Nickel (Ni) was not considered an important element for plant growth, but now research has concluded that it is an essential element for plant growth. The normal range for nickel in most plant tissue is between 0.05-5 ppm. Due to its low requirements (often in parts per billion), it is found in sufficient levels as a contaminant in the soil, water, fertilizer, etc. Nickel deficiency is unusual and is often misdiagnosed as it initially shows no symptoms in plants. This explains why most labs do not test for it and why it is not included in most fertilizers.
Function of Nickel: Nickel is a component of some plant enzymes, most notably urease, which metabolizes urea nitrogen into useable ammonia within the plant. Without nickel, toxic levels of urea can accumulate within the tissue forming necrotic legions on the leaf tips. In this case, nickel deficiency causes urea toxicity. Nickel is also used as a catalyst in enzymes used to help legumes fix nitrogen. There is evidence that nickel helps with disease tolerance in plants, although it is still unclear how this happens.
Deficiency: Minor nickel deficiency displays no visual symptoms, but can reduce growth and yield of plants. Significant nickel deficiency will display visual symptoms typically in the old leaves of the plants as nickel is a mobile element. Deficiency symptoms in legumes are exhibited as whole leaf chlorosis along with necrotic leaf tips (caused by the accumulation of toxic levels of urea). In woody ornamentals, symptoms occur in the spring in new emerging growth and may include shortened internodes (giving a rosetting appearance to the plant), weak shoot growth, death of terminal buds and eventual death of shoots and branches. In pecans, the symptoms are similar to woody ornamentals, but also include decreased expansion of the leaf blade and necrosis of the leaf tips. The leaves develop a condition called “mouse-ear” in which the leaflets are small with rounded tips vs. long and pointed.