Aroids are a genus of semi-epiphytic plants. In nature these plants grow up and on trees, rock formations and decaying organic matter that has fallen to the forest floor.
In the humid, damp forests and jungles where most Aroids grow nutrients and water are provided through water running over the roots that grow widely across the bark of a tree, or into small, shallow crotches and holes which fill with water and decaying plant matter.
Many common issues with growing Philodendron or other Aroids in the home stem from a poor quality soil that retains too much water, causing these specialised epiphytic roots to rot or grow weakly.
Making a good quality Aroid substrate
There are many different ways to create a good Aroid mix. What matters is that water drains quickly through the substrate, it dries relatively quickly even in winter or dormant periods, and that it doesn't compact and break down too quickly.
1/3 good quality potting mix. This mix has some perlite and larger bits of organic material already mixed in, but that's not really necessary.
1/3 perlite. Perlite is puffed volcanic rock, it's light and won't break down and helps keep the mix from compacting over time.
1/5 orchid bark. The orchid bark helps create 'pockets' of matter and prevents the perlite floating to the top and soil being washed out of the bottom of the pot.
1/5 horticultural grit or sand. This is a fine grit which won't break down over time. Perlite is lighter than water and floats, whereas the grit is heavier. Having both ensures that with time the soil won't settle at the top or bottom of the pot and compact.
The finished mix
The finished mix. Your mix should look similar to this. It's light and airy with plenty of pockets for the plants roots to grow between. Aroids love to attach their roots to the orchid bark like they would a tree in nature.