The right plants catches nitrogen from the air. Fungus and microorganisms make minerals, like potassium, available from sand and rocks. This works great until

The right plants catches nitrogen from the air. Fungus and microorganisms make minerals, like potassium, available from sand and rocks. This works great until someone destroys the soil by killing all microorganisms with pesticides or fertilisers. Forests are very good at producing a lot of biomass without help fertilisers, even when a lot of trees are "harvested".