A spirit root didn't just act in a vacuum. It was affected by one's physique and constitution, as well as their cultivation techniques.
Since Alex's cultivation technique was the Five Yang Divine Path, the current one being the strongest variant of it in the Divine grade, every test he performed henceforth would be polluted with Yang aura entirely.
It would be a fair assessment of its effect on his own body, but it would certainly make it harder to use the same information and apply it to help the others.
Since he wasn't doing it just for himself, Alex decided to change his practice and rely on a neutral cultivation method. As such, Alex had made sure to find a good one before he started his tests.
His mother had been given a particularly strong but neutral cultivation method, which he had borrowed from her for this particular test, which was also what he had used for the baselines.
