Why dig carefully?
Soil, to be fertile, must be porous, moist and hold nutrients in a form that can be absorbed by plants. Careful digging can create and/or improve these conditions by:
breaking up soil to improve drainage where the ground is too wet improving the texture of the soil by exposing it to sun, rain and frost improving its fertility by mixing in while digging, soil conditioners like peat, compost, leaf mold and organic and inorganic fertilizers in the Far North, exposing the soil to warmer air temperatures and thus reducing the level of the permafrost below.
When to dig
To allow for settling, site should be dug at least 2 weeks before planting; longer is better and there is a distinct advantage in digging new beds a whole season before they will be needed - in fall for spring because frost action is an excellent conditioner of soil; in spring for fall because a crop of grass can be grown as green manure and turned in before permanently planting.
How to dig
A large bed: trenching is the most effective method and well worth the time it takes; divide space into sections 15-18 inches wide, take one spade's depth from the top of the first section and pile it beside the last section; in the first hole, turn over another spade's depth, at the same time mixing fertilizer and conditioners into the soil; next, shovel the top of the second section onto the top of the first, again mixing fertilizers and conditioners into the soil as you work; proceed similarly to the end of the bed, one section at a time and at the end add the topsoil from the first section to the top of the last (see drawing).
One planting hole: dig out soil to required depth, reserving the top soil to put on top again when finished; mix fertilizers and conditioner with the soil you have dug out and replace it in the bottom of the hole, add a layer of unfertil¬ized soil that will be in immediate contact with the roots of the plant, set the plant in place, add soil tamping it gently around the roots to eliminate large air spaces, add top soil till the hole is just slightly below the level of the sur¬rounding ground, water deeply; when plant is growing well, add more topsoil till the hole is level with the surrounding ground, or a little above it.
Why improve soil?
Few garden soils are of perfect tilth (20% sand, 40% clay, 40% humus); most need additives to be fertile.
Clay soils are heavy and hard to work; they hold water (an asset in dry areas but not in wet), are slow to dry out and warm up in spring, and they form a crust in hot summer sun making it difficult for rain and air to enter. Sandy soils are easier to work and they warm earlier than clay, but water runs through them, they dry out too quickly and nutrients leach out.
HOW AND WHEN TO IMPROVE SOILS
Clay: by digging in each year, to a depth of at least 1 foot, quantities of humus, which may be compost made from your own leaves and garden refuse and/or vegetable scraps, peat moss, sand or ashes, commercial conditioners; at the same time add organic fertilizers, such as manures and bone meal, and inorganic commercial fertilizers; clay soils are best dug up in the fall and left rough - frost will help to break up heavy lumps.
Sandy: dig out bed completely to a depth of 2 feet; on the bottom place either a layer of clay soil, a thickness or two of black polyethylene, fibreglass batts or a 6-8-inch layer of shredded newspaper; replace sandy soil, mixing in the same kinds of humus and fertilizer as for clay soil; work on sandy soils is best done in fall or spring.