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Gardening with Alan: Soil Preparation

Digging maybe good exercise, but is it good for your soil? You might be surprised to learn that many successful organic gardeners never dig their plots at all. I like digging over soil that is just being taken into cultivation, but after that I'm a great believer in light forking and heavy mulching allowing the worms to take down the organic matter for me. Digging is hard work and not to be undertaken lightly, especially if there is a large area of ground to tackle. Although digging is an important method of improving some soils, for others it can actually damage the soil structure. If you are to dig, wait until conditions are suitable avoid working the soil when it is too dry and impenetrable, or too wet and sticking to your tools and boots.

Clay soil
Digging, though hard work, can be of benefit because improving drainage is the key to success with clay soil. You can do this by by digging thoroughly, incorporating plenty of well-rotted organic matter and horticultural grit as you go. Some gardeners recommend double digging, which means cultivating the soil to the depth of two spade blades. I reckon that for most soils and most gardeners, single digging is adequate.Digging heavy clay soils before Christmas will increase the surface area exposed to the weathering effects of frost, which help to break it down by the spring. Alternatively, you can improve the drainage by raising the growing level using no-dig fixed beds. It is important to avoid walking on the soil until it has had time to drain in spring. For an early start, try protecting small areas of prepared soil with black polythene during the winter to keep off heavy rainfall, and replace with clear polythene in spring to help dry the soil out and encourage surface weed seeds to germinate. Hoe off the weed seedlings before sowing begins.

Sandy Soil
Digging is normally unnecessary and maybe impossible if the soil is very stony as well. Simply apply loads of organic matter, such as well-rotted farmyard manure or garden compost, to the soil surface in early spring and let the worms do the rest. If supplies are limited, concentrate your efforts on important areas, such as the vegetable plot, rather than spreading the organic matter thinly over the entire garden. Also, grow green manures on vacant ground. In certain circumstances, double digging maybe necessary if an impervious layer (known as a hardpan) has formed below the surface.

Silty soil
Digging can be damaging to silty soils which are easily compacted under foot, leading to poor plant growth. Avoid this by keeping off the soil as much as possible, especially when the soil is wet. Consider installing a no-dig bed system if growing vegetables. Also, cultivated areas can be prone to surface compaction (known as capping) as the result of heavy rain. Growing green manures on all vacant ground will prevent compaction by rain and improve its long-term fertility. Mulch all other bare soil areas with a loose organic material.

Digging: Step-by-Step
Digging breaks up compacted layers in the soil promoting better drainage and aeration. You can also take the opportunity to remove debris and weeds as well as incorporate organic matter. The secret of success with digging it to do it at the right time, when the soil is not too wet or too dry. The best time to dig heavy soils is in early autumn to allow frost action to weather the clods of earth, breaking them down into a crumbly tilth by the spring. Light soils are best cultivated in early spring. Do not walk on the soil after it's been dug.

To dig, or not to dig?
Pros

  • Good exercise
  • Breaks up compacted layers
  • Improves drainage and aeration on heavy soil
  • Buries annual weeds allows the removal of perennial weeds
  • Exposes large clumps of heavy soil to the effects of weathering
  • Exposes soil-dwelling pests to birds
  • Allows organic material to be incorporated into the soil
Cons
  • Hard work
  • Damages the soil structure if carried out when the soil is too wet or too dry
  • Allows heavy rain to compact the surface of silty soils where vegetation is removed
  • Breaks up the natural structure of good soils
  • May bring infertile subsoil to the surface
  • Brings buried weed seeds to the surface
  • Exposes useful creatures such as earthworms to birds
How to cultivate your soil
If you are going to cultivate your soil, you will need to invest in the right equipment. Don't skimp on this, because cheap tools don't last and are uncomfortable to use. I like the traditional spade with a T- or D-shaped handle to turn over decent earth, but my own soil is so stony that I tend to use a fork for digging as it slides between the stones more easily than a spade.

Forking
Simply push a border or garden fork into the soil to the full depth of its prongs, levering it back and turning it over, breaking up clods and removing weeds as you go. Repeat until the whole area has been cultivated.
Good for Recently cultivated or light, sandy, and stony soils.

Simple digging
The basic form of digging with a spade that's pushed into the soil to the full depth of its blade (known as a spit), before levering back and turning it onto the same area. Chop up large clods and remove perennial weed roots by hand.
Good for Most soils and for removing perennial weeds.

Single digging
Similar to 'simple digging' but a 30cm (12in) wide trench is created with the soil placed to one side. A layer of organic matter is then added to the bottom of the trench. By moving back 15cm (6in) the next strip of soil can be dug up, turned and thrown forward on top of the organic matter. After two 15cm (6in) wide strips have been dug, the first trench will have been filled and a new 30cm (12in) wide trench created. Repeat until entire plot has been cultivated, filling the last trench with the soil from the first.
Good for Impoverished soils and for removing perennial weeds.

Double digging
After creating the first trench as for 'double digging', use a garden fork to the full depth of its prongs in the bottom of the trench to loosen the subsoil and break up any compacted layers. Incorporate organic matter and/or grit as required to improve drainage, before following the procedure for 'single digging'.
Good for Poorly drained soils and deep-rooting crops.

Mechanical digging
Powered machinery can make light work of large areas of ground, but such equipment is very heavy and noisy to operate and difficult to manoeuvre. I've put them through their paces many times on Ground Force, where they can give instant results, but in my Hampshire garden, a hired rotavator doesn't get an outing more than a couple of times a year. Repeated use of a mechanical digger can also damage heavy soils because it's always cultivated to the same level, creating an impervious layer just below the maximum depth of the blades. If the ground is full of perennial weeds you will need to kill these off first or run the risk of spreading the weed problem; couch grass, bind weed and ground elder love being chopped up by a rotavator; it spreads them about even better than nature herself