Zinc-based agronomic bio-fortification strategies for soybean in the North Eastern Himalayas of India

Zinc (Zn) deficiency is an emerging health issue in the North-Eastern (NE) Himalayas of India. Agronomic bio-fortification of soybean offers a promising solution as it is widely consumed throughout the NE regions. Therefore, this study aimed to assess the response of soybean varieties to different Zn sources and methods in terms of Zn bio-fortification, and bio-availability. The results revealed that, the soybean variety JS-97-52 recorded greater seed (1.67-1.87 Mg ha⁻¹ and stover yields (1.77-1.90 Mg ha⁻¹; however, the JS-335 had higher Zn content (31-31.5 mg kg⁻¹, lower phytic acid (PA) levels (569 and 585 mg 100 g⁻¹), and a lower PA: Zn ratio (18.2 and 18.9). Among Zn fertilization methods, the combined soil + foliar spray of ZnSO₄·7 H₂O (SA + FA-ZS) significantly (p < 0.05) increased total chlorophyll (17%), nodule number (32%), dry weight (32%) and seed yield (21%) compared to other treatments. The foliar spray of ZnSO₄0.7 H₂O (FA-ZS) had significantly higher Zn content (24-34%), Zn uptake (52%), oil (8-9%), protein content (10-11%) and farm profitability. Moreover, the SA + FA-ZS, reduced PA content by 13-15% and the PA: Zn ratio by 29-33% as compared to control. The FA-ZS also led to a 2.8-fold increase in partial factor productivity, 4.1 to 4.2-fold increase in agronomic efficiency and 5.0-to-5.4-fold increase in crop recovery efficiency of Zn. The SA + FA-ZS improved DTPA-extractable Zn and soil microbial activity across both years. Therefore, the foliar application of ZnSO₄·7 H₂O, combined with soil application, can enhance yield, Zn biofortification, bioavailability, and soil fertility in soybean cultivation.