Trees (and other plants) are almost entirely air and water. Very little soil gets incorporated, and almost all of that is trace minerals dissolved in the water.

Curious. I've heard this about trees. What about tomatoes? Cucumbers? etc? Or is this comment only referring to the "substance" of a plant and not its fruit(or similar)?

It applies to fruit and similar. All the sugar and other carbohydrates are air+water primarily. There's a bit of nitrogen from the soil, and everything else is extremely small trace elements. Carbon, Hydrogen, Oxygen, Nitrogen (CHON) are the four most common elements in all Earth life. Plants get the hydrogen and oxygen from water, and carbon (and more oxygen) from CO2 in the air. Cellulose is CHO only. Same for glucose, fructose, etc. Nitrogen is mostly found in the proteins, with a bit in DNA, RNA, and ATP. Humans are about 3% nitrogen by mass, plants are a bit less (lower protein concentration).