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How to Dig a Vegetable Garden - Soil Science and Preparation
by Don Boekelheide
Before you start digging or tilling, ask yourself two questions:
What will be growing in this bed, and when was it last tilled?
If you don’t plan to grow vegetables or annual flowers, or if you aren’t establishing a bed of bulbs or asparagus, you may not want to dig it at all.
Did you do a lot of digging last year here?
If so, it’s best to simply spade or single dig this time around, and only as needed.
Digging a New Bed
But let’s say this is a new bed for the vegetable garden, or one that hasn’t been deeply worked in three or more years. Classic double digging makes sense. Here's how to double dig your vegetable garden.
With really tough sites the first time around in the Carolina Piedmont, a 5-pound pick is often the best tool for the first year. Rototillers deserve their own article, but for larger market gardens they can certainly be appropriate tools, if used properly.
Tools and Materials:
You’ll need a shovel or spade, a digging fork, a metal-toothed rake, a hoe and a wheelbarrow.
A tamping board is helpful, as are stakes, string, a hammer and a measuring tape.
1. Begin by clearing the area of rocks, weeds and debris. All appropriate organics go straight into the compost pile.
2. Mark the edges of the bed with stakes and twine (or, if you are curving, whatever technique works for you. (Garden hoses make good temporary curved bed outliners.) If you like, measure with the tape; if you know the size of your bed, it takes the guesswork out of planning plantings and fertilizing. Be sure you can easily reach the center of the bed from either side; for me, that’s a distance between 1 to 1.2 meters.
3. Spread 2-3 cm (an inch) of high-quality compost (best) or composted large animal manure evenly over the entire top of the bed. (You can go as high as 10 cm of plant-based compost—not mature—but there’s no need to overdo it.) A 1 meter x 3 meter bed (about 3 feet x 10 feet) needs about 90 liters of compost (about 3 cubic feet).
Note: This is also a good time to add lime and phosphate fertilizer, but check first with a soil test to find how much to apply.
Phosphate rock and bonemeal are two traditional organic agriculture sources of P.
4. Park the wheelbarrow beside the bed. With the shovel or spade, remove a trench about 30 cm deep (about a foot, the depth of a regular shovel blade), the width of the bed (roughly a meter). Put it in the wheelbarrow, and forget about it for a while.
5. With the spading fork, loosen the soil in the bottom of the trench. Don’t turn it over or mix it with the top soil, just waggle the fork around.
6. “Roll” the topsoil from the next 30 cm (about 1 foot) swathe into the trench, covering the forked soil. That makes a new trench.
7. Repeat step 5 in that trench.
8. Continue down the bed to the end, alternating steps 6 and 7 until the end of the bed. Work backwards, so you never step when you’ve just double dug (i.e., spaded, then forked).
9. Fill the final trench, after forking, with the soil in the wheelbarrow.
10. Add a complete organic fertilizer that contains nitrogen, such as Espoma Garden-tone or an OMRI equivalent. You can follow the bag recommendation from Espoma or calculate the amount of nutrients you need, remembering to take into account any phosphorus you already added.
11. With the hoe (a large, sturdy field hoe is best), work the fertilizer into the top 10 – 15 cm (about 4 inches) of soil, breaking up clods as you go.
12. With hoe and rake, smooth the bed and form it into a gently-rounded shape so it is higher than the neighboring paths. Rake any tough clods into the paths.
13. Optional: Sprinkle another 1-2 cm (about ½ inch) of quality compost on the top of the bed and gently rake it in to the very top layer of soil.
14. Optional: With a roller or tamper (a piece of plywood the width of the bed and a meter or so long), gently roll or tamp the soil (by standing on the plywood for a minute).
You are ready to plant!
Hints:
Remember, you only do this every three to five years, in most garden situations.
In between years, add only 1-2 cm compost (about ½ inch). In most cases, you will also want to add appropriate fertilizer for the specific crop since, unlike on a farm, most urban gardeners cannot utilize recycled nutrients from livestock or fallows with cover crops.
Some gardeners do very little “in between” tillage, often relying on deep mulching techniques. Others single dig annually (skipping the fork step), especially to incorporate cover crops. You can also use a U-bar or broad fork to lift the soil without turning it.
There are other sensible soil preparation methods, of course. With really tough sites the first time around in the Carolina Piedmont, a 5-pound pick is often the best tool for the first year. Rototillers deserve their own article, but for larger market gardens they can certainly be appropriate tools, if used properly.
Whatever technique you choose, you’ll be well served to remember the basics: keep topsoil on top, add organic matter (good compost), watch pH and correct with lime based on a soil test, and don’t ever work clay soils when they are wet. © Don Boekelheide 2011 all rights reserved
Editor: Find more Soil preparation techniques by Don Boekelheide in Soil Science and Fertility.
Don Boekelheide
For the past five years, Don Boekelheide, has taught a hands-on food gardening class at Central Piedmont Community College, modeled on his Peace Corps training. Don holds a Master of Science degree in agriculture from Cal Poly and formerly served as a Peace Corp Ag teacher in Togo. He is a former Extension Master Gardener volunteer for Mecklenburg County, NC.
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