FArming purity decreasing?

Hey guys i have a quick question. I understand all of the purity stuff about farming.But i bought a 99 percent mercury based cereal.I planted it with 50 percent upkeep and i hear to use 300 plenty and 300 speed (which is supposed to raise purity faster) and it went down to 88. Then i did it again went down further. Is there like apurity cap considering the level of the crop. Im getting just basic oatmeal im not sure if that is why it is going down. Any ideas? whats he best way to raise purity? thank you for any input.