Philodendrons, Monsteras, and all plants in the Araceae family are also known as arums or aroids. Aroids thrive in mildly acidic soil mixes with excellent drainage and good water retention. They want large air pockets in the soil for their roots to grow into and access plenty of oxygen. Coco coir and aged / fine bark provides some water retention while chunky bark and perlite create those large pockets of aeration.