One of the fundamental principles of organic gardening is to build your soil. Using a garden tiller to work your soil may be a good way to avoid back-aches, but using it too often, or under the wrong conditions (too wet or too dry), can actually break your soil down rather than help build it up.
In the 18th century, a British agriculturalist named Jethro Tull (famed for improving numerous agricultural implements, most notably the seed drill), was largely responsible for spreading the notion that the roots of plants physically ate the soil particles they were planted in.
It was Mr. Tull's belief that in order to free up nutrients in the soil, the soil must first be pulverized (broken up) by means of the plow. Today, soil and plant scientists know differently. Unfortunately, Mr. Tull's idea caught on and eventually became the norm. Even today, many farmers and gardeners remain under the false impression that they have to dig up or turn over the soil before they can plant.
One of the best ways to ensure success in gardening is to take your cues from nature. When you think about it, nature uses a "no-till" method of building and feeding the soil. Rather than radically disturbing the soil, it is top-dressed by layers of leaves and other debris that slowly accumulate on the surface of the soil.
By adding these organic material in layers, the underlying soil surface remains spongy, which makes it easy for the young roots of newly planted seedlings to work their way through the soil. In addition, a complex world of microorganisms that live beneath the soil break down organic matter, which slowly releases nutrients to the plants and contributes to a healthy soil structure. This is how soil is formed in nature.
Soil contains a complex community of living (and non-living) organisms. Collectively, these organisms are referred to as the "microherd." For example, earthworms are one component of a microherd. Insects, bacteria, fungi, nematodes, and hundreds of other microscopic organisms are also part of the herd.
All members of the herd have a specialized job--either to break down organic matter in the soil or to eat other soil organisms--and excrete the nutrients plants need, in a form that plant roots can take up. As a whole, the herd works to convert organic nutrients from one form into another that plants can use, in the same way that the food we eat is changed by our digestive enzymes into nutrients our bodies can use.
As a gardener, think of your "herd" as your livestock. Your job is to keep your herd supplied with enough raw food (by adding organic materials to the soil) to keep them healthy and happy so they can feed your plants.
The microherd in your soil lives primarily in the top few inches, the same top inches of soil broken up during tilling. As the blades on the tiller slice through the soil, they break up beneficial fungi, kill worms, and rip and crush other mircoorganisms, some of which probably took years to become established in your soil. Members of the microherd have a symbiotic relationship, so once some of these microorganisms die other members will leave too. Their departure impacts the nutrition your plants receive, as well as the structure of the soil.
The spongy texture is lost, and watering and compaction start to become a problem. Eventually, pests and diseases find it easier to move in and establish themselves. The soil stops functioning as a factory of nutrients, which forces you to add fertilizers. Unfortunately, fertilizers (as well as pesticides and herbicides) irritate microorganisms like worms, and will prevent them from becoming re-established so the chemical cycle continues to repeat.
Using methods like Patricia Lanza's, "Lasagna Gardening" or Lee Reich's, "Weedless Gardening," or simply by gardening in raised beds, it's possible (and tempting) to turn away from tilling (and digging) completely. But if garden size or other physical limitations make it necessary to use a tiller, try to keep the following in tips in mind:
When preparing a new garden bed or incorporating a new layer of organic matter, try to accomplish the task with just a single pass of the tiller. Most garden soils contain weed seeds that lay dormant until the soil is disturbed and the seeds become exposed to light. Less tilling means less weeds. Avoid using tillers for routine soil cultivation, especially when conditions are too wet or too dry. Never till your soil without adding more organic matter to it at the same time.