The soil is full of microscopic organisms which have a symbiotic relationship with your plants. They help plants absorb nutrients and extend the plants' root systems while in turn feeding on sugars produced by the plants. A recent article by Osteopath Dr. Mercola on his Natural Health website explains recent discoveries about this relationship, which includes many other benefits to your plants.
While the article also leads to a pitch for purchasing an inoculant, you can cultivate these beneficial organisms which occur naturally in the soil by applying compost, compost tea, and worm castings, all of which have their own microscopic populations and inoculate your soil with new species of fungi and bacteria. A substantial layer of mulch also nurtures these essential ingredients for soil and plant health.
Tilling the soil disturbs and substantially destroys the colonies of fungi and other microscopic organisms flourishing in your soil. After tilling, a new population of fungi must form over time, retarding the nourishment and growth of the plant until equilibrium is restored. For more about the miracle of micorrhiza and related topics view this series of articles in The Furrow magazine published by John Deere.