Any poop can be used as fertilizer, it just needs to compost for a while. Some poop longer than others. While I've never tried it, I'd imagine you would have to let human dump break down with some other organic matter for quite a while. It's all about the diet of the animal. Take cow manure for example : Cows have a very simple diet of hay which gets broken down by multiple stomach chambers before coming out. Humans, on the other hand, eat meat, dairy, vegetables, grains, things with preservatives, flavorings, food dyes and other artificial ingredients abundantly. Combine that with the fact that our poop is basically a biological hazard left unattended and you've got an awful fertilizer not worth composting.