Anyone have any experience using rubber mulch around the base of their fruit trees as a slow release zinc fertilizer? I put a bunch of rubber tire mulch around my trees about ten years ago because I was lazy having to remulch with cypress. I then read recently that rubber does break down in the environment despite the claims from the Vigaro rubber mulch bag. The leachates are rendered nontoxic during the rotting process (I read the research done on children's playgrounds), but some heavy metals are released, namely zinc with some trace aluminum. I'm not worried about lead as car tires typically don't have much and lead isn't taken up into fruit tree tissues anyway. The main warnings online from garden people are about zinc toxicity, but nobody provides any evidence. I did see some evidence from a soil test on a playground that had tire mulch that was 15 years old and piled 4 inches high. The soil tested at 20ppm while toxicity usually occurs around 200ppm. The amount of zinc leaching also slows down as the tire is broken down in the environment.
Anyway, I read a study done in India that suggested using tire crumb as a low cost slow release zinc fertilizer for tropical fruit trees -- namely because high PH sandy soils are usually deficient in zinc. In fact, the leaves on my tropical fruit trees couldn't be any bigger or greener.
I'm in the process of deciding whether to remove the tire crumb mulch or keep it. What would you do?