Zinc is a micronutrient with low mobility within the plant, but it has numerous critical functions. The structure and functionality of many enzymes depend on the presence of Zn in the plant. It is estimated that approximately 2,800 proteins rely on Zn for their synthesis and action. Zinc is necessary for carbohydrate synthesis during photosynthesis and in the conversion of sugars into starch. It is involved in hormone metabolism by regulating auxin levels through the synthesis of the amino acid tryptophan. In seed maturation and production processes, Zn promotes pollen formation and fertility; thus, zinc deficiency has a greater effect on grain yield than on vegetative development.
Zinc also helps maintain cell membrane integrity and provides tolerance to plants against pathogens, especially soil-borne ones. Currently, zinc is perhaps the most deficient micronutrient in the world. Additionally, its deficiency can reduce crop yields by 20% without showing symptoms.
The Dynamics of Zinc in the Soil
Zinc deficiency is more commonly observed in sandy soils, while it is less frequent in clayey soils due to their adsorption and retention capacity. Of the total zinc content, only that which is in the soil solution and can be easily desorbed is available to plants, but it is also easily leached, as happens in tropical soils. Zinc is primarily absorbed in the form of Zn2+ or, under high pH conditions, as ZnOH+. The availability of Zn decreases as pH increases, particularly noticeable after pH > 7.4, due to an increase in adsorption capacity, the presence of hydrolyzed forms of Zn, and possible adsorption by calcium carbonate. For this reason, alkaline and calcareous soils tend to show zinc deficiencies more frequently.
Soil organic matter helps improve Zn availability by forming mobile organic complexes that the plant can absorb. Conversely, its availability decreases with high levels of phosphorus and copper. Other factors that have contributed to exacerbating the Zn problem include the use of increasingly pure fertilizers, experiences with higher crop yield potentials, and the exclusion of this micronutrient from fertilization plans.
