Only lasts 13 weeks in soil. It presumably lasts a whole lot longer when you don't bury it in the ground.

I agree. https://en.wikipedia.org/wiki/Galalith is made of casein and formaldehyde. It was popular 100 years ago and there are plenty of pieces still around. I searched formaldehyde in the article but I din't find it, I'm not sure if they hide it or they are using a very different method.