Soul cultivation technique is much more different than body refining technique and qi cultivation technique.
Having a powerful soul is a must need for martial cultivators and masters of supplementary paths.
However, cultivating the soul is also harder than the other two type of cultivation techniques. One needs to have a soul powerful enough to sustain the technique. But cultivating soul cultivation technique has huge merits.
For example, let's take two alchemists of the same rank and same soul strength. While refining medicinal pills, the one practicing a soul cultivation technique would have a natural advantage over the other one.
Moreover, with enough soul qi, they might be able to concoct pills that surpass the limits of their ranks. That's why soul cultivation techniques are heavily sought by the supplementary masters.
