I heard that dead animals can be used as fertilizers and can be helpful for making a tree grow taller and more healthy.
Once I had buried a dead dog under a lemon tree, which at first did not result in more fruits. But after after some years the tree started bearing more fruit than before.
Is it sustainable to use this type of technique to make plants grow more? Can you use dead animals as fertilizers?
It's part of nature that the corpses of dead animals are recycled by other organisms. But a corpse degraded by bacteria releases many dangerous toxins, for example botulinum toxin, which can contaminate the water and be dangerous for people and other animals.
Please note what happens to the corpse of the animal in the natural environment. Firstly, most of it is eaten by scavengers. The rest is eaten by insects and their larvae, and only a small portion of it actually decays.
So if you want to dispose of a corpse in the most sustainable way, I'd consider feeding it to scavengers (they may be rare in your environment) or feed it to maggots, which would be eaten by other animals, whose fecal matter would fertilize the ground.
botulinum toxin doesn't convince me by itself that burying animals is unsustainable. For one, the question gave no information about proximity to a water supply. In general, both rock/soil and plants tend to be really good at filtering out many toxins, so it's not clear to me that a few buried animals are going to pose any noticeable health risk. Remember, that leaving carcasses out in the open can also create pathogens, but in that case, they're not buried. I may be wrong about this, but it would be nice to see supporting evidence.
Depending on the size of the animal, you can try hot composting it. This should keep any toxins out of the water supply, while also giving you a good fertilizer. The basic steps are start with lots of wood chips. You want a base layer of at least 18 inches. (Again it depends on the size of the animal, larger animals need a larger base.) Add the animal and build the pile up in a conical shape. You want at least 18 inches (more is better) of woody material all around the animal.
Depending on what you're growing and what parts you are using (for instance, I deer hunt and eat most of the deer, but I do bury the bones in the garden). Some plants need more calcium than others. In the past, people used to plant animal skulls under apple trees to provide calcium for the plant.
