As the roots of a healthy plant grow and expand, they absorb some of the nutrients and the soil itself. The soil doesn't disappear, but the ratio of roots to soil changes and that is a good thing. Professional nursery growers will tell you that the ideal ratio of roots to soil is 4 or 5 to one. Yes, a well--potted plant will be 80% roots and 20% soil. If the root ratio expands much beyond that, there will not be sufficient soil to retain nutrients and water and repotting is recommended.
Tiny root hairs that few people even notice when they examine root systems, do the vast majority of the work for plants. They are fragile and easily damaged when soil is replaced or washed away. Proper repotting means gently loosening the outer edges of the intact rootball before moving it to a pot one size larger with new soil added to the bottom and sides of the pot, but not on top.
Plants use nutrients in surprisingly minute quantities and rarely need to be replenished. Unless a plant is severely potbound, the best way to replace depleted nutrients in older soil is by using fertilizer, not repotting unnecessarily.
Plant growth rate is mostly determined by the amount of available light. Most indoor plants are in less than optimum light and their growth rate slows as they age. Thus, older, larger plants can often stay in the same pot indefintely and supplemented occasionally with fertilizer to replace depleted nutrients.
