Use a broadfork in your permanent garden beds to relieve soil compaction while conserving the soil food web.
ILLUSTRATION: ELAYNE SEARS
One of the basic tenets of organic gardening is to put as much effort into improving soil fertility as you put into growing your crops. When you use permanent garden beds and pathways, you can concentrate on building soil in deeply worked beds that will improve over a period of years, all the while growing robust, disease-resistant vegetables. Permanent vegetable beds also make more efficient use of water and fertilizer, and soil compaction is limited to pathways where repeated footsteps can naturally inhibit the growth of weeds.
In the 2008 article Gardening for Keeps, I addressed the practical aspects of designing a garden using permanent beds, pathways and green access corridors (which also produce nutrient-rich grass clippings). In this article, I will cover some benefits of permanent garden beds, plus discuss the invisible soil food web and how you can conserve and enrich the life-forms that create superior soil fertility.
Limiting Soil Compaction With Dedicated Beds
One of the big advantages of working in permanent garden beds is that you can limit some forms of soil compaction. Wet soil is especially prone to compaction because water acts as a lubricant. The first time you step on wet soil, it can become 75 percent compacted. After the fourth step in the same spot, 90 percent of the pore spaces are gone. Soil compaction restricts root growth physically because overly dense soil is often impenetrable, plus it handicaps any roots already present by depriving them of oxygen and access to soilborne nitrogen.
You should always try to avoid stepping on your garden beds, but some operations — such as building trellises or using a broadfork — require standing space. In these cases, you can prevent unnecessary soil compaction by standing on a wide board to distribute your weight. Standing on a board or using boards as walking bridges over garden beds will reduce your body’s ground pressure from about 8 pounds per square inch to less than 1. Just as snowshoes make it possible to walk on snow by distributing your weight, boards, wide steppingstones or flakes of baled hay can help protect permanent vegetable beds from the compaction caused by footsteps.
Surface compaction caused by driving rain also can be troublesome, especially for clay soils. When bare soil is subjected to heavy rain, the larger particles and organic matter often wash away in the mud, leaving behind a surface layer of fine particles that dries into a crust. Organic mulches will cushion the soil from the pressure of heavy rain and protect it from erosion. In situations where you can’t mulch — in a newly seeded bed, for example — use a sheet of burlap or other porous cloth to prevent surface compaction. Row cover tunnels cause raindrops to shatter and disperse before they reach the soil, so such covers prevent surface compaction, too.
Permanent Garden Bed Options
Garden experts talk mostly about raised beds, but they are not essential and can be expensive to build. Our point here is that rather than plowing or tilling your entire plot each year, it’s better to lay out and tend permanent areas for dedicated beds and paths. The beds can be raised if you are in a rainy climate, or sunken if your site is dry and windy. Flat beds are also fine, with markers to distinguish the beds from the paths.
If you do choose raised beds, framing is optional. The plants will grow just as well in unframed beds, although you’ll probably need to do a bit of rebuilding each spring. Framing does make the beds look sharp, but lumber for framing can be expensive. Low-cost or free options such as logs or recycled cedar fence rails work fine — just lay them along the long sides of your beds as shown in the Image Gallery. No need to enclose the ends; if you leave them open it will be easier to adjust the beds’ positions if you need to in the future.
Many gardeners like framed garden beds because they can use the frames to support or anchor row covers, plastic, bird netting or other protective covers. For example, pipe brackets installed on the outside of wood-framed raised garden beds can be used to hold the ends of hoops made of wire or plastic pipe, or the ends of wire hoops can be pushed into the ground just inside the frames to provide a firm anchor. The trend toward more extensive use of protective covers is so prominent that many gardeners regard bed framing materials primarily as structural foundations for tunnels or plastic-covered boxes that do the work of small greenhouses.
Improving Soil Fertility
Every soil has an innate texture based on the size of indigenous soil particles. Clay particles are small, so clay soil feels slippery when wet. Sandy soils have much larger particles, so they feel gritty. The tiny size of clay particles means the spaces between them are small, too. In comparison, the spaces between sand particles are huge. To increase the sponginess of either type of soil and prepare it for the task of growing vegetables, a thorough digging is in order to incorporate spaces for air and organic matter, such as grass clippings or compost.
After starting new gardens in several different states, I have yet to encounter soil that was not vastly improved by deep digging in the garden’s first two years, with constant inputs of compost and mulch to raise its organic matter content. With time, the soil’s most important component evolves — a dynamic community of bacteria, fungi and insects. These soil critters exude natural glues and enzymes that bind soil and organic matter particles, creating loamy structure (tilth). After five years of thoughtful organic management, any soil should show notably improved structure, with a profile of 50 percent indigenous soil particles; 25 percent pore spaces for air, water and roots; and 25 percent biosphere of bacteria, fungi, earthworms and other life-forms that call your garden home.
This biosphere is crucial because it’s what improves your soil structure, and it’s the means through which most plants get essential nutrients. Of particular interest are a talented group of soil-dwelling fungi that form dynamic relationships with plant roots. These fungi, collectively known as mycorrhizal fungi, form colonies around plant roots that enhance a plant’s ability to take up nutrients and water, and increase the plant’s resistance to soilborne diseases.
Cultivation has the unfortunate side effect of ravaging beneficial fungal colonies, so when you cultivate your garden beds is important. For example, turning up and shattering spadefuls of soil in fall destroys colonies of many beneficial fungi, and they won’t have time to recover before cold winter weather ends their season of active growth. As a result, spring crops get planted into soil that’s in microbial disarray. The story changes if you stop short of cultivating when cleaning up beds in fall, and instead either plant a cold-hardy cover crop, such as hairy vetch or cereal rye, or cover the soil’s surface with a winter mulch. These are the preferred methods for conserving healthy populations of beneficial fungi, including strains that help legumes capture and accumulate nitrogen, the main nutrient plants need for vigorous new growth.
After coaxing along permanent garden beds with infusions of organic matter for a few years, most gardeners will switch from deep digging to cultivation methods that are less traumatic to the soil’s structure. Shallow cultivation at the surface is often needed to prepare seed beds, mix in organic fertilizers or control weeds. In a small garden, this can easily be done with a sharp hoe, and in a big garden, with a tiller set to go no more than 4 inches deep.
A more vexing problem often lurks below ground, as natural forces will cause the soil in permanent beds to gradually compact and form a “hardpan.” As water percolates downward, fine soil particles will accumulate where drainage slows, much as fine soil remains in your sink after you wash vegetables. This hardpan is known in Australia as “hostile subsoil” — a good phrase to describe subsoil that drains too slowly and is too tight to be penetrated by plant roots.
Research done by cooperating farmers at Cornell University’s Reduced Tillage in Vegetables Project has found that cutting a narrow slit — 1 to 2 inches below the hardpan layer — is the least destructive way to restore the drainage capacity of compacted subsoil, which in turn boosts plant productivity. In a small raised bed, you can do this using a digging fork. In larger gardens, a broadfork is the best tool for preventing or relieving compacted subsoil.
MOTHER EARTH NEWS first published plans to build your own broadfork in 1980 (read How to Build a Broadfork). The broadfork has since been embraced by many organic gardeners as the best way to reintroduce air to the deepest sections of a permanent bed without tearing up the soil’s structure and thus setting back the soil-building process. Along with air, soil amendments such as compost, wood ashes or organic fertilizers will naturally sift into deep holes made by a broadfork.
I use a broadfork often, but in my clay soil, it isn’t entirely able to take the place of deep digging to mix in organic matter and a fresh supply of air. I try to limit my forking or digging frenzies to warm weather — for example, when renovating beds after I’ve harvested garlic and cool-season spring crops. Soil recovers faster from digging under warm conditions, and nutrients released as the shredded fungal mycelium and roots decompose can be utilized by the next crop.
When you keep garden walkways in vegetation or mulch while minimizing cultivation in adjoining beds, you’re practicing what is called “zone tillage.” Walkways become refuge areas for night crawlers and other soil-building creatures, while the beds get cultivated only as much as needed to accommodate changing crops. Even compacted pathways can host numerous earthworms as long as there is vegetation or mulch at the surface. In a 10-year study done in West Lafayette, Ind., earthworm counts under pathways planted with a bluegrass-clover mixture averaged 400 per cubic meter, compared with 340 in an open dairy pasture and only 20 in a no-till field planted in continuous corn. Another study from Germany showed that soil compaction had little effect on the production of earthworm castings. Rather, having plenty of rich vegetation at the soil surface — either as mulch or mowed greens — will transform pathways into earthworm havens.
Recently I heard a soil-minded gardener joke that growing cauliflower takes about five years — four for soil-building and one to grow the crop. He was right, because cauliflower and many other veggies won’t prosper in low-integrity soil. Growing in wisely nurtured and maintained permanent garden beds guarantees better results, season after season.
Read more: You can learn more about how to manage your soil’s food web in Barbara Pleasant’s book The Complete Compost Gardening Guide.
Contributing editor Barbara Pleasant gardens in southwest Virginia, where she grows vegetables, herbs, fruits, flowers and a few lucky chickens. Contact Barbara by visiting her website or finding her on Google+.