Nickel (Ni) is an abundant element found in soils at an average of 50 mg/kg. Soil Ni can commonly range from 5 to 500 mg, and serpentine soils have the highest natural levels of Ni. Elevated Ni levels may occur due to industrial activity such as metal refining or the agricultural application of heavy metal-contaminated sewage sludge.

Nickel can be found in oxidation states ranging from -1 to +4, but Ni2+ is the most common state in biological systems. Nickel availability in soil may be as low as 0.2 mg/kg, but proper soil Ni deficiencies are rare. Nickel availability may be considerably lower than total nickel concentration. Above pH 6.5, Ni forms insoluble hydroxide complexes with iron, aluminum, and manganese. Liming has been demonstrated to reduce Ni uptake in acidic soils.

Below pH 6.5, most Ni compounds are relatively soluble. Organic soils adsorbed more Ni than sandy soils, and amino acids and other organic acids can mobilize Ni. Thus, acidic, organic-rich soils may promote Ni uptake.

Ni enters plants via passive diffusion and active transport. Ni may share a common transport system with other ions such as Cu2+ or Zn2+. Thus, Ni cations carry a double positive charge which can induce deficiency via excessive fertilization with competing ions (Zn2+, Cu2+, Mg2+, Mn2+, Fe2+, Ca2+).

Low concentrations of Ni (0.01 to 10 mg/kg soil) have been shown to affect plant growth positively. Nickel was not considered an essential nutrient until the 1970s, when it was proven to be a component of urease, the enzyme that converts urea into ammonia. When plants are Ni deficient, they cannot process urea, disrupting nitrogen assimilation and causing urea to accumulate to toxic levels, often presenting as leaf-tip necrosis on pale green leaves. Proper Ni nutrition is also essential for plant response to disease; Ni application has been shown to reduce fungal infections by 50%. Ni is essential for leguminous plants’ healthy root nodule growth and seed yield.

In pecan trees in the southeastern US, a disorder known as “mouse-ear,” causing early-season leaf chlorosis and stunted leaves and buds, has been attributed to Ni deficiency. Mouse-ear has been successfully treated with a 100 mg/L application of Ni. Ni deficiency in pecans is caused by excessive applications of copper and zinc fertilizers inhibiting Ni uptake on soils with low Ni concentrations. Cold temperatures have also been implicated because Ni uptake is decreased at low temperatures.

While Ni is essential in low concentrations, it can cause toxicity symptoms similar to other heavy metals. 10 mg/kg is considered the limit for sensitive species and 50 mg/kg for tolerant species. Nickel toxicity can cause altered leaf morphology, increased cell wall permeability and ion leakage, reduced transpiration, disrupted photosynthesis and increased oxidative stress. Excessive Ni may also inhibit the uptake of other essential ions, such as Fe2+ and Mg2+. Similar to other heavy metals, the oxidative stress produced by excess Ni can cause chromosomal damage and mutations to cell nuclei.