Spiritual root are in a sense a magnet that attracts and controls Qi.
The better your spiritual root is, the better you are at controlling Qi and absorbing Qi; It also decides how far you can go in the path of immortality.
Mortal (1-7) -These are the lowest and most common spiritual roots. They go from 1 to 7 and each stage higher is 1.5X faster than the Previous.
Earth (1-7) -1st stage Earth is 2x as fast Mortal Stage 7 and every stage after is 1.3x as fast as the previous stage.
Sky (1-7) -pretty much the same as the previous one except it is 1.4x every stage instead of 1.3x.
Heaven (1-7) - same except 1.5x every stage.