How Zinc Behaves in Tropical Soils with Heavy Vegetable Farming

David Palenski
25th January, 2024

How Zinc Behaves in Tropical Soils with Heavy Vegetable Farming

Zinc sample

Photographer: Ben Mills
Maintaining healthy soil is crucial for food production, particularly in tropical regions where soils are often naturally nutrient-poor. Long-term use of fertilizers and lime – common practices to improve crop yields – can alter the soil’s chemistry in complex ways, impacting not only essential nutrients but also potentially harmful trace elements. Researchers from the University of Delaware, University of São Paulo, and Rutgers University[1] investigated how these changes affect zinc (Zn) in tropical soils, a micronutrient vital for both plant and human health. Zinc deficiency is a widespread problem, affecting both crop yields and human nutrition[2]. Plants need zinc for enzyme function and protein production, and humans require it for a healthy immune system and overall growth. However, zinc’s availability to plants is heavily influenced by soil conditions. Factors like soil pH, organic matter content, and the presence of other elements – particularly phosphate – can either bind zinc, making it inaccessible, or release it, potentially leading to losses. The study focused on comparing agricultural soils, which have been heavily fertilized for years, with soils under native vegetation that haven’t experienced such intensive management. The researchers wanted to understand how the conversion from natural ecosystems to farmland altered zinc’s behavior in the soil. They measured the soil’s capacity to adsorb – or hold onto – zinc, and how quickly zinc moved between different chemical forms. Adsorption is important because it influences how much zinc is available for plant uptake. The results showed that agricultural soils had a 30% higher capacity to adsorb zinc compared to native soils. However, a surprising finding was that a smaller proportion of zinc was in a mobile form in the agricultural soils (21% versus 35% in native soils). This means that while the agricultural soils could hold more zinc overall, less of it was readily available to be leached away or taken up by plants. This seemingly contradictory result is explained by changes in how zinc interacts with other soil components. Using a technique called X-ray absorption spectroscopy, the researchers determined the speciation of zinc – essentially, what chemical forms it was present in. They found that in the agricultural soils, zinc was increasingly associated with phosphate. This is significant because the formation of zinc phosphate minerals, like hopeite, can reduce zinc’s availability to plants[2]. While phosphate is an essential plant nutrient, excessive levels can tie up zinc, creating a deficiency even if the soil contains sufficient total zinc. The study also revealed that aluminosilicate minerals – common components of weathered tropical soils – were the primary “sink” for zinc, accounting for 34% of total zinc retention in both soil types. This means that zinc readily binds to these minerals, effectively removing it from circulation. However, the shift towards zinc-phosphate associations in agricultural soils represents a change in the dominant zinc retention mechanism. These findings build upon earlier research demonstrating that manure application can alter the speciation of phosphorus and associated metals in soil[3]. That study showed how manure could transform iron-phosphorus minerals into less crystalline forms, impacting phosphorus availability. Similarly, the current research highlights how long-term fertilization can shift zinc from being bound to aluminosilicates to being bound to phosphate, with potentially negative consequences for plant nutrition. Furthermore, research on remediating contaminated soils has shown that phosphate amendments can sometimes increase zinc mobility[4]. This seemingly counterintuitive effect is due to the complex interactions between phosphate, zinc, and other soil components. The current study adds to this understanding by demonstrating that phosphate association is a significant factor in zinc speciation in fertilized tropical soils. The researchers emphasize that understanding these complex interactions is crucial for developing sustainable agricultural practices. While fertilizers are essential for maintaining crop yields, their long-term effects on soil chemistry and nutrient availability need careful consideration. Managing phosphate levels and potentially incorporating strategies to enhance zinc solubility could be important for ensuring both crop productivity and nutritional quality.

AgricultureEnvironmentPlant Science

References

Main Study

1) Zinc speciation in highly weathered tropical soils affected by large scale vegetable production.

Published 22nd January, 2024

https://doi.org/10.1016/j.scitotenv.2024.170223

Related Studies

2) Soil factors associated with zinc deficiency in crops and humans.

https://doi.org/10.1007/s10653-009-9255-4

3) Long-term manure application effects on phosphorus speciation, kinetics and distribution in highly weathered agricultural soils.

https://doi.org/10.1016/j.chemosphere.2014.07.029

4) Desorption kinetic and sequential extraction of Pb and Zn in a contaminated soil amended with phosphate, lime, biochar, and biosolids.

https://doi.org/10.1007/s11356-023-30643-0


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