Assessment of nickel's sufficiency critical levels in cultivated soils, employing commonly used calibration techniques

Although Ni has been officially recognized as an essential micronutrient for all higher plants since 2004, research on assessing its sufficiency critical levels with different soil tests is missing in the literature. The objective of the study was to determine Ni critical levels in unpolluted cultivated soils utilizing four methods, employing three commonly used calibration techniques. Ten soils with different physical-chemical properties and low Ni content were treated with Ni at rates of 1, 2, 4, and 8 mg kg(-1). After equilibration for one month, the soils were analyzed for extractable Ni by four methods, namely DTPA, AB-DTPA, AAAc-EDTA, and Mehlich-3. Response to soil-applied Ni was assessed by a greenhouse pot experiment, with the untreated and Ni-treated soils in three replications, using ryegrass (Lolium perenne L.). The aboveground biomass of ryegrass was harvested two months after sowing, dry weight of biomass was measured and relative biomass yield was calculated. Nickel's critical levels were determined employing the: (a) graphical technique of Brown and co-workers, (b) Mitscherlich-Bray equation, and (c) Cate and Nelson graphical technique. According to the first technique, Ni critical levels were approximate to 2 mg kg(-1) for the DTPA and AB-DTPA methods, and 6.0 and 5.3 mg kg(-1) for the AAAc-EDTA and Mehlich-3 methods, respectively. Similar levels were obtained by the Mitscherlich-Bray equation. However, the critical levels assessed by the Cate and Nelson technique were lower and ranged from 0.5 to 1.3mg kg(-1) for all four methods. Conclusively, Ni sufficiency critical levels for all four methods are expected to range at levels of a few mg Ni kg(-1) of soil. As far as the three calibration techniques are concerned, a distinct boundary between Ni response and non-response was accomplished by none. However, the fact that 60-74% of the soils were correctly separated into responsive and non-responsive to added Ni by the graphical technique of Brown and co-workers suggests that this is the most suitable technique.