Trees Are Made Mostly from Air, Not Soil
Through photosynthesis, trees get most of their mass from carbon dioxide in the air, not from nutrients in the soil. The wood is primarily made of carbon captured from the atmosphere.
About this fact
Most people assume trees get their mass from soil nutrients, but this is largely incorrect. Trees obtain the majority of their mass from carbon dioxide in the air through photosynthesis. During photosynthesis, trees absorb CO2 from the atmosphere and combine it with water to create glucose and cellulose - the main components of wood. The carbon atoms from atmospheric CO2 become the carbon backbone of all organic molecules in the tree. While trees do absorb some minerals and nutrients from soil (like nitrogen, phosphorus, and potassium), these represent only a small fraction of the tree's total mass. The vast majority - about 95% - comes from air and water. This is why a tree can grow to massive size without significantly depleting the soil around it. When trees die and decay, they release most of their carbon back into the atmosphere, completing the carbon cycle.