Iron (Fe) deficiency in humans, particularly in pregnant women and children, is caused by inadequate dietary Fe intake and is a global nutritional problem. Foliar fertilization is a cost-effective agronomic approach to increase Fe bioavailability in the human diet. We evaluated the effects of different Fe formulations (Fe-citrate, Fe-EDTA, FePO4, nano-Fe oxide, and humic acid (HA) with and without Fe) on growth, yield, and Fe accumulation in the edible parts of rice, soybean, and lettuce crops. Rice and soybean received multiple sprays at different growth stages, i.e., tillering, anthesis, and grain filling in rice as well as flowering and pod filling in soybean, while lettuce received a single foliar spray. In rice and soybean, the seed Fe accumulation increased proportionally as the number of foliar sprays increased; however, the grain yield did not show this relationship. Among Fe treatments, HA+Fe was identified as the best treatment in terms of improving overall plant growth, yield, and Fe accumulation in the edible parts of all three crops. We found a significant positive correlation between the shoot/stover Fe content and the grain Fe content, but HA+Fe showed an opposite trend, i.e., minimal Fe retention in shoots/stovers and maximal increases in the seed Fe contents in both crops, suggesting better Fe mobilization efficiency from shoots to developing seeds. We strongly recommend that HA with Fe can be used as a foliar Fe fertilizer to improve the growth, yield, and Fe status in different crops.
