Comparative study of the effectiveness of nano-sized iron-containing particles as a foliar top-dressing of peanut in rainy conditions

The high loss of foliar-applied fertilizers in the wet season and other extreme weather conditions reduces fertilizer use efficiency remarkably low. Emerging nano-iron materials, owing to their highly desirable properties such as exceptionally small size, particle surface and interface attributes, strong adhesion and high iron ratio, could overcome the limitations of traditional fertilizers. However, their anti-leaching property, leaching resistance mechanism, iron nutrition enrichment capacity and cost effectiveness remain unclear. In this study, four nano-iron materials (Fe nanoparticles (Fe-NPs), Fe3O4-NPs, α-Fe2O3-NPs and γ-Fe2O3-NPs) were compared with traditional iron fertilizers (FeSO4 and Fe-EDTA). The results showed that the efficiency of traditional iron fertilizers decreased greatly under simulated rainfall conditions due to their poor leaching resistance. In contrast, nano-iron fertilizers performed exceptionally well under simulated rainfall conditions with significant increase in leaf iron concentration, photosynthesis and growth of peanut. This was mainly attributed to their decreased contact angle, increased wetting capacity and increased adhesion on plant surface, which collectively decreased fertilizer leaching remarkably. The adhesion capacity of Fe3O4-NPs (53.5 mN/m), was nearly three times that of traditional iron fertilizer (18.3 mN/m). Therefore, only a small amount of Fe3O4-NPs (4 mg/plant) was needed relative to traditional iron fertilizers (FeSO4, 59 mg/plant) to achieve comparable high leaf Fe concentration (1076.96 μg/g) under rainfall conditions. Cost evaluation suggests that the cost-competitiveness of Fe3O4-NPs will become comparable to that of FeSO4 when its price comes down from the current cost of $139/kg to $36/kg, the cost of FeSO4, which is likely considering the pace of nano-material technology development. Together, these results demonstrate the remarkable potential of nano-iron fertilizers in mitigating the escalating fertilizer cost and improving the environmental outcomes of peanut, as well as potentially other crops grown in wet season.