Broken glass waste usually just ends up in landfills, but that may not always be the case. A new study shows that ground-up glass particles can be mixed with soil to create a plant growing medium that’s actually better than soil alone.

Although relatively large glass items can be recycled into more glass of the same color, small pieces of glass of different colors are too small and cumbersome to efficiently sort for recycling. Therefore, although there are efforts to change this fact, large quantities of these small pieces are usually simply discarded.

Since both glass and sand are primarily composed of silica, scientists at the University of Texas Rio Grande Valley wanted to see if ground glass particles could replace sand in a plant-growing medium. This would not only keep the glass out of landfills, but also reduce the need for sand from mining, which is becoming increasingly scarce.

Led by professors Julie Vanegas and Teresa Patricia Feria Arroyo, the researchers sourced glass particles from a company that crushes bottles destined for landfills and then tumbles the resulting particles to round off their sharp edges. Three sizes of these particles were mixed with commercial potting soil, in ratios ranging from 100% glass particles to 100% soil.

The scientists then began growing cilantro, pepper and jalapeño plants in pots with different growing media under otherwise identical greenhouse conditions.

First, it was found that particles about the size of coarse grains of sand were best at getting oxygen to the plant’s roots and maintaining optimal moisture levels. And importantly, because the particles had been tumbled, they did not cut off the roots growing around them.

The researchers found that plants grown in mixtures containing primarily potting soil contained higher amounts of nutrients such as nitrogen, phosphorus and potassium, which are necessary for plant growth. However, plants in mixtures containing over 50% glass particles (by weight) actually grew growth faster than those in 100% potting soil, and they also retained more water. The pH values ​​were in the same range for all mixtures.

The faster growth rate was probably due to the fact that all pots with 100% soil had developed a fungus known to inhibit plant growth. This fungus did not occur in any the pots contained a lot of glass particles – scientists are currently trying to find out why this was so.

Whatever the answer, it is hoped that the commercial use of recycled glass on agricultural soils will reduce the need for problematic fertilizers, pesticides and fungicides.

“I think it’s really important to minimize the use of chemicals that can negatively affect our health,” says chemistry student Andrea Quezada, who is presenting the team’s findings at the American Chemical Society’s fall meeting. “If we are able to reduce them and help the community by collecting recyclable materials, we can provide people with a better quality of life.”

The influence of the glass particles on factors such as yield and taste becomes apparent when the plants are harvested.

Source: American Chemical Society