The doubts about the technique seems warranted. On the other hand there is a simple reason why it might be true : if large herds destroyed the herbs on which these herds depend on, they would die off. So from an evolutionary point of view, it makes a lot of sense for large herds to fertilize soil. They NEED grass to thrive.
In particular, this Descartes quote
"I should like you to consider that these functions (including passion, memory, and imagination) follow from the mere arrangement of the machine’s organs every bit as naturally as the movements of a clock or other automaton follow from the arrangement of its counter-weights and wheels."