Double Digging

By Marie Shallcross

Double digging is a method of deep soil preparation in which the soil is loosened to a depth of 60 cm (2 feet) or two times the length of a spade’s blade. The blade length as a measurement is also referred to as a “spit”. This soil is then improved, by adding organic matter, and possibly grit.

Whilst double digging is labour intensive, it is a less exhausting method of soil preparation than triple digging. This is where the soil is excavated to three times the spade depth. The Victorians were great ones for cultivating the soil and used to refer to double digging as ‘bastard trenching’ being an inferior method of introducing organic material to improve the soil when compared to triple digging.

There is of course, the whole dig / no-dig argument. It is often possible to create what is effectively a new border on top of the old soil rather than double digging. No-dig gardening is eminently suited to cultivating flowers, vegetables and fruit in raised beds. It is a subject I’ll re-visit in another article.

But most people when faced with borders in their garden or an allotment, will want to make use of what soil is there already and improve it. Learning about soil preparation and how to dig properly with the correct tools for you and your garden is an early part of the Plews Basic Gardening course.

Where the borders or allotment have been cultivated and organic matter added on a regular basis, even if not for a couple of years, single digging is usually sufficient. The principle of digging out a trench and adding compost is fundamentally the same for both single digging and double digging.

Whilst many ‘old school’ gardeners will be conversant with the whole double digging process, there are many gardeners who have never had the opportunity or need to carry out a gardening task which is centuries old. The following method is for them.

Soil preparation where double digging is required or advisable –

• cultivating soil in new gardens, where the builders may not have left much top soil
• where there is a heavy, unworkable clay soil
• where nutrients need to be added by incorporating organic matter at root level rather than as a mulch
• creating new borders; perhaps in an area that has been lawn
• if planning an orchard or even a few fruit trees planted together in one area a good deep layer of top soil is useful
• where there are invasive, perennial weeds such as bindweed or ground elder; as it’s a more thorough weeding operation than digging them up individually
• on an allotment, where it hasn’t been cultivated for a while
• regenerating a perennial or mixed herbaceous flower border

Advantages of soil preparation using double digging: –

• It increases the amount of space in the soil for air and water, which in turn improves plants growth
• Soil structure is improved, especially when compost or manure are added
• It encourages earthworms and the activities of soil microbes
• It breaks up soil crusts and hard pans, and increases the speed that water that can seep into the soil
• Used with enough organic fertilizer, it helps the soil retain water and nutrients

These advantages are the same for single digging if organic matter / compost is added.

Tools needed

• hoe
• digging fork
• rake
• digging spade
• wheelbarrow/s
• tarpaulin or similar to put soil onto.

Method for Soil Preparation by double digging and adding organic matter

NB: Do not mix the topsoil and subsoil.

The difference should be fairly obvious – top soil is generally darker in colour than sub soil as it has more nutrients.

• Hoe off any annual weeds; if any perennial weeds will lift out easily with the hoe then remove those too.
• Dig a narrow trench, across the width of the bed, about 30 cm (1 foot) wide, and about 1 spit/ 1 spade blade / 10” deep (you may or may not be able to see the subsoil)
• Remove any perennial weeds as you dig. These can be composted separately on site in a sealed container. If allowed to dry out and die, they can be added to the compost bin. Or turned into a liquid feed. Alternatively, hot compost on or off site. Invasive weeds such as Japanese Knotweed have specific requirements for disposal.
• Set aside the topsoil you have dug out, either onto the tarpaulin or in the wheelbarrow; you will need it later.
• Using the digging fork, loosen the soil/subsoil along the bottom of your first trench to a depth of another 10-12”, ie the length of the fork tines/ prongs.
• Now add some compost to cover the base forking/ mixing it in lightly.
• Dig another trench next to the first, in-filling the first trench with this soil; mixing in some more compost as you go.
• Continue this process until you have finished/ reach to the end of your border. You will notice that the level of the topsoil has been raised up from all the organic material being incorporated, as well as through breaking up the subsoil.
• You will have an empty trench at the end of the bed. Add compost as above, then bring the topsoil you set aside and put it into this trench.
• Lightly rake over the border when done, to level the soil.

Weeds seeds will probably germinate within a few days, hoe these off. Repeat as necessary until ready for planting. If any perennial weeds appear, dig out.

Two weeks is the usual time to leave a newly prepared bed before planting. This gives time for the weeds to be dealt with as above, for the worms and micro-organisms to start working their magic and for the soil level to settle.

I have to say, although it is hard work, there is something distinctly satisfying about soil preparation by double digging. There is a visible result in all that turned over soil that can be quite beautiful when caught by the late afternoon sun.

Not that I dig everywhere; I’m also very fond of no-dig gardening in its various forms!

Marie Shallcross
Plews Garden Design