This paper deals with soil genesis and distribution of Ni and Cr in soils formed along a toposequence of mainly ultramafic rocks in an aridic soil moisture regime in Northeastern Iran. Three pedons in the summit (Xeric Haplogypsids), backslope (Lithic Xeric Torriorthents), and footslope (Lithic Calcigypsids) of a toposequence were described and sampled. Hornblende was the main mineral of the parent materials, while chlorite and albite were the associated minerals. The calcic and gypsic horizons were the diagnostic horizons in shallow solums of the studied pedons. Despite the low amount of Ca in the parent materials, the Ca/Mg ratio in the soils reached to 33, owing to the high amount of carbonates and gypsum with aeolian origin. Released iron from the ferromagnesian minerals has created a brown color in the soil groundmass. The amount of Fed (CBD-extractable Fe) in the studied soils (3.5 to 6.8 g kg–1) was significantly lower than soils formed in humid areas. Iron oxides features were as nodule in the groundmass, coating, and hypocoating in carbonate nodules and voids, as well as coating along microcracks of coarse fragments. The total amount of Ni and Cr in the parent materials was 106–1485 and 244–2653 mg kg–1, respectively. The aqua-regia extractable concentrations of Ni and Cr in the soil layers varied from 52.6 to 191.8 and from 35.2 to 127 mg kg–1, respectively. Fed, Ni, and Cr concentrations and the presence of chlorite reflect the nature of ultramafic rocks. On the other hand, high amount of gypsum, carbonates, and silt contents, presence of mica and palygorskite in the clay fraction, and high values of Ca/Mg ratio in the studied soils were the evidence of aeolian addition.

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