Rubber Mulch and Heavy Metal Contamination of Soil

Dr. Curtis Swift, Colorado State University Extension


Abstract: 

Rubber mulches are used throughout the Green Industry in place of wood mulch.  Unlike wood chips or bark mulch, rubber mulch is reported to stay put in rainstorms, not attract termites or carpenter ants or rot.  Zinc, cadmium and other heavy metals that leach from rubber mulch are a concern as high levels of these elements can be toxic to plants or leach into ground water resulting in problems for aquatic life.

The objective of this study was to determine if rubber mulch applied to the high phosphorus, high pH soils in Western Colorado creates a soil environment toxic to plant growth

Methods and Material:

Rubber mulch from recycled rubber tires, shredded cedar, pallet mulch and mini-bark mulch were applied to a depth of two inches in  the same planting bed for comparison. The control plot was mulch free.

The soil was a blended planting mix.  Roses were planted and uniformly fertilized.  Hand watering was done to ensure plant establishment after which time water was accomplished by means of a bubbler placed near the base of each plant.  All plants were appropriately watered throughout the experiment.

Mulches were applied in the summer of 2003 and soil samples collected in the summers of 2006 and 2007.  Several soil samples were taken from each treatment at a depth of 0-15 cm, combined, air dried and submitted to the Soil, Water and Plant Testing Laboratory at Colorado State University in Fort Collins, Colorado for analysis. 

Results:

Total and plant available Zn, As, Cd, Pd and Ni were examined for each treatment.  The total and plant available Zinc levels in the rubber mulch treatment were 21.2 and 19.0 parts per million (ppm) as compared to a range of 5.6 to 6.8 ppm respectively for the other treatments. The total and available levels of As, Cd, Pb and Ni were similar for all treatments. Copies of soil test reports are available from the author.

Conclusion:

The use of rubber mulch in Western Colorado under bubbler irrigation is not detrimental to plant growth. The test area will be converted to overhead irrigation and reevaluated after three years to determine if a change in irrigation method affects soil levels of Zn and other heavy metals.

Placed on the Internet, August 4, 2009