A living thing’s soul is its capacity to engage in the activities that are characteristic of living things of its natural kind. What are those activities? Some are listed in DA II.1; others in DA II.2:
Movement and rest (in respect of place)
So anything that nourishes itself, that grows, decays, moves about (on its own, not just when moved by something else), perceives, or thinks is alive. And the capacities of a thing in virtue of which it does these things constitute its soul. The soul is what is causally responsible for the animate behavior (the life activities) of a living thing.