According to the principle of mass conservation I think the amount of the soil should be reduced when plants grow. But this reduction is not typically observed. For example, some trees (big trees) grow so fast that if the soil was reducing due to the growing trees, it seems like no soil should be left around them (i.e. the soil surface had to go noticeably down). But it didn't occur. So, which matters are reduced by growing plants?
The vast majority of the mass of a plant is carbon-based which is obtained directly from the air via photosynthesis. So trees are, in a loose sense, solidified air!
And most of the mass that comes from the ground is water which, of course, is constantly being replaced when it rains (or by Charlie with her watering can).
The soil is the source of a small quantity of vital elements like magnesium (component of the chlorophyll), phosphorus (DNA), sulphur (some proteins) and more. As the plant grows, these are removed from the soil, so its mass shrinks.
As said in other answers, CO2 is pulled from the air, and H2O is constantly resupplied. Then there is Nitrogen, which is either taken from the ground, or from the air, depending on the plant.
Leaving meat eating plants aside, all the other stuff has to come from the soil, but how much is it? This is actually pretty easy to estimate yourself:
Just burn the tree/plant, and all the water will evaporate and all the carbon will be burned, and all that is left are the minerals that that plant pulled from the soil.
So if you ever had a nice campfire burning for quite a while, you get a pretty good idea that while being a fraction of the trees mass/volume, it is still quite some material.