In an effort to find materials to supplement pine bark media, some growers have considered using ground rubber from recycled tires. Ground rubber is free of most disease pathogens, provides good drainage, resists decomposition and is both inexpensive and widely available.
Tire manufacturing involves addition of zinc oxide to strengthen the rubber.
Ground rubber contains 1.55 percent zinc. Under acid conditions in the soil, zinc bound in the rubber becomes available for plant uptake. Although zinc is essential for plant growth, most landscape soils and media used for container-grown nursery stock already contain adequate levels. Therefore, addition of zinc from any source may cause more problems than it resolves. Ground rubber, as either a mulch or a media amendment, increases the potential of zinc toxicity, especially when coupled with application of micronutrients in the fertilizer.