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Comment by rini17 | Hacker News Reader

As I understood they put up to 1 gram of polystyrene per kg of growth medium. Which is easily achievable concentration when composting biomass contaminated with plastics, or near unregulated landfills.

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So, no impact on agriculture. That's what I figured.

Is 1 in 1000 an unbelievable concentration in topsoil? Microplastics in the atmosphere and in water supplies could result in an ever growing amount in the soil, and since the only way it leaves is by being washed away to contaminate someplace else, perhaps those concentrations will exist in a decade or so.

It kind of is an unbelievable concentration except in certain exceptional circumstances. A 0.1% concentration by weight for just the first 30 cm of topsoil would require about 5 tons of microplastics per hectare (or about 2 tons per acre). That's a lot of microplastics. It would be the equivalent of about 52 plastic water bottles per square meter. Unless the farm is on an extremely mismanaged landfill, I don't think that kind of concentration is likely.

Edit: I'm not saying microplastics aren't a problem, they're just not likely much of a problem specifically for agricultural yields.

Sadly, mismanaged landfills are probably more common than you think. Even if having diminished yields might not be an issue, microplastics concentrating in edible parts of crops might be.

Farms are very rarely next to landsfills, mismanaged or not.

I agree. I also have been wondering a lot lately about where does all the tires that wear away each year go? Street sweepers, the oceans? Millions of tires each year. It’s frightening to think what that does.

Rubber is a naturally occurring compound. Vulcanizing it makes it a little tougher to break down, but it will still decompose in 50-100 years.

Which then produces oils, heavy metals, and mineral toxins absorbed by plant material and aquatic systems. An additional danger before decomposition are uncontrollable toxic fires from large piles of disposed tires. Only 35% of tires get reused or recycled, as it's difficult to do so cost-effectively.

Tires are also filled with carbon black, which is fairly similar to soot/ash from fires, so I wouldn’t say it’s a completely unprecedented compound in nature, either.

Tires are made with nontrivial amounts of plastic as well - this [1] link says 24%

[1] https://www.nationalgeographic.com.au/nature/tires-the-plast...

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