Waste tires to fertilizer: Carbon black zinc fertilizer for maize in calcareous soils
Hopkins, B. G., E. Bremer, K. Greer, C. J. Seely, R. L. Haymore and E. Jones. 2026.
Abstract
Waste tires accumulate in massive quantities worldwide, posing significant environmental challenges. Pyrolysis under high vacuum offers a potential disposal solution,
producing recovered carbon black (CB) enriched in zinc (Zn) from zinc oxide used
in tire manufacture. Our objective was to evaluate recovered CB from pyrolysis of
mining tires (CB4000) as a Zn fertilizer for maize (Zea mays L.) in calcareous soil
with low plant-available Zn. This material contained 90 g Zn kg-1 and increased Zn
bioavailability as measured by ion-exchange membranes (plant root simulator [PRS]
probes). In 2 years of field and glasshouse trials, CB4000 enhanced Zn uptake and
grain and silage yields. The effectiveness of CB4000 to increase stem width, yield
and Zn uptake of maize was equal to traditional zinc sulfate (ZnSO4) fertilizer when
applied at double the rate of Zn. Concentrations of toxic organic compounds were
below detection or well below regulatory limits for use as a fertilizer and maize tissue concentrations of heavy metals of concern (lead [Pb], chromium [Cr], and nickel
[Ni]) were unaffected. These results demonstrate that recovered CB from pyrolyzed
mining tires can function as a safe and effective Zn fertilizer, while also offering a
sustainable recycling pathway for end-of-life tires.
