Say your root base travels 10 inches down and there are nutrients 20 inches down that have not yet been absorbed, when it rains and the water soaks down to the 20" mark the nutrients will move towards the top of the soil. I am a botanist but i grow MJ for fun because of the many unique characteristics of the plant. It is the same concept as folding a paper towel and dipping 1 inch into a cup of water and watching it climb.