A method of producing chemically stable foamed glass for use as a plant growth medium, comprising the steps of: a) combining a foaming agent, a pH control agent, and particulate waste glass to define an admixture having from about 1 weight percent to about 5 weight percent foaming agent and from about 1 weight percent to about 3 weight percent pH control agent with the remainder being particulate waste glass; b) drying the admixture at temperatures between about 204 degrees Celsius (400 degrees Fahrenheit) to about 232 degrees Celsius (450 degrees Fahrenheit) and then arraying the resulting dry mixture into rows of powder mixture for firing and foaming; c) after drying, sintering the admixture at temperatures between about 704 degrees Celsius (1300 degrees Fahrenheit) and about 816 degrees Celsius (1500 degrees Fahrenheit); d) foaming the admixture at temperatures ranging from between about 788 degrees Celsius (1450 degrees Fahrenheit) and about 871 degrees Celsius (1600 degrees Fahrenheit) to yield a soft foamed glass body; e) curing the soft foamed glass body at temperatures between about 849 degrees Celsius (1560 degrees Fahrenheit) and about 860 degrees Celsius (1580 degrees Fahrenheit); f) cooling the soft foamed glass body at temperatures between about 760 degrees Celsius (1400 degrees Fahrenheit) and about 843 degrees Celsius (1550 degrees Fahrenheit); and g) immediately after f), quenching the softened foamed glass body with flowing air at room temperature to yield a foamed glass substrate; wherein the foamed glass substrate includes at least about 65 volume percent interconnected pores; wherein the interconnected pores have diameters between about 0.2 mm and about 2 mm; wherein the interconnected pore walls have a crazed microstructure; and wherein the interconnected pores include a pH buffering agent available for aqueous dissolution; wherein the dry mixture is in the form of a powder, and wherein the powder is supported by a fiberglass mesh fleece to prevent fines from spilling as the powder is moved in a continuous process via conveyor through a tunnel kiln, with the fleece being burned away as the powder sinters. a) combining a foaming agent, a pH control agent, and particulate waste glass to define an admixture having from about 1 weight percent to about 5 weight percent foaming agent and from about 1 weight percent to about 3 weight percent pH control agent with the remainder being particulate waste glass; b) drying the admixture at temperatures between about 204 degrees Celsius (400 degrees Fahrenheit) to about 232 degrees Celsius (450 degrees Fahrenheit) and then arraying the resulting dry mixture into rows of powder mixture for firing and foaming; c) after drying, sintering the admixture at temperatures between about 704 degrees Celsius (1300 degrees Fahrenheit) and about 816 degrees Celsius (1500 degrees Fahrenheit); d) foaming the admixture at temperatures ranging from between about 788 degrees Celsius (1450 degrees Fahrenheit) and about 871 degrees Celsius (1600 degrees Fahrenheit) to yield a soft foamed glass body; e) curing the soft foamed glass body at temperatures between about 849 degrees Celsius (1560 degrees Fahrenheit) and about 860 degrees Celsius (1580 degrees Fahrenheit); f) cooling the soft foamed glass body at temperatures between about 760 degrees Celsius (1400 degrees Fahrenheit) and about 843 degrees Celsius (1550 degrees Fahrenheit); and g) immediately after f), quenching the softened foamed glass body with flowing air at room temperature to yield a foamed glass substrate;