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Glass fertilizer beads could be a sustained nutrient delivery system

Agricultural fertilizers are critical for feeding the world's population, restoring soil fertility and sustaining crops. Excessive and inefficient use of those resources can present an environmental threat, contaminating waterways and generating greenhouse gases such as nitrous oxide. Now, researchers reporting in ACS Agricultural Science & Technology have addressed those challenges with glass fertilizer beads. The beads control nutrient release, and the researchers say they're environmentally compatible.

"The results show that glass fertilizers can be tailored to plant needs, slowly and sustainably releasing nutrients to boost productivity without harming soil quality," says Danilo Manzani, a co-author of the study.

Over time, the use of agricultural chemicals has increased. In 2020, the Food and Agriculture Organization of the United Nations estimated that global demand for fertilizers would surpass 200 million metric tonnes. Fertilizers contain nitrogen, phosphorus and lower amounts of other elements like calcium. Unfortunately, the benefits of these nutrients are lost through leaching into groundwater and emissions into the air, necessitating frequent reapplication and creating downstream environmental problems like toxic algal blooms. A potential solution could come from tiny glass beads that previous researchers used to improve plant growth. To improve the efficiency of nutrient delivery, Manzani, Eduardo Ferreira and colleagues developed a water-soluble, multicomponent glass fertilizer designed for controlled nutrient release.

The researchers synthesized glass consisting of several micro- and macronutrients, such as phosphorus, potassium and calcium. They ground the glass into small (less than 0.85 millimeters wide) and large (0.85 to 2 millimeters wide) particles. In an initial test, the particles were added to either water or a buffer solution that mimicked soil conditions. They found that each nutrient released from both sizes of glass particles and diffused into the solutions steadily over 100 hours with minor fluctuations.

They then applied a nutrient solution or different amounts of the glass beads to soil seeded with a typical lawn and fairway grass, and they compared the plants' growth in the two treatments. The nutrient solution, which was applied only once, immediately stimulated plant growth, but the effect quickly diminished. However, the single application of glass fertilizer sustained plant growth regardless of particle size, though overall growth depended on the bead dose.

Manzani, Ferreira and colleagues also examined the possible ecotoxicity of the glass fertilizer by exposing lettuce and onion seeds to the beads. Seeds exposed to glass fertilizer had roughly the same germination rate and cell health as those never exposed or those treated with soluble nutrients. The researchers say that these results indicate an efficient and sustained alternative to conventional fertilizers with lower environmental impact.