Nickel Concentration in Soils

Nickel is abundant in the crust of the Earth, comprising about 3% of the composition of the earth. Nickel averages 50 mg Ni kg^-1 in soils and commonly varies from 5 to 500 mg Ni kg^-1 but ranges up to 24,000 to 53,000 mg Ni kg^-1 in soil near metal refineries or in dried sewage sludge, respectively. Agricultural soils typically contain 3 to 1000 mg Ni kg^-1, whereas soils derived from basic igneous rocks may contain from 2000 to 6000 mg Ni kg^-1 (62).

Total nickel content is, however, not a good measure of nickel availability. At pH<6.7, most of the nickel exists as sparingly soluble hydroxides, whereas at pH^-6.5, most nickel compounds are relatively soluble (48). Depending on the soil type and pH, nickel may also be highly mobile in soil and is further mobilized by acid rain. The role of pH in nickel availability was illustrated by Van de Graaff et al. (63), who observed that long-term irrigation with sewage effluent increased heavy metal loading in soil, but that plant metal contents did not increase, apparently owing to the increased soil pH, iron complexation and coprecipitation, and precipitation of phosphorus–metal complexes.

Truly nickel-deficient soils have not been identified to date; however, Ni deficiency can occur as a result of excessive use of competing ions (Zn, Cu, and MgO and unfavorable growth conditions (2)). Nickel is the 24th-most abundant element in the crust of the earth, and plant nickel requirement (<0.05 mg kg^-1 dry weight) is the lowest of any essential element. Although a large number of analyses have been conducted for nickel in plant tissues, no recorded levels have been below 0.2 mg kg^-1 dry weight in field-grown plants. Nickel can be supplied by atmospheric deposition, at rates that easily exceed the removal from the crops in the field (64). The ubiquitous nature of nickel is illustrated by the experiments that established the essentiality of nickel (1). In these experiments, the authors went to extraordinary lengths to purify or re-purify all chemical reagents, equipment, and water and to maintain contaminant-free growing conditions. Even under these conditions, it required three generations of crop growth to deplete the nickel carried over from the grain before the first evidence of nickel deficiency was observed.

The possibility that nickel-deficient soils exist, however, cannot be discounted particularly as purity of fertilizers is improved, the use of urea is increased, and atmospheric deposition of pollutant nickel is decreased. Plants grown under specialized conditions (greenhouses and tissue culture), particularly with urea as a nitrogen source, may be especially susceptible to nickel deficiency (40).

Nickel toxicity, which is usually associated with serpentine soils, sewage-sludge application, or industrial pollution, is a well-described constraint on crop production in many parts of the world. In serpentine soils (derived from basic igneous rocks), nickel concentrations may range from 1000 to 6000 mg kg^-1 dry weight and are frequently associated with high concentrations of iron, zinc, and chromium and an unfavorable ratio of magnesium to calcium. Values for ammonium acetateextractable nickel in these soils varies from 3 to 70 mg kg^-1; however, it is not always clear that poor plant growth can be ascribed to any single factor concerning nickel.

Similarly, in sewage-amended soils or in contaminated soils, it is often difficult to relate total nickel load with plant productivity as factors such as the chemical properties of the contaminant and base soil, pH, and oxidation–reduction state affect results (48,65). Indeed, the importance of considering soil pH is well illustrated by Kukier and Chaney (65 and references therein), who demonstrated that addition of limestone to raise soil pH is highly effective in immobilizing nickel in situ and in reducing phytotoxicity. Plant species also differ in their ability to obtain nickel from soils and hence any measurement of soil nickel must be interpreted with consideration of the plant species of interest.