The current state of understanding of microplastics is far too inadequate for there to be any useful lab testing available to consumers.
For example TFA talks about 6PPD-q which is acutely toxic to fish, but "no one has studied the impact of 6PPD-q on human health." So we don't know if you should worry about that one. What we do know is that toxicity for aquatic animals is extremely different from toxicity in mammals. Soap, for example, is very toxic to fish.
Another example that caught my eye: in one sentence there is a claim that 3.5 trillion tire microparticles are washed into the SF Bay per year. Then a couple of sentences later, there is a claim that a car emits 1 trillion microparticles per kilometer driven. So the cumulative distance driven across everyone in the Bay Area is around 2 miles per year??
When two different estimates disagree by nine orders of magnitude, you're either doing astronomy or you're doing something very wrong.
Unfortunately the field of microplastics research is filled with work that is not just overly sensationalist, but that is very clearly wrong if you do a little napkin math. And almost without exception, the people involved are just plain bad at chemistry. I mean basic stuff like not understanding that a monomer has very different properties from the corresponding polymer. SMH.
Edit:
For soil specifically, there has been ample research showing that burying large amounts of tires in the ground is safe:
"There is no evidence that tyre bales embedded in the ground represent a real or potential source of pollution, or pose a threat that water and soil quality in the vicinity will deteriorate."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7377248/