|“|| Souls manifest in 3 forms:|
CURRENCY, HEALTH & WRATH.
— Ingame description
Souls are gathered after War defeats an enemy, opens a Chest or destroys a piece of the environment. There are three types of souls in Darksiders, all of them gather automatically without the need of touching them nor interacting in any other means. As soon as they appear they will target War to be harvested, even at the great distance. In Darksiders III they are known as Soul Lurchers.
|“|| The souls of the dead will be your currency. Some enemies are more easily bought than killed.|
— The Charred Council, to War
The blue CURRENCY souls are the most basic kind and are used as currency for purchasing items from the Demon Vulgrim. They are gained from killing any enemy and often found in chests. The other way to gain them very easily is to destroy environment elements like cars, lamps etc. The interesting thing is that for example killing a Wicked earns two souls and destroying a car might earn three. This might prove useful at the very beginning.
If one wants to harvest much of these Souls quickly using The Harvester (Console only) might be a good idea. There are also Enhancements that augment Soul harvest: Reaper and Death's Blessing. Using Ravager at the early game might also prove useful.
The green HEALTH souls are much less common and replenish War's Health. They are gained for killing an extra strong enemy like a Fleshburster for instance and also found in chests. The other way to gain them is to kill lesser enemies like Wicked or Broodling with an action button.
Apart from finding a chest, probably the most efficient way to harvest those and quickly replenish War's Health is to hunt and kill Crows as they give considerable amount of Health Souls. Probably the best place to do so is Anvil's Ford's main square, from which one goes directly to Ulthane, followed by The Crossroads' Plaza as a second best.
The yellow WRATH souls are the least common and are also gained for killing some stronger enemies, they replenish War's Wrath. Like other Souls, they can also be found in chests.