Gardening & purity

Can someone explain how this works to me please?
is this right?:
1. The pot purity seems to combine the purity of the clay, granite, and lime in equal proportions.
2. The purity of the output will be some combination of input plant, pot, humus, and water+ a random factor
If anyone knows how the purities are combined in 2., and what the random factor is, I'd really appreciate it. Is it possible to grind plant purity beyond the purity of the pot/humus/water (like in haven you can grind farm quality beyond soil quality)?
is this right?:
1. The pot purity seems to combine the purity of the clay, granite, and lime in equal proportions.
2. The purity of the output will be some combination of input plant, pot, humus, and water+ a random factor
If anyone knows how the purities are combined in 2., and what the random factor is, I'd really appreciate it. Is it possible to grind plant purity beyond the purity of the pot/humus/water (like in haven you can grind farm quality beyond soil quality)?