If the white roots are seen to be growing around the side of the pot and the soil is hardly to be seen, then the plant certainly needs re-potting.
This bacterial activity breaks down the soil into the necessary chemicals which the plant roots can absorb. So with the addition of humus our soils come to life and soil should be a very live substance. Uncultivated soil, whether it be woodland or meadow, is normally sufficiently supplied with humus for its needs because of the dropping of leaves, the rotting of vegetable matter such as tree trunks and grass, the degeneration of wild life of all forms and the droppings of animal manures.
Little of this natural activity takes place in the cultivated garden, for leaves are swept up as they fall, grass is cut and removed, animals are either nonexistent or so few in number that no appreciable amount of animal manure is deposited. All the emphasis in the garden is on taking from the soil and none is on putting back into the soil.
The makers claim in fact that any plants that will grow in John Innes mixtures will grow better in Levington and I am not qualified to dispute this claim. I have used it extensively and have never found trouble.
We need comparatively small amounts of soil mixtures for our pot plants and we are fortunate in having readily available from garden stores and from many chemists, ironmongers and even chain stores regular, standard supplies of John Innes Potting Composts. These are not proprietary compounds. They are named after the John Innes Horticultural Institution where the standard recipes were carefully worked out by their scientists.
But to make certain, pick up the plant and with one hand over the soil surface, the stem of the plant going between the fingers, turn it upside down and knock the edge of the pot sharply on any hard surface. The root ball should fall cleanly out of the pot into the hand.