There should be no land to put pebbles under the plant pots.
There are several reasons why you shouldn’t put pebbles in the bottom of flower pots, including the following.
The BBC says it’s wrong to put pebbles under plant pots. It is better to pour a layer of sand underground, which will allow water to penetrate into it, and then, if necessary, be absorbed by the roots.
Adding gravel for drainage reduces the amount of soil available to plant roots. Essentially, this means that you are narrowing the pot and these conditions are unfavorable for the plant. University of California gardener Sue McDavid says the plants love good drainage, especially in pots. If water stagnates around plant roots for an extended period of time, root rot will damage and possibly kill the plant.
Instead of running water straight through the soil, over gravel or other material, and through drainage holes, the water completely saturates the soil so there is no air space left. This can take a long time and temporarily the plant roots will be deprived of oxygen.
A layer of gravel or pot bedding at the bottom of the container adds unnecessary weight because it is heavier than most light clay soils and difficult to move.