Dr. Rob
To keep soil at its optimum best, you have to use some nutrients or fertilizers. Plants require 16 essential nutrients - 3 from water and 13 from the soil. That said, soil does not always have the same levels of all nutrients for proper plant growth. So you need things like carbon, nitrogen, calcium, hydrogen, phosphorus, magnesium, potassium and sulphur. But you also need to have micro organisms like insects, worms or arthropods to create and develop organic matter that is absorbed by the root of living plants. You need energy-rich sugars and carbon to grow. Go and buy some nutrients like from Advanced Nutrients or General Hydroponics - Heavy Harvest or Iguana Juice. But be careful, too much fertilizer can be dangerous for soil and microorganisms. So take care with pesticides and fertilizers but note that you do need them.
Happy Growing!