Garden Soil Preparation Techniques: Sheet Mulching and Double Digging

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Getting ready to sheet mulch the lawn at a public park.
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The double digging process enhances soil tilth and productivity by creating open space in the soil for downward root growth. Once your bed is dug, avoid stepping directly on it, as that compacts the newly aerated soil.
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Once you've sheet mulched, shape and form your beds as needed. This entire yard was sheet mulched after cement was removed and soil was amended. Bringing fresh compost, mulch and straw to the site created these garden beds.
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Sheet-mulching is a layering of compost, cardboard, and mulch over a lawn or other area in preparation for making a garden. Sheet mulching harnesses the power of worms and composting microorganisms in the soil. Cardboard keeps the light out to kill the lawn or weeds underneath; worms eat through the cardboard to get to the compost, turning and conditioning the soil as they go, thus doing most of the hard work for you.
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Piling on the cardboard over the first layer of mulch goes quickly when done in community. This front lawn was sheet mulched in less than three hours by this homegrown guild.
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“Urban Homesteading” is a must-have handbook for city folk with a passion for the simple life. Readers will find concise how–to information that they can immediately set into practice, from making solar cookers and growing tomatoes in a pot to raising chickens on a tiny plot and maintaining the mental serenity of country life in the fast–paced city environment.

Across the country, city dwellers are taking part in the urban homesteading movement by ripping up their lawns and installing food-producing gardens. In Urban Homesteading: Heirloom Skills for Sustainable Living(Skyhorse Publishing, 2011), authors Rachel Kaplan and K. Ruby Blume explain how two gardens soil preparation techniques, sheet mulching and double digging, can transform a bare yard into a new garden with great yield. This excerpt is taken from the Chapter “Urban Dirt Farmer.” 

Two Methods of Garden Bed Prep: Sheet Mulching and Double-Digging

There are many ways to start a garden, even on a spot that has never been gardened before. If you have a patch of bare land, or a bit of lawn, you can reclaim and redesign it as a food, herb or flower growing landscape. Sheet mulching and double-digging are two methods for reclaiming the land for growing. They tend to the soil in different ways but don’t think of them as an either-or proposition. Sheet mulching can be used to quickly transform a lawn or bare yard in a garden. Double digging can be used subsequently to increase the yield and productivity of the garden. Using both methods will be the most thorough way to prepare whatever soil you’ve got.

Sheet Mulching

The front lawn must go the way of the dodo. No longer will we spend our time in submission to the manicured lawn, wasting water and energy. Turn your lawn into your lunch! Sheet mulching reclaims the lawn with an organic “lasagna” of cardboard, compost, and mulch. This is a low-effort way to create a garden bed while adding fertility to the soil. You can do this project in an afternoon with some friends.

Easy Sheet Mulch Recipe 

After a rain, or in the morning after one hour of watering, lay down 2 to 3 overlapping layers of cardboard, 3 to 5 inches of compost, and 8 to 12 inches of tree mulch. For every 100 square feet that you are covering, you will need 1 to 2 cubic yards of compost, and 3 to 4 cubic yards of mulch.

You can sheet mulch any time, but the best time to do it is in the fall, or before your biggest rainy season. You want your sheet mulch to get nice and wet so the worms will come up and do the rest of the work for you.

Materials for Sheet-Mulching 

Cardboard: The cardboard smothers out all the weeds and provides a foundation for your sheet mulch. Get the largest pieces possible. Try looking behind furniture stores or bike shops and driving around to the more industrial parts of town to scavenge cardboard. Remove all the staples and tape and use brown cardboard if you can find it. Layer your cardboard, making sure to overlap the edges. This is important if you are trying to get rid of grass or other noxious weeds. Two layers will work, but for particularly tenacious or noxious weeds use three.

Compost: In some places, the best resource for compost is the dump, where the green garbage materials collected at the curb are turned into compost and sold back to the consumer. If you don’t live in an area that has this kind of program, see if you can find a nearby farm that will sell (or give) you well-composted horse manure. As a last resort, buy compost in bags at the garden supply store, but it will cost you a lot to do this project that way. A cubic yard is a lot of compost. A regular pick-up truck with normal suspension can probably handle half a yard at a time.

Tree Mulch: You can often access this by calling a tree trimming company and having them dump the contents of their truck at your house. They often have to pay to dispose of their waste, so they are happy to give it away for free, but beware: some places insist you take a full truckload, which can be between 10 and 20 cubic yards of material. Share it with your neighbor who is going to be so inspired by your sheet-mulching project that they’re going to do one too. Check the yellow pages for arborists and tree doctors. Try to get mulch that is a mixture of wood and leaves, and make sure the mulch comes from a disease-free tree. Avoid highly acidic tree mulch (like redwood) or trees that are full of seeds and will sprout throughout the garden.

When Is it Ready? 

Sheet mulch needs at least one full rainy season to do its magic. This is when the worms come out and eat through the cardboard to get to that delicious compost. If you are starting in the beginning of spring, you can plant in the same season. Try planting simple veggies like zucchini or potatoes in your sheet mulch. For zucchini, scoop out a hole in the compost and mulch about the size of a soccer ball and fill it with rich potting soil. Plant your starts into this and re-cover with a bit of mulch. Water thoroughly and often. For potatoes do the same, using a bit less dirt around each piece of seed potato.

If you are patient or your soil is really bad, try a two-year program. Sheet mulch in the fall, and rather than turning your mulch in the spring, plant zucchinis or potatoes or fava beans, which bring nitrogen to the soil. At the end of that growing season, water deeply or wait for the first rain; then use a fork to lightly mix the layers, but don’t turn under or completely cultivate. Add another inch or two of compost and another 3 to 4 inches of tree mulch. The following spring, cultivate your soil as you normally would; you will have created 6 to 8 inches of gorgeous topsoil. Be sure to get under the lowest layer of what you added and mix the native soil in before you start planting your garden.

This kind of project is best done with friends and neighbors. You can do it in rounds: first my house, then yours, then yours, until the entire neighborhood has forsaken its lawns for the possibility of growing its own lunch.

Double Digging

Double digging is the energetic opposite of sheet mulching, a much more labor intensive but equally useful gardening technique that enhances soil fertility by intensively digging the area where you will be planting. Developed by British horticulturist Alan Chadwick, double digging has been used in gardens throughout the world to increase soil fertility and garden yield to an incredible degree. John Jeavons extended this practice at his research and development site in Willits, California, where micro farming in largely inhospitable terrain was refined, and small double-dug beds became highly productive. Double digging maximizes production yield in small spaces and has been used to great effect all around the world, especially in Third World countries where people are more dependent on their small home gardens.

Part of the bio-intensive method of gardening, a double-dug deep bed of 100 square feet can produce from 200 to 400 pounds of vegetables a year. According to USDA statistics, the average American eats 322 pounds of veggies a year, so just one bed—20 feet by 5 feet—can keep an adult supplied with vegetables for an entire year. A backyard micro farm located on as little as 1/8 of an acre can produce a large portion of a family’s vegetable needs for the year. Bio-intensive methods use less water per unit of land than commercial agriculture and still less per pound of food produced. With the double-digging method, plant roots can go down as far as ten feet if the soil is loose enough, rather than spreading horizontally, so there is less competition between plants for nutrients at the surface.

How to Do It 

• Begin by spreading a layer of manure over the top of the bed.
• Starting at one end of the bed, dig a trench about a foot deep.
• Put the removed soil into a wheelbarrow.
• Loosen the subsoil by using a garden fork and plunging it into the soil and moving it around.
• Dig a second trench next to the first, throwing the topsoil and manure into the first trench. Be like a gopher and work the subsoil in the bottom of the second trench.
• Dig a third trench and repeat the process.
• To make a path, throw all the pebbles and rocks you dig up to the side of the bed.
• Dump the soil in the wheelbarrow (from the first trench) into the last trench.

When you finish, throw the topsoil from the path-to-be on the top of the bed, and line the path with the stones and rocks you’ve uncovered.

You’ll want to avoid stepping on this bed because that will undo all the hard digging work you’ve done, and recompress the soil. Make your path spacious enough to accommodate your walking, and design your bed to be the right dimensions so that you can reach across it from either side without having to step upon it. Four feet across is about as big as you’ll want it to be if you want to be able to reach across the bed from either side.

This excerpt has been reprinted with permission from Urban Homesteading: Heirloom Skills for Sustainable Living by Rachel Kaplan with K. Ruby Blume, published by Skyhorse Publishing, 2011. 

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