Zinc (Zn) may be found in soils as soluble Zn2+, adsorbed to clay and other negatively charged matter, or as insoluble minerals. Like many mineral nutrients, the availability of zinc is related to the pH of the soil. Above pH 7.5, Zn is mostly unavailable, but every time the pH is reduced by 1, the Zn availability increases five-fold. Root and microbial exudates also break up parent material and release available forms of Zn. Due to poor agricultural practices, up to 50% of global arable soil may suffer from reduced Zn levels. Soil types most likely to suffer from Zn deficiency are calcareous, sandy, saline or salt-affected soils, and waterlogged soils.
Zinc Management
This element is required for protein synthesis and other enzymatic processes.
Zinc in the Soil
Zinc in the Plant
Plants take up Zn primarily in the form of Zn2+ and complexes formed with organic acid chelates and root exudates. Zn is a cofactor for hundreds of plant enzymes and is also a structural constituent of many proteins. These enzymes and proteins play a role in photosynthesis, DNA and RNA synthesis, plant defense and stress tolerance, carbohydrate metabolism, and other important physiological processes.
Figure from Cabot, C., Martos, S., Llugany, M., Gallego, B., Tolra, R., and Poschenrieder, C. (2019). A role for zinc in plant defense against pathogens and herbivores. Frontiers in Plant Science.
Photo of zinc deficiency symptoms in citrus leaves. Image from Flickr.
Zinc Deficiency
Plants deficient in Zn suffer from reduced enzyme activity and disrupted photosynthesis, often resulting in chlorosis. Protein synthesis slows down, causing Zn to accumulate, which may attract higher rates of insect herbivory. When Zn is deficient, reactive oxygen species (ROS) are considered to be the primary factor limiting growth. Plants vary in their tolerance to Zn as well as their ability to accumulate it in their tissues. But when Zn is deficient in the soil, applying it as fertilizer or a foliar spray can improve numerous aspects of plant health, such as pathogen defense and stress tolerance.
Zinc Toxicity
Because Zn interacts with such a variety of molecules, its concentration in the cytosol is tightly regulated by various proteins. Zn becomes toxic when an overabundance of Zn2+ ions prevents other important ions from binding to their proper sites. High concentrations of Zinc in soil may stunt plant growth. In plant tissues, 300 ppm Zn is typically considered toxic.
Further Reading
Alloway, B. J. (2008). Zinc in soils and crop nutrition. International Zinc Association.
Cabot, C., Martos, S., Llugany, M., Gallego, B., Tolra, R., and Poschenrieder, C. (2019). A role for zinc in plant defense against pathogens and herbivores. Frontiers in Plant Science.
Cakmak, I., McLaughlin, M. J., and White, P. (2017). Zinc for better crop production and human health. Plant and Soil.
Noulas, C., Tziouvalekas, M., and Karyotis, T. (2018). Zinc in soils, water and food crops. Journal of Trace Elements in Medicine and Biology.
Sturikova, H., Krystofova, O., Huska, D., and Adam, V. (2018). Zinc, zinc nanoparticles and plants. Journal of Hazardous Materials.
Wang, X. P., Li, Q. Q., Pei, Z. M., and Wang, S. C. (2018). Effects of zinc oxide nanoparticles on the growth, photosynthetic traits, and antioxidative enzymes in tomato plants. Biologia Plantarium.
Yusefi-Tanha, E., Fallah, S., Rostamnejadi, A., and Pokhrel, L. R. (2020). Zinc oxide nanoparticles (ZnONPs) as a novel nanofertilizer: Influence on seed yield and antioxidant defense system in soil grown soybean (Glycine max cv. Kowsar). Science of the Total Environment.
