Double-digging is one way to create good soil structure for healthy plant growth.
Decades before the terms “eco-friendly” and “sustainable growing” entered the vernacular, How to Grow More Vegetables (Ten Speed Press, 2012) demonstrated that small-scale, high-yield, all-organic gardening methods could yield bountiful crops over multiple growing cycles using minimal resources in a suburban environment. The concept that John Jeavons and the team at Ecology Action launched more than 40 years ago has been embraced by the mainstream and continues to gather momentum. Today, How to Grow More Vegetables, now in its fully revised and updated 8th edition, is the go-to reference for food growers at every level: from home gardeners dedicated to nurturing their backyard edibles in maximum harmony with nature’s cycles, to small-scale commercial producers interested in optimizing soil fertility and increasing plant productivity. In this excerpt from chapter 1, “Deep Soil Creation and Maintenance,” discover how to create good soil structure for your raised bed garden.
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Properly preparing your raised beds is a great way to get healthier soil. A well-prepared bed will rejuvenate your soil structure, and proper soil structure and sufficient nutrients allow uninterrupted and healthy plant growth. Loose soil enables roots to penetrate the soil easily, and a steady stream of nutrients are allowed to flow into the stem and leaves. Plants close together in narrow beds are easy to cultivate and harvest, and closely spaced plants benefit from a good microclimate and healthy soil.
The initial preparation and planting of a raised bed may take 6 1/2 to 11 hours per 100 square feet. If you are lucky enough to have loose soil, the time commitment will be less. No matter how long it takes, the time invested pays off with increased yields and healthier soil and plants.
As you become more skilled at double-digging, and your soil becomes healthier, the time invested is greatly reduced. Often a 100-square-foot bed can be prepared in two hours or less. We estimate that only 4 to 6 1/2 hours should be required on an ongoing basis for the entire bed preparation and planting process as the soil develops better structure over time with correct care and compost.
We recommend investing in quality tools from the beginning — poor tools can be tiring and discouraging — but do whatever gets you started. Experiment with short D-handled and long, straight-handled tools to figure out which you like best — some folks find that D-handled tools allow them to work more efficiently, while some like the extra leverage and upright posture that long handles allow. A 5/8-inch-thick plywood board, 2 to 3 feet long by 3 to 5 feet wide, serves as a “digging board” to stand on. A digging board will spread your weight out so that you compact the soil less when standing on the bed area. You can treat the board with linseed oil to protect it against soil moisture, if you like. A bow rake makes leveling and forming the bed easy. If you don’t have one, hula hoes are the perfect tool for cultivating the upper 2 to 4 inches of soil around closely spaced plants.
Carefully choose a place for your raised beds that has access to water and sunlight — preferably 7 to 11 hours of direct sunlight each day. To begin, mark out a bed 3 or 4 feet wide and at least 3 feet long. A 3-foot-square space is big enough to form a microclimate beneficial to your plants and the soil. Longer beds are popular, but make sure you can easily reach the entire bed without standing on it.
The best time to double-dig the soil is early morning or evening during spring or autumn. The cooler air temperature helps preserve soil life. Dig only when the soil is evenly moist — it’ll be easier for you and better for the soil. Soils that are too wet are easily compacted, and compaction destroys friable structure and minimizes aeration. These conditions harm the beneficial microbiotic life that your plants need to thrive. Soils that are too dry tend to be hard and difficult to dig, and good soil structure will be difficult to maintain if your digging results in just so many brick-like clods.
Test the soil by forming a ball in your palm. Soil is too dry for digging when it is loose and will not hold its shape after being squeezed in the palm of your hand (in cases of sands or loams) or when it cannot be penetrated by a spade (in the case of clays). Soil is too wet when it sticks to the spade as you dig.
The goal of double-digging is to loosen the soil up to 2 feet below the surface. The first year, you may be able to reach only 15 to 18 inches with reasonable effort — that’s just fine. You don’t have to hurt yourself to have a good garden. Nature, the loosened soil, worms, and plant roots will further loosen the soil with each crop, so digging will be easier each year and the depth will increase over time. Be patient in this soil-building process. It takes 5 to 10 years to build up a good soil (and one’s skills). Actually, this is very rapid; it takes nature thousands of years to build the 6-inch layer of topsoil needed to grow a good crop of food. After the soil has been initially prepared, you will find that the double-digging method requires very little work per unit of food produced. The Irish call this the “lazy bed” method of raising food.
After good soil structure is established, you can simply cultivate the top 2 to 4 inches. Another way to keep soil light and airy is to single-dig, or loosen the top 12 inches with the spading fork. Single-digging between crops in the same growing year will help improve your soil faster.
First, check the soil moisture. The soil should be evenly moist to facilitate digging, but not saturated. If needed, water the area to be dug. For hard, dry clays that have not ever been cultivated, this may mean up to 2 hours with a sprinkler. Begin the next steps when soil is evenly moistened.
Spread a layer of compost over the entire area to be dug and place the digging board on the bed, leaving a foot or two from the end of the bed for the first trench. Be sure to dig trenches across the width of the bed. Remove soil from the top foot of the first trench with a spade. Set this aside, and try not to turn it over — most of the beneficial microbes are in the top 6 inches of the topsoil.
Standing in the trench or on the digging board above the trench, dig down another 12 inches (or as deep as possible) with a spading fork, a few inches at a time if the soil is heavy or tight. Try not to turn the soil over too much, just lever the tines through, loosening large chunks. If necessary, toss large clods a little bit, allowing them to fall on the tines and break; you can also stab clods with the fork and lever them apart.
After loosening the first trench, move the digging board back and dig the next trench, moving each spadeful of the top 12 inches of soil forward into the first trench. When digging, make as few motions and use as little muscle as possible in this process. As you dig the soil, you will discover a smooth rhythm in which you are virtually just shifting your balance and weight rather than straining your back and arms. Repeat the subsoil-loosening process in each trench before moving on.
Leveling the topsoil with a rake every three to four trenches will help you maintain an even depth and good drainage. Notice how, as you dig, some of the compost you spread on the bed slides down into the trench. This approximates the way nature adds organic matter and will quickly enrich the entire depth of your soil.
Always make sure that the upper layer of soil (the top 12 inches) is not turned over during the double-dig. Try to aim for a balance of loosening the soil, while not mixing it too much, to preserve the natural structure of your soil.
After digging, the bed will often be 2 to 10 inches higher than the soil’s original surface.
A good soil can contain around 50 percent air space, allowing for increased diffusion of oxygen (which the roots and microbes depend on) into the soil and of carbon dioxide (which the leaves depend on) out of the soil. The increased “breathing” ability of a double-dug bed is a key to improved plant health. To prevent erosion and promote more even water saturation in a clay bed with a high rise, you can create a small soil lip around the top of the bed.
You may choose to add compost at different points when double-digging. Instead of applying compost only after the double-dig, consider spreading a 1/2-inch layer over the bed before digging and/or a 1/2-inch layer into the trench during the dig.
The U-bar, or broadfork, can be used as a substitute for the ongoing double-dig for soil that is in reasonably good shape. The broadfork is quicker and easier than using a spade and a spading fork. The broadfork tines don’t disturb the soil as deeply as a double-dig, but the lower region of the soil compacts more slowly than the top, so it’s often sufficient. It aerates the soil less than a full double-dig, which is an advantage in looser, sandier soil and can be a problem in tighter clays. If you use a broadfork regularly, only double-dig as often as required to keep your soil light and fluffy.
As your soil improves and the large clods disappear, your bed may not raise as high as initially. Do not worry about this. It is a sign that you and your soil are successful. The goal of double-digging is not the height of the bed, but a reasonable looseness and good structure of the soil.
To check your structure, squeeze some slightly moist soil firmly in your hand, then open your hand. If the soil falls apart easily, it does not have good structure. If it holds the shape of your hand even when you press it gently with the fingers of your other hand, it does not have good structure. If the soil breaks apart into small clumps when you press it with your fingers, it probably has good structure.
Remember that structure is different from texture. The texture is determined by its basic ingredients: silt, clay and sand particles. The soil structure is the way these ingredients hold together. With your assistance, sticky “threads” exuded by microbial life and the roots produced by the plants help to loosen clay soil and improve sandy soil.
Once the bed is prepared, you will truly appreciate its width. A 3- or 4-foot-wide bed can be fertilized, planted, weeded and harvested from each side with relative ease, and insects can be controlled without walking on the bed.
This width also allows a good mini-climate to develop below closely spaced plants. You may wish to use a narrower bed, 1 1/2 to 2 1/2 feet wide, for easier harvesting of trellised plants like tomatoes, pole beans and pole peas.
Try not to step on the growing beds once they have been prepared. This compacts the soil and makes it more difficult for the plants to grow. If the bed must be walked on, use the double-digging board. This will displace your weight over a large area and minimize the damage. Plants obtain much of their water and nutrients through the contact of their root hairs with the soil. If they do not develop an abundant supply of root hairs, less water and fewer nutrients are taken in. The root hairs are more numerous and vigorous in looser soil, so keep your soil loose.
When weeding, note that the entire weed root usually comes up out of loosened raised bed soil. This is easy weeding, and if you get the entire root, next year will be even easier.
The living mulch shade cover provided by mature plants helps to keep the soil surface moist and loose. If the soil compacts between young plants before the microclimate takes effect, you can cultivate to fix the problem.
Once your beautifully alive bed is prepared, keep it evenly moist until and after planting so the microbiotic life and plants will thrive. The bed should be planted as soon as possible after digging to take advantage of the new surge of life made possible by bringing together the soil, compost, air, water, sun and fertilizers.
This excerpt is reprinted with permission from How to Grow More Vegetables: And Fruits, Nuts, Berries, Grains, and Other Crops by John Jeavons and published by Ten Speed Press, 2012.
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