Repeated foliar zinc applications enhance shoot biomass, zinc accumulation, and mineral composition of sorghum across contrasting soil-moisture regimes in a calcareous soil

Water deficit and low zinc (Zn) availability frequently co-occur in calcareous soils, constraining sorghum growth and tissue mineral status. This greenhouse study evaluated whether the timing and repetition of foliar Zn application improved aboveground biomass, whole-shoot Zn accumulation, and mineral composition of grain sorghum (Sorghum bicolor (L.) Moench) cv. Tagamdyk-2017) under contrasting soil-moisture regimes. A completely randomized 3 × 8 factorial experiment with three replicates combined soil moisture maintained continuously from sowing to harvest at 100% (T1), 75% (T2), or 50% (T3) of field capacity with eight foliar schedules: a water-sprayed control (D0); single applications at the 4–5-leaf (D1), panicle-initiation (D2), or flowering-initiation (D3) stage; three two-stage combinations; and a three-stage D1+D2+D3 program. After 105 d, soil moisture and foliar Zn schedule significantly affected fresh and dry biomass (P < 0.01), whereas their interaction was nonsignificant. Mean dry biomass declined from 34.3 ± 0.5 g pot⁻¹ at T1 to 27.9 ± 0.4 and 20.4 ± 0.3 g pot⁻¹ at T2 and T3, respectively. Across moisture regimes, D1+D2+D3 produced the highest mean fresh biomass (133.0 ± 3.2 g pot⁻¹), dry biomass (29.9 ± 0.8 g pot⁻¹), shoot Zn concentration (34.0 ± 0.8 mg kg⁻¹), and Zn uptake (1.04 ± 0.03 mg pot⁻¹), compared with 114.2 ± 2.5 g pot⁻¹, 25.5 ± 0.6 g pot⁻¹, 14.2 ± 0.3 mg kg⁻¹, and 0.37 ± 0.01 mg pot⁻¹, respectively, in D0. The soil-moisture × foliar-Zn interaction was significant for shoot Zn concentration (P < 0.05) and Zn uptake (P < 0.01). Repeated Zn application was also associated with higher shoot N, P, K, Fe, Cu, and Mn concentrations but did not eliminate the adverse effects of severe soil-moisture limitation. Under the conditions of this pot experiment, repeated foliar Zn application improved whole-shoot Zn status and mineral composition, while soil moisture remained the dominant determinant of biomass production.