Any working of a (not-too-wet) soil tends to condition it by loosening it for deep root penetration, incorporating air and blending any surface organic matter into the lower layers. From the crooked stick of primitive man to the modern rotary tiller is a long, long way, yet both these implements had a common purpose-to condition soil and make it a better place for plants.
Except for tillage to kill weeds, the other benefits obtained from working the soil belong under the heading of soil conditioning. All these benefits result, of course, only when the gardener digs or plows at the proper time, as determined by the mud pie test, or by the experienced eye.
Perhaps the ultimate in soil preparation and conditioning is the operation our British cousins call trenching. Trenching is sometimes confused with double digging, a somewhat less exacting and laborious operation. Trenching is an elaborate procedure during which the soil in the entire garden area is completely inverted, with the lower layer on top and the topsoil buried at least six inches beneath the surface. Underneath this "upside-down cake" the underlying subsoil is worked to an additional depth of several inches.
The operation begins with a trench dug to the depth of a standard garden spade entirely across one end of the garden (or lawn) area. The soil from this trench is wheeled to the far end of the area, where the work is to finish. Next the soil from the layer below the surface in the first trench, which may include some of the subsoil, is also dug out to another spade's depth and wheeled to form a pile alongside first. This leaves a trench, two spades deep, along one edge of the intended lawn or garden area.
Into the bottom of this trench a layer of organic matter is dumped and spread to a depth of two or three inches. Fertilizer is then sprinkled over this to help decompose the mass.
Because of the depth of this trench, it makes an excellent place to dispose of any weeds or other vegetable trash that may be around. Even if weed seeds germinate there is no chance that they will survive to reach the surface. The only thing to avoid are plants like Canada thistle and bindweed that have deep perennial roots which can grow through many inches of soil. Either fresh or composted organic matter can be used.
If the trenching is done in fall, it offers a good way to dispose of fallen leaves. Too, working during cool fall weather is much more pleasant than during the warm days of late spring and summer.