Tilling simply means turning over the soil. This can be done with a spade or a fork. But this is not as simple a matter as it sounds. There are many reasons not to till your soil at all. Tilling the soil has several possible disadvantages. For example, tilling will expose dormant weed seeds which will then germinate. Also, tilling will disturb and perhaps destroy the earthworms and other organisms that naturally till and fertilize the soil. Finally, tilling will tend to destroy the humus in the soil by exposing it to light.

That said, there are some situations in which tilling is helpful. Most importantly, soil that has become compacted by foot traffic or machinery should definitely be tilled. Also, if your soil is poor then adding amendments (compost and/or fertilizer) by digging them into the soil will be beneficial. 

The new thinking about tilling is: no tilling. In a 2014 book, The Soil Will Save Us, Kristin Ohlson documents how soil scientists are experimenting with cover crops, composting, no-till techniques.

Courtesy of NYBG Plant Information Service