alt Hacker NewsBecause cotton underwear is arguably the right answer:
"Buy this item made out of this material, bury two of them in the ground" requires no further cutting or measuring, and most people would have to go to the store to buy a cut of cotton fabric anyway.
This particular clothing item also contains a more resistant skeleton of fibers against which to compare when the test material is heavily decomposed. This is how the "10 point decomposition scale" functions, and a single cotton square would not facilitate that.
Finally, it's novel, which entices potential participants to complete the survey.
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there's also the comparative usage: if you personally go buy a 10 pack and bury each item at equal depth in different locations in your yard then you can find the spot that decomposes cotton underwear fastest. This is a good enough proxy for soil activity.
The photos in the team's actual paper (linked in OA) are hilarious and lovely - real pride there. Citizen science projects have educational value as well as the data.