Technical Abstract: The influence of essential micronutrients on the endogenous bioavailability of Ni is unknown. This study examines the linkage between Ni deficiency and endogenous foliar concentration of Ni, Zn, and Cu. It was hypothesized that expression of morphological symptoms of Ni deficiency by pecan [Carya illinoinensis (Wangenh.) K. Koch] is influenced by the ratio of either Zn, or Cu, to Ni in symptomatic foliage. The hypothesis was tested in a greenhouse study using ‘Desirable’ seedlings trees growing in an orchard soil known to cause Ni deficiency in potted trees. Amendment of the potting soil with various amounts of either Zn-sulfate or Cu-sulfate produced seedling trees possessing a variety of Zn:Ni and Cu:Ni concentration ratios, and growth/morphological symptoms exhibiting different degrees of Ni deficiency. Symptomatic trees were later foliar-treated with Ni-malate, from Alyssum sp. biomass; hence, correcting deficiency symptoms in the subsequent year’s growth. Severity of Ni deficiency was unrelated to foliar Ni concentration, but strongly linked to foliar Zn:Ni or Cu:Ni ratios. Deficiency symptoms increased sigmoidally with increasing Zn:Ni or Cu:Ni ratio, and were correctable, regardless of the Zn:Ni or Cu:Ni ratio in seedling trees by a single foliar application of Ni-malate extracted from Alyssum biomass. Soil Zn or Cu supplements did not detectably affect foliar Ni concentration. These data indicate that Ni deficiency occurring in pecan orchards is potentially caused by either Zn or Cu fertilization induced reductions in the physiological availability of Ni. A reduction in Ni’s bioavailability to physiological process might be due to either competitive inhibition or sequestration.