[thedoc (OP)]

Chris Martin asked the Naked Scientists:
   
If you weighed a pot of dry soil, then grew a plant in and dried it out again, would the total weight of the soil and the plant together be more than when you started, and if so, where does the extra weight come from?

What do you think?

[evan_au]

A very similar experiment was performed by van Helmont around 1650. He found that a growing willow plant put on a lot of mass, but the soil lost almost no mass. He (incorrectly) assumed that all the weight came from the watering that he had been doing for 5 years.

Stephen Hales came to the modern view that plants extract some of their matter from air.

We now know that plants absorb carbon dioxide from the air, and, powered by photosynthesis, combine this with water to produce the biomass of the plant. The small loss in the soil's dry weight comes from minerals absorbed by the plant.