[JennyG (OP)]

Marcus asks,

How can oak trees and others grow so huge without making a great hole in the earth? Where does their mass come from, if not from the dirt.

What do you think?

[evan_au]

An early scientist carefully measured the mass of a plant and the mass of the soil in which it grew. He was surprised to see that the total mass actually increased over time.

Most of the mass of a tree is water and cellulose:
- Cellulose consists of long chains of carbon, oxygen and hydrogen, and is built from Carbon Dioxide, Water and Sunlight.
- Carbon Dioxide comes from the air
- Water comes from the soil, rain and mist. The water moves through the soil, and is not the solid mass of the soil
- The water has a small amount of salt, which does come from the soil

There are smaller contributors to the dry mass of the tree: proteins & enzymes
- Proteins need Nitrogen in bio-accessible form (unfortunately, trees cannot use it directly from the atmosphere).
- Nitrogen comes from the decay of other plant and animal matter (helped by fungi and bacteria), and also by "fixing" nitrogen from the atmosphere (done by specialist bacteria)
- proteins need smaller quantities of sulphur, and enzymes also need more exotic elements like copper (for chlorophyll)
- If there is not enough rotting vegetation, these elements must come from minerals in the soil

So the loss of mass in the soil is a very small percentage of the mass of the tree.
- If the soil becomes depleted in minerals, they must be replenished from fertiliser

[chiralSPO]

water and carbon dioxide from the air are the raw materials that account for most of the mass of a tree.

[chiralSPO]

photosynthesis: https://en.wikipedia.org/wiki/Photosynthesis

[Petrochemicals]

Depending on what you mean by 'eat' and 'soil', peat can be put onto borders almost continually without adding much to them, other than the nutrients. Peat is almost entirely constituting of organic matter. A forest will actually increace this ammount of organic matter, leading to fertile soil, which someone will cut down to utilise for a short time, then the land will be very substantially less productive. Seen in many places around the world including the UK.