Optimization of Soil and Foliar Zinc and Iron Application for Enhanced Growth and Yield of Mungbean under Agronomic Biofortification in an Arid Region

In many parts of the world, people rely heavily on a diet of grains and pulses, which are excellent sources of energy and protein. However, these foods often lack sufficient amounts of essential trace minerals like iron and zinc. When the soil itself is poor in these nutrients, the plants growing in it cannot absorb enough to make their seeds nutritious, leading to a condition often called hidden hunger in the people who eat them. This is particularly true in arid regions where the soil is alkaline, meaning it has a high pH level that locks up these vital minerals, making them unavailable to plant roots. To solve this, scientists have developed a strategy called agronomic biofortification. Instead of waiting for the soil to naturally provide these nutrients, farmers apply them directly to the crop. This can be done by adding minerals to the soil or by spraying a nutrient solution directly onto the leaves, allowing the plant to absorb them quickly. The goal is to boost both the amount of food a plant produces and the nutritional quality of its seeds.

Researchers in the arid western plains of Rajasthan, India, set out to find the most effective way to apply these nutrients to mungbean, a short-duration pulse crop often called the "poor man's meat" due to its high protein content. They conducted a field experiment over two growing seasons to test how different methods of adding iron and zinc affected the growth and yield of the crop. The team worked with two distinct varieties of mungbean, one known for its shorter growing period and another that takes slightly longer to mature. They divided their field into sections to test a combination of treatments. For iron, they compared doing nothing against applying a specific amount of iron sulfate to the soil, followed by spraying the leaves with a dilute iron solution at two critical times: when the flowers first appeared and when the pods began to form. For zinc, they tested a range of approaches, including doing nothing, adding zinc sulfate to the soil, and using a special bacterial inoculant that helps dissolve zinc in the soil, all combined with foliar sprays.

The results showed that the way nutrients were applied mattered just as much as the amount used. The researchers found that the variety of mungbean known as GM-4 consistently outperformed the other variety, GM-7, growing taller and producing more biomass. However, the most significant gains came from the specific combination of treatments. The plants that received a moderate amount of iron applied to the soil, followed by two sprays on the leaves, grew significantly taller and developed more branches than those that received no iron or a higher dose without the sprays. Similarly, for zinc, the most successful approach involved treating the seeds with a zinc-solubilizing bacteria, applying a moderate amount of zinc sulfate to the soil, and spraying the leaves with a zinc solution. This integrated method produced the tallest plants, the most dry matter, and the highest number of branches.

When it came to the final harvest, the benefits of these treatments were clear. The plots that received the combined zinc and iron treatments yielded significantly more seed and straw than the untreated plots. The highest seed yield recorded was over 1,000 kilograms per hectare, a substantial increase compared to the control groups. The study demonstrated that simply adding more fertilizer was not the answer; rather, the timing and method of application were crucial. The combination of soil application and foliar spraying allowed the plants to access nutrients when they needed them most, overcoming the limitations of the alkaline soil. The research suggests that for farmers in similar dry, alkaline regions, using a specific variety of mungbean alongside a tailored strategy of applying iron and zinc can significantly improve both the quantity of the harvest and the nutritional value of the seeds, offering a practical path toward better food security.