Why are your Philodendron leaves turning yellow?

The plants require nitrogen from the soil in order to manufacture pigments used for photosynthesis including chlorophyll. Since chlorophyll is green and requires nitrogen to be made, the lack of nitrogen causes chlorophyll and thus the color green to slowly leave the plant and allows other pigments to become exposed making the leaves turn yellow. This can be solved by adding nitrogen to the soil, i.e. giving it fertilizer.