no dig gardening
After cultivating once, a mulch (farmyard manure, compost or other organic matter) is spread over the newly dug surface, and left for the worms to take down into the earth naturally. Unless you want to become a slave to the spade again, you should always avoid walking on these beds. To ensure weeding is not a problem, make borders as small as possible. The feeding roots of most plants grow within the top three to four inches of the soil, so in theory this is all that needs to be cultivated.
The no dig gardener relies totally on earthworms to do all the digging. Digging the ground is completely unnatural and actually harms the soil's structure and microlife. Digging breaks down the soil tilth, increases the loss of nitrogen and the breakdown of organic matter. It also kills a great deal of the microlife critical to plant/soil nutrition interchange, and it gives man a backache.
In nature, the earth is not tilled, and fertilisers (dead plants and animals, fallen leaves, etc.) begin as mulches on the soil's surface. Under this mulch undisturbed soils develop two features central to the interactions between plants and soils. One of these features consists of the soil layers which result from the tendency of the soil to separate into mulch, topsoil, and subsoil. The other is a valuable, spongy condition called "crumb structure." Digging both mixes the soil's layers and destroys its crumb structure, interfering with the processes and organisms which have evolved to depend upon these features.
The mulch layer consists of plant and animal remains, the main reason for the soil's fertility. The topsoil consists of minerals, huge numbers of soil organisms, dissolved nutrients, organic matter brought down from the mulch, and humus. Humus results from the breakdown of organic matter and has the capacity to absorb moisture, hold nutrients, and acts as a buffer for ph extremes. It is in the topsoil that the most of the soil's fertility is stored, in the form of soil life and as nutrients held in solution by the soil's humus. The subsoil consists mostly of mineral particles, leached nutrients, and some plant roots.
Digging exposes the topsoil's extremely valuable and slowly-forming humus to the air where it can be oxidized and lost, and destroys the soil's existing crumb structure. Digging reduces the earthworm population, thereby slowing reestablishment of a new crumb structure, and can cause fine soil particles to be washed downward toward the subsoil where they collect and form a dense sedimentary layer. In short, naturally occurring soil layers and crumb structures serve vital functions which are not aided by digging. And since nature always begins to repair itself immediately once disturbed, the digging gardener has to work constantly to prevent their return.
From these facts, it is arguable that tilling is an ineffective and at best, temporary approach, to increasing soil tilth and fertility. We are better advised to cooperate with nature and use the soil's own natural method to improve fertility - a thick, rich mulch of organic materials to feed soil organisms and plants, which then work toghether to establish natural soil tilth (and as previously mentioned "digging gives you backache").