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The original was posted on /r/permaculture by /u/Transformativemike on 2024-03-28 22:08:29.
SHEET MULCH REDUCES PFAS RISK?!?!?!
This chart clearly shows that sheet mulching with cardboard and woodchips REDUCES contamination from PFAS—even under the worst-case scenario.
Wait, what? Isn’t this the chart that’s going around gardening subs to “debunk” sheet-mulching with cardboard? Aren’t people claiming this PROVES sheet-mulching causes unnecessary PFAS contamination of soil?
Yes, people are incorrectly interpreting the data that way.
This is why in credible communication, we shouldn’t pull stuff out of context from a study about chicken litter to make claims about sheet mulching, which that study did not intended to prove. It get’s really easy to make mistakes and draw wrong conclusions.
Don’t take my word for it, actually look at the numbers yourself.
It’s important to note that this study DOES NOT CLAIM TO DEMONSTRATE PFAS IN CORRUGATED CARDBOARD SHIPPING BOXES SUCH AS THOSE USED IN SHEET MULCHING. That’s a misrepresentation that’s really problematic and misleading in itself. This study tests a shredded mix that includes materials known to have much higher levels of PFAS than shipping boxes. We generally recommend avoiding those materials for sheet mulching.
But let’s use these exaggerated numbers, anyway, because they‘re a good worse-case scenario and still demonstrate that sheet mulching reduces potential PFAS contamination.
As you see, these numbers report chemicals like PFAS by the weight of the bedding material.
Many people are pointing out that the “shredded cardboard” material gets a 15, while the baseline, raw virgin wood chips, gets a little less than half that 6.3.
Now, the point of sheet-mulch is that it allows us to use a very, very thin layer of cardboard, and substitute in more home-grown materials like grass clippings, and to use a much smaller quantity of mulch overall. The sheet-mulch critics are now advocating for using a foot of chips to reliably accomplish the same goal as you can with 2-4 inches of chips in a sheet mulch.
Since we’d be using 4+ times the amount of chips, that means we’re getting nearly twice is much PFAS from our mulch, using the numbers in this study. By volume, it would contribute maybe <1-5% of the PFAS as the chips.
But the sheet mulch is even better that then, since this is by a whole-weight basis, not a volume basis. First, a layer of cardboard is going to require a much higher volume to get to the same weight as wood chips. For wood chips, Google tells me the weight per volume is 23.72 lbs per square foot. For cardboard it’s .026 lbs per square foot.
The PFAS contributed by the cardboard gets to be so small that we’d need a bunch of decimal points to even add it on. Worst case scenario using exaggerated numbers.
So, the sheet mulch roughly cuts the amount of potential PFAS in half, using these worst-case scenario numbers.