognizing the Physical Signs of Deficiency
The most distinctive visual sign of nickel deficiency is the accumulation of toxic levels of urea in the leaf tissue, resulting in necrotic lesions. This tissue death typically begins at the leaflet tips and margins, often resembling a burn or scorch. In severe cases, high concentrations of urea can directly kill the cells.
Necrosis is often accompanied by chlorosis, or yellowing, which first appears in young leaves, even though nickel is considered a mobile nutrient. The chlorosis can be mistaken for nitrogen deficiency, but the distinctive necrotic spots from urea toxicity help differentiate the two conditions. General symptoms also include stunted growth, an abnormal, bushy appearance, and reduced leaf size.
In pecans, nickel deficiency produces a characteristic symptom known as “mouse-ear.” This condition is identified by leaflets that are smaller than normal, have rounded tips, and often show marginal necrosis. Legumes are also sensitive, frequently exhibiting whole-leaf yellowing coupled with necrotic tips on their leaflets. These symptoms often appear in the spring on new growth as the plant attempts to mobilize depleted nickel reserves.
Environmental Factors Contributing to Deficiency
The availability of nickel is dictated by soil chemistry, particularly soil pH. Nickel is absorbed by plants as the divalent cation (Ni²⁺), which becomes less soluble and available as the soil pH rises above 6.7 or 7.0. In alkaline or over-limed soils, the Ni²⁺ cation forms precipitates that plant roots cannot absorb, leading to a conditioned deficiency.
A second factor is the competitive interaction with other metal cations, such as zinc (Zn²⁺), copper (Cu²⁺), and iron (Fe²⁺). These elements share a common uptake system in the roots, and high concentrations of any one can inhibit nickel absorption. Excessive application of fertilizers containing these competing metals can inadvertently induce a nickel deficiency, even if the soil contains sufficient total nickel.
Soil type also influences availability; sandy soils with a low cation exchange capacity are more prone to deficiencies. Although total nickel content may be high, only the available Ni²⁺ fraction matters for the plant. Cool and dry soil conditions, especially early in the growing season, can temporarily reduce uptake, leading to transient deficiency symptoms in spring growth.
Practical Strategies for Correcting Deficiency
Addressing a suspected nickel deficiency requires confirmation through tissue or soil testing. Applying an amendment without confirmation risks creating a toxic environment, as the gap between deficiency and toxicity is narrow. The application of soluble salts, such as nickel sulfate (NiSO₄), is the standard method for correcting a confirmed deficiency.
For immediate correction, a foliar spray application is the most effective method, especially when the deficiency is induced by high levels of competing metals. The leaves bypass the root-uptake system, allowing the nickel to be absorbed directly into the plant tissue. A very dilute concentration, often 0.03 to 0.06 parts per million of nickel, is sufficient to reverse symptoms like leaflet necrosis.
Soil application of nickel sulfate can be used as a preventative or long-term measure, though it is less common for annual crops. Adjusting soil pH is also a preventative strategy, as lowering high pH levels increases the natural solubility and availability of the Ni²⁺ cation. Monitoring the application rates of competing micronutrients, such as zinc and copper, helps maintain the necessary balance for nickel uptake.